Download * Porphyrins. XXVII.

THE JOURNAL OF CHEMICAL PHYSICS
VOLUME 59,
NUMBER 2
1 5 J G L Y 1 9 7 :l
Porphyrins. XXVII. * Spin-orbit coupling and luminescence of Group IV complexes
Martin Gouterman, Frederick P. Schwarz, and Paul D. Smith
Department of Chemistry, University of Washington, Seattle. Washington 98195
D. Dolphin
Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138
(Received 25 September 1972)
Luminescence studies are reported on compounds M(IV)X,P: M is Si, Ge, Sn, Pb; X is F, Cl, Br, I,
OH, benzoate; P is etioporphyrin or octaethylporphin. (One tetraphenylporphin is reported for
comparison.) We find fluorescence yields 2 X 10 1 :;, <p/ :;, 2 X 10- 4; phosphorescence yields
7 X 10- 2 :;, </>p :;, 3 X 10- 1 ; and phosphorescence lifetimes 100 msec :;, Tp :;, 1 msec. The contrasting
vibronic envelopes of phosphorescence for octaethylporphin and tetraphenylporphin derivatives is
explained by attributing the former to transitions 10± 1 -. 10c;"D and the latter to 10± 9 ~ 10c;ND' where
± 1 and ± 9 are pseudoangular momentum quantum numbers. The spin-orbit interaction is calculated
by the extended Hiickel mode!, and it is found that the ligands have far more effect than the metal, in
agreement with the data. However a simple relation between decay rates and spin-orbit coupling fails
quantitatively, and the extended Hiicke! model appears to exaggerate the contribution of the ligand to
the spin-orbit coupling.
I. INTRODUCTION
Spin-orbit coupling in organic molecules has been
studied extensively over the past two decades. 1 A
prototype system used in these studies has been
halogen substituted napthalene molecules in an alkyl halide or alkane solvent. 2,3 Enhancement of
spin-orbit coupling is indirectly observed in the
shortening of the triplet state lifetime and in an increase of the phosphorescence yield upon either increasing the atomic weight of the halogen, of the
external halide, or of both. These studies have
been extended to the nitrogen heterocycles, 4,5 halogenated phenanthrenes 6 charge-transfer complexes' 7,8 heavy atom-aromatic molecular complexes,9 and doped crystals. 10 ,11 The results of
these studies may be divided into two categories:
(1) identification of those states and orbitals coupled by spin-orbit interaction 4 -6 and by the combination of spin-orbit and vibronic interactions ll - 13 ;
(2) attempts to estimate the effect of spin-orbit
coupling on the two radiationless transitions S l
T1
and T1~~/Y- So and on the radiative tranSition T1
_SO.7,10,14-19 Results in this latter category have
generally been based on assumptions: (i) All radiationless decay from the Sl state proceeds through
the Sl'~' T1 route. 16-19 (ii) The radiative transition
rate T 1 - So is unaffected by perdeuteration. 7,10,14
Although the first assumption is clearly questionable, the second assumption seems well founded.
However, Johnson 2o has shown recently that even
this second assumption fails in benzene, where
perdeuteration does affect the radiative rate T 1 - So
through the Franck-Condon integrals. Clearly the
problem of radiationless decay is still not fully understood and can benefit from systematic studies on
"V/V-
other systems where the effects of heavy atom substitution can be experimentally studied and theoretically analyzed.
In this paper we report some systematic studies
of the heavy atom effect on radiationless transitions
of Group IV metalloporphyrins. That the emission
yields of porphyrins are far more affected by metal
substitution than are the absorption spectra, has
been clear since the pioneering studies of Becker
and Allison. 21 Results of later studies in this field
have been the subject of fairly recent reviews. 22,23
Metal effects can arise both because of spin-orbit
coupling and because of paramagnetic effects due
to unpaired metal d electrons. Ake and Gouterman24 ,25 analyzed the lowest excited state wavefunctions for the electronic and spin-orbit coupling elements for VO, Co, Ni, Cu, and Zn complexes.
More recently the effect of spin, vibronic, and environmental crystal field on the zero field splitting
of the zinc porphyrin triplet state has been discussed. 26,27 A growing interest in this subject can be
expected as the result of recent successful studies
by optically detected magnetic resonance (ODMR)
of the zinc porphyrin triplet state. 26,28
Although there have been many studies on porphyrin luminescence, 21-23 the Group IV compounds
have not attracted systematic study. While Becker
and Allison 21 reported on Sn(IV) and Pb(II) complexes, we know of no previous reports of Si(IV),
Ge(IV), and Pb(IV) porphyrin luminescences. Recently a theoretical study of the electronic structure of Group IV porphyrins 29 has clarified the effect of metal oxidation state on the spectra. In this
paper we report the fluorescence and phosphorescence quantum yields of the Group IV metal complexes of octaethylporphin and etioporphyrin, two
676
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PORPHYRINS. XXVII
skeletons whose electronic properties are essentially the same. We show the effect on luminescence
not only of the central metal but of axial ligands,
whieh also introduce heavy atom effects. We shall
attempt to relate the observed effects on Sl-vA- T l ,
T l "'''" So, and T 1 - So to the revelent spin-orbit coupling matrix elements. These studies should complement the similar studies going on with the halogenated aromatics. Our findings may also have
some biological relevance, for the coupling elements we uncover here may be playing a role in the
funetioning of the heme enzymes.
II. EXPERIMENTAL
A. Apparatus
The room temperature absorption spectra were
recorded on a Cary 14 spectrophotometer. Low
temperature and room temperature excitation and
emission spectra were detected by an apparatus
described previously30 with the following modifications: (i) all excitation was provided by a 1000 W
G. E. type DXW tungsten-halogen lamp; (ii) both
RCA phototubes 7265 and 7102 were used. Allluminescence spectra were corrected by determining
the detection sensitivity with respect to the known
output of a standardized Eppley spectral irradiance
lamp.
The room temperature and liquid nitrogen temperature quantum yields were measured with an
additional instrument. A blank was inserted in the
sample holder and the intensity of the excitation
light transmitted through the sample was measured
by either an Optics Technology Model 610 power
meter or a Hewlett-Packard 8330A radiant flux detector. Then the sample was inserted and both the
transmitted excitation light and the luminescence
of the sample were measured. The integrated corrected luminescence spectrum was compared to the
integrated fluorescence of Zn etioporphyrin I, which
has a fluorescence quantum yield of 0.04. 31 In addition the number of photons absorbed by the standard was also determined and compared to that of
the unknowns. The unknown Group IV octaethylporphins and etioporphyrins and the Zn etioporphyrin were excited by 530 nm light. A check was
made against another standard, Pt etioporphyrin
I, which has a quantum yield of 0.9 at 77 OK. 32 All
quantum measurements were reproducible to within
10% error.
Lifetime measurements were made on the same
apparatus as previously described. 30 A gated photomultiplier mode was used in all the lifetime determinations.
B. Synthesis
I Dichlorooctaethylporphinatosilicon (IV) [Si(IV)CI 2 0EP)33
Octaethylporphin (500 mg) was placed in a Carius
tube (100 ml capacity) and covered with 30 ml of
677
dry pyridine (distilled from barium oxide). Argon
was passed over the surface of the mixture and the
tube was cooled in liquid nitrogen until the pyridine
froze. Silicon tetrachloride (2 g) was then added
and the tube sealed after cooling in liquid nitrogen.
The cold tube was then placed in a Carius oven, allowed to warm to room temperature and then heated
at 170 for 12 h. After cooling to room temperature the contents of the tube were poured into cold
water. The aqueous phase was extracted with
methylene dichloride and the methylene dichloride
was washed with IN HCl until all the pyridine had
been removed. After a further wash with water the
organiC phase was dried over CaC1 2, filtered, and
the product (310 mg) crystallized from hot CH2 C12/
cyc1ohexane. An analytical sample was recrystallized from chloroform/ cyc1ohexane.
Analysis: Calculated for C36H44N4C12Si: C, 68.43;
H, 7.03; N, 8.87; CI, 11.22. Found: C,68.01;
H, 7.27; N, 9.15; Cl, 11.64.
0
2. Dichlorooctaethylporphinatogermanium (IV)
[Ge(lV)CI 2 0EP)33
This compound was prepared as above using germanium tetrachloride (2 g) and octaethylporphyrin
(500 mg). An analytical sample was recrystallized
from chloroform/ cyc1ohexane.
Analysis: CalCUlated for C36H44N4C12Ge: C,
63.93; H, 6.57; N, 8.29; CI, 10.48. Found: C,
64.36; H, 6.52; N, 8.07; CI, 10.70.
.1. Dichlorooctaethylporphination (IV) [Sn(IV)CI 2 0EP)
Octaethylporphin (500 mg) was placed in a Soxhlet
extractor, and extracted with boiling glacial acetic
acid (200 ml) which contained sodium acetate (5 g)
and stannous chloride (1 g). After 24 h the mixture
was cooled, filtered, and the filtrate washed with
hot water and then methanol. The residue was dissolved in a minimum amount of chloroform and this
solution was washed with ION HCI, and then dried
over CaCI2. The dry solution was filtered and the
product (480 mg) recrystallized from chloroform/
cy c10hexane .
Analysis: Calculated for C36H44N4C12Sn: C,
59.85; H, 6.15; N, 7.76; Cl, 9.81. Found: C,
60.21; H, 6.06; N, 8.17; Cl, 10.15. Thethreedimension structure of this sample has been measured by x-ray diffraction. 34
4. Octaethyiporphinatolead (II) [Pb(II)OEP) 33
Octaethylporphin (500 mg) was suspended in refluxing DMF (100 ml). Lead acetate (2 g) was added
and the mixture was refluxed for 10 min. The hot
solution was then filtered into ice cold water (500
ml). The preCipitate was collected by filtration
and washed with cold water. The solid was then
dissolved in a minimum of methylene dichloride
and dried over sodium sulfate. The mixture was
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678
GOUTERMAN, SCHWARZ, SMITH, AND DOLPHIN
filtered and the product (410 mg) precipitated by
adding pentane to the refluxing methylene dichloride
solution.
Analysis: Calculated for CS6H44N4Pb: C, 58. 68;
H, 6.03; N, 7.61. Found: C, 59.02; H, 6.44; N,
7.68.
5. Dichloroetio (I) porphinatotin (IV) [Sn(lV)(OHhEtio]35
This compound was made from the free base etioporphyrin I and stannous chloride using a Soxhlet
extraction procedure in glacial acetic acid, similar
to that described for Sn(IV)ClzOEP.
Analysis: Calculated for CszHssN4ClzSn: C,
57.69; H, 5.45; N, 8.41; Cl, 10.64. Found: C,
57.94; H, 5.60; N, 8.27; Cl, 11.00.
6. Dihydroxoetio (l) porphinato tin (IV)
[Sn(IV)(OHh Etio]
Approximately 10 mg of Sn(IV)ClzEtio prepared
above was dissolved in 100 ml of refluxing methanol, followed by the addition of tml of saturated
aqueous NaOH. Examination with long wave uv
light shows a dramatic increase in the orange fluorescence indicating the successful displacement
of the chloride by the hydroxide. Ten ml of water
was then added, and the methanol removed by rotary evaporation. The precipitated porphyrin was
suction filtered, washed with hot water, and recrystallized from CH2 Clz•
Analysis: Calculated for C32HssN40zSn: C,
61. 07; H, 6.09; N, 8.90. Found: C, 60.4; H,
6.1 (assumed); N, 7.3.
7. Dibenzoatoetio (I) prophinatotin (IV) [Sn(lV)(BzhEtio]
A 100 ml solution of 10- 4M Sn(IV)(OH)2Etio in
chloroform was refluxed with 50 ml of a saturated
aqueous solution of benzoic acid for several hours.
The two solvent layers were then separated and the
chloroform layer was washed several times with
hot water to remove the last traces of benzoic acid
and finally dried over anhydrous NazS04.
8. Difluoroetio (l) prophinatotin (IV) [Sn(lV)F 2Etio]
A 100 ml solution of Sn(IV)(OH)zEtio in chloroform was shaken with 50 ml of freshly prepared
6M HF (aq) until examination by long wave uv light
showed no further decrease in the orange fluorescence. The two layers were then separated and the
chloroform layer dried over anhydrous NaZS04.
9. Dibromoetio(l)prophinatotin(IV) [Sn(lV)Br2Etioj.
Diiodoetio (l) porphinatotin (IV) [Sn(IV)I2Etio]
These samples were prepared by the procedure
described for Sn(IV)FaEtio except 6M HBr and 6M
HI, respectively were shaken with the chloroform
solution of Sn(IV)(OH)aEtio. The reactions were
assumed complete when examination with long wave
uv light showed no further decrease in the orange
fluorescence. The layers were separated and dried
over anhydrous NazS04 as described above.
Unlike the other stannic etioporphyrin derivatives
mentioned, the bromide and iodide derivatives are
quite labile and are easily displaced as axial ligands
by most any nucleophile. Basic solvents or solvents containing alcohol or ether groups will dissociate the compounds. Attempts at direct synthesis from stannous bromide and stannous iodide and
free base etioporphyrin (1) resulted in the formation
of the dihydroxide complex. Dichloromethane
seems to be the best solvent for spectroscopic studies of these compounds.
10. Dichlorooctaethylporphinatolead(IV)
[Pb(IV)CL,OEP]
This compound, previously unreported in the literature, was prepared by oxidizing the Pb(II)OEP
with chlorine gas. Approximately 5 mg of
Pb(II)OEP was dissolved in 10 ml of dry 3-methylpentane. A chlorine solution was made by bubbling
chlorine gas through 10 ml of dry 3-methylpentane
until a distinctly yellow solution was obtained. The
chlorine solution was then added dropwise (stirring
after each drop) to the Pb(II)OEP solution, causing
immediate color change and preCipitation of
Pb(IV)ClzOEP. After the addition of 5-10 drops of
the chlorine solution the reaction mixture was allowed to stand for 15 min and was then centrifuged,
the 3-methylpentane decanted and the ppt washed
several times with cold 3-methylpentane and three
times with acetone. The bright red microcrystal
Pb(IV)ClzOEP ppt seemed to be quite stable to air.
The mass spectrum showed a molecular ion for
Pb(IV)ClzOE P. In some solutions the initial reddish
pink color gradually fades to green, suggesting reduction back'to Pb(II)OE P.
Good luminescence and excitation spectra were
obtained by the following technique. A small
amount of the bright red Pb(IV)ClaOEP preCipitate
was dissolved in a O. 01M solution of sodium methoxide in methanol and frozen in liquid nitrogen immediately. The red -pink color typical of the other
members of the Group IV octaethylporphyrins was
maintained. Samples prepared this way give luminescence and excitation spectra (Fig. 5) as well
as the other luminescence data (Table II) that are
consistent with the assignment of these spectra to
a Pb(IV)OEP species. However, it is possible that
in these solutions methoxide replaced chloride as
the axial ligand.
C. Solvents
The CHaCla was either Mallinkrodt spectra AR or
MCB spectroquality grade. CHCls was Merck reagent grade. The CHsOH was MCB spectroquality
grade. EPAF was fluormetric grade from Hartman-Leodon Co. The 2-MeTHF was MCB chrom-
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679
PORPHYRINS. XXVII
rather large metal effect reported for the Group
ill series Ga(III)CI, In (III)CI, THill)CI38 : Along this
series there is a significant red shift and decrease
in the Q(O, O)/Q(1, 0) intensity ratio. As seen in Table I, we have found a similar trend with change of
ligand on Sn(IV)X2Etio along the series X= F-, OH-,
benzoate, CI-, Br-, r.
Figure 4 gives the near uv absorption spectrum
for SnClzOEP beyond the B(O, 0) band. A distinct
B(1, 0) is observed approximately 1200 cm-1 to the
blue of B(l,O). There is also a distinct N band as
expected38 further to the blue. No other peaks are
ooserved down to 250 nm. The spectra for
Si(IV)ClzOEP and Ge(IV)ClzOEP are essentially the
same as that of Fig. 4. An interesting spectral
feature is the broadening of the B(O, 0) band along
the series SnF2 < SnClz < SnBr2 < SnIz. This first
caught our attention as a decreasing ratio of B(O, 0)
over N(O, 0) peak molar extinction coefficients. Absolute molar extinction were not measured. But as
shown in Table I, the ratio of Q(l, 0) over N(O, 0)
peak molar extinction coefficients remains constant.
Since most absorption intensity is in the B band,
conservation of oscillator strength suggests that the
changing ratio of B(O,O) over N(O, 0) peak molar extinction arises from the broadening of the B(O, 0)
band rather than any intensity redistribution among
Q, B, and N bands.
The Pb(IV) spectra do not seem to have previously been reported. As discussed in Sec. II B,
Pb(IV)OE P was made but was only stable at low
temperature. An excitation and an emission spec-
oquality reagent grade. All of the solvents were
used without further purification. At 77 oK the
EPAF and 2-MeTHF formed glasses whereas the
other solvents formed snows. We might note that
there is little solvent effect expected, and we found
that the CHzClz snow and 2-MeTHF glass gave the
same values (within experimental error) for if!p/if!,
and 'Tp for Sn(IV)ClzEtio and Sn(IV)ClzOEP. In general oxygen is not expected to affect results in rigid
media at 77 OK, and so the solutions were not degassed. The concentrations of the solutions were
on the order of 10-5M, thereby excluding dimer formation and concentration quenching.
III. RESULTS
A. Absorption and Excitation Spectra
Figures 1-3 give the absorption spectra of the
molecules Si(IV)ClzOEP, Ge(IV)ClzOEP,
Sn(IV)CI20EP. All show a normal porphyrin spectrumZ3 • 36 : a relatively weak visible Q band showing
a vibrational progression of apprOximately 1200
cm -1 and a very intense B band in the near uv. The
wavelength and intensity of the peaks is given in
Table I. The metal has some effect on the spectra:
there are very slight shifts in wavelength and slight
changes in the ratio of intensity Q(O, 0) to intensity
Q(l,O). Similar spectra were reported by stern
and Dezelic37 for Ge(IV)CI2 and Sn(IV)CI2 mesoporphyrin in benzene, although their molar extinction
coefficients were some 30% higher than those reported in Table I. This rather small metal effect
for the Group IV porphyrins contrasts with the
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WAVELENGTH (NANOMETERS)
FIG. 1. Si(IV)C120EP: absorption spectrum in dichloromethane at 298 OK (dashed line). Emission spectrum in
2-Me-THF at 77 ~ (solid line).
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680
GOUTERMAN, SCHWARZ, SMITH, AND DOLPHIN
Molecule
TABLE I.
Wavelength and intensity of principal maxima for group IV porphyrins. a
Solvent
N(O,O)
B(l,O)
B(O,O)
Q(2,0)
Q(l,O)
==================
~
Q(O,O)
N(O,O)
B(O,O)
N(O,O)
0.513
13.40
0.488
14.42
B(O, 0)
Ll. Al/2(nm)
Absorption b
Wavelength (nm)/optical densityc
SiCl 20EP
CH2 Cl z
GeCl 20EP
CH 2 Cl z
SnCl 20EP
CH 2 Cl 2
337 d
1.54
340d
2.02
348 d
2.98
Sn(Bz)2 Etio
CHCl 3
Sn(OH)2 Etio
CHCl 3
SnI'2 Etio
CHCl 3
SnCl 2Etio
CHCla
SnBr2Etio
CHCl 3
SnlzEtio
CHCl 3
PbX 2OEp·
CHaOH
350
1. 78
359
2.00
368
3.63
385
5.07
385
4.61
383
5.60
403
26.7
404
26.9
403
40.0
387
3.44
377
3.05
383
3.92
393
3.27
407
24.1
397
25.7
403
28.0
412
15.3
410
11.80
500
0.13
500
0.16
501
0.19
500
0.09
500
0.10
495
0.10
501
0.13
503
0.12
505
0.25
~500
534
0.80
535
1. 21
538
1. 53
535
0.90
537
1. 04
533
0.87
538
1. 07
543
1. 07
543
1. 77
~535
0.93
Pb(II)OEpf
CHCl 3
SnClzTPP
CH z Cl 2
367
6.59
460
14.6
418
GO.O
397
6.74
521
0.33
560
1. 80
571
0.83
571
1.16
575
1.43
572
1. 00
574
1. 00
569
1. 00
575
1. 00
578
1. 00
581
1. 00
8
8
0.534
7.G5
11
0.487
3.25
24
~570
1. 00
580
1.40
599
1.30
Emission g
Wavelength (nm)/relative No. of photons
Molecule
Solvent h
SiCl 2 0EP
l\ITIII'
GeCl 20DP
1\1 TlI I'
SnCl 20EP
l\TTHI'
Sn(Bz)zEtio
CHCI}
Sn(OI-!)2 Etio
EPAF
SnI'zEtio
l\TTHI'
SnCl 2 Etio
SnBr2Etio
l\JTHF
CHCl 3
SnI2Etio
CnCI:;
PbX 2OEp·
l\IcOH
Pb(II)OEpf
SnCl,TPP
;Jl\leP
l\TTHF
Q(O,O)
Q(O,l)
T(O,O)
575
G35
710
1. 00
0.50
0.02
575
630
703
1. 00
1. 00
0.77
575
638
703
1. 00
15.8
1.14
579
635
712
1. 00
1.13
5.88
570
625
700
1. 00
0.91
3.70
570
620
700
1. 00
0.80
5.70
Essentially the same as SnCl 20EP
575
G30
715
1.00
0.94
15.0
575
630
712
1. 00
0.94
23.0
580
620
705
1. 00
1.27
15.2
795
605
655
705
1. 00
1.12
0.08
T(O,l ')
755
0.009
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0.018
740;
1.20
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0.4
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0.12
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1. 74
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0.007
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0.020
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5.38
775
1. 56
aAll metals are valence IV except where indicated.
bAbsorption data taken at room temperature except for PbX20EP.
"When OD of Q (0,0) is not 1.00, then the measurements were done for a 1 x 10-4M solution in a 1 cm cell.
dAbsorption in CH30H determined relative to Q(I, 0).
·"Absorption" data determined from the excitation spectrum in a methanol snow including Na methoxide. X is probable methoxide.
fVery broad Q (0,0) band; emission taken from Becker and Allison (Ref. 21).
'Emission data taken at 77 "K. MTHF, EPAF, 3MeP form glasses at 77 "K while CHela and CHaOH form snows.
lIgolvent abbreviations: MTHF (2-methyltetrahydrofuran); EPAF (12 ether: 10 isopentane: 6 ethanol: 1 N, N-dimethylformamide); 3 MeP (3-methylpentane).
IThese measurements made with a RCA 7102 phototube.
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681
PORPHYRINS. XXVII
Ge (IV) CI
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I
,.
0.2
8
, I
x 20
,,
03
<!
10 175
Cl.
"
I
w 04
OEP
9
"
iJi 07
z
w
2
WAVELENGTH (NANOMETERS)
FIG. 2. Ge(IV) ClzOEP: absorption spectrum in dichloromethane at 298 oK (dashed line). Emission spectrum in 2
Me-THF at 77 oK (solid line).
trum are given in Fig. 5. The spectra are similar
to Sn(IV)Cl20EP but there is a red shift and the intensity ratio Q(O, O)/Q(1, 0) is reduced. The spectrum of Pb(n)OEP is given in Fig. 6 and is distinctly different from Pb(IV)OEP. The Soretband is
strongly red shifted and is anomalously weak; the
N band is red-shifted and is anomalously strong;
the Q band lacks the customary vibronic structure.
Excitation spectra in the region of the Q bands
were taken for all the emission spectra. In all
1.0
"
0.7
0.6
Cl.
0
w 0.4
-..J
I
,
I
r
I
I
I
,,I
I
,
I
I
I
I
I
\
\
\
01
\
400
~
I
,
I
............ "
450
___ /
", ...... /
500
I
,
0
I
n.
>-
8
f-
7
iJi
z
w
x 12
I
,'
,,
>
.:; 03
I
, 1,
I
II
I I
I I
I I
I I
;:::: 0.5
02
x 22
,' I
I I
<t
W
,, ,,
I
II
U
a:
,I
,,
0
-J
f-
II
II
f-
w
0
9
,I
>- 0.8
Z
10 Ul
z
Sn(IV)C'20 EP
•
0.9
(/)
cases the agreement between excitation of the fluorescence and phosphorescence was very good.
Comparison of the excitation and absorption spectra
showed a systematic error in the ratio of Q(1, 0)/
Q(O,O) of 10% which could easily be attributed to a
solvent effect in going from CCl2H2 to 2MeTHF or
EPAF. In fact the Sn(IV)Br2Etio and Sn(IV)(Bz)2Etio
excitation spectra agreed with the absorption spectra, where the solvent was unchanged. The peak
maxima in all cases show suffiCiently good agree-
f-
?;
6 w
u
z
I
1
I
5 uw
I
4 z
(/)
w
\
~
I
3
\
I
\
,
?
2
V
550
:::l
-J
W
f-
<!
-..J
W
a:
600
650
700
750
800
850
WAVELENGTH (NANOMETERS)
FIG. 3. Sn(IV)C120EP: absorption spectrum in dichloromethane at 298"K (dashed line). Emission spectrum in 2
Me-THF at 77 "K (solid line).
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682
GOUTERMAN, SCHWARZ, SMITH, AND DOLPHIN
Sn (IV)CI
r
l..,
z
w
Cl
0
OEP
2
7-
06,
FIG. 4. Absorption spectrum of
Sn(IV)Cl 20EP in methanol in the
near uv at 298 oK.
3 05~
6: 04~
c
N
I
03'02r-
oil
i
!
I
L_~
_______ - L _ _ _ _ ..._ _ _L_
250
_ ____ ._ .. ~_~ _ _ ~ _____ .J._~~
300
350
400
WAVEI_ENGTH (NMJOMETERS)
trast to the absorption spacing of ~ 1200 cm-1 • The
triplet emission T(O, 0) begins - 3200 cm-1 to the
red of the Q(O, 0). One or more vibrations of the
triplet are observed and some have been listed in
Table I. In most cases the lowest energy T(O, 1)
peak is not accurately known because it was measured on a RCA 7265 phototube. In the case of
Sn(IV)CI20EP, the phosphorescence peaks were observed with a RCA 7102 phototube and due to the
more uniform photosensitivity of this tube in the
ment with the absorption spectra that we could conclude that the emission was from the principal absorbing species.
B. Luminescence Spectra
Typical luminescence spectra are given in Figures 1-3 and 5. The peaks are listed in Table I.
We see that a Q(O, 0) and Q(O, 1) emission spectra
is mirror image to the Q(O, 0) and Q(l, 0) absorption. The emission spacing is ~ 1500 cm-1 in con-
~
U;
'W
1----
10
w
11::
8
tl
7
If)
,__
\
I I
I
I
I
I
1
/
I
o
~ :t!
I
Q
/
~
I
o 3
w
I
I
I
I
I
I
\:
\
I
I
I
I
I
/
x5
/
/
f
\
\I
\
\
\
r\
1\
/ \
\
/
I
J
:
I
I
(j)
,/
t5z
\,
4
~
w
z
,
3
5
2
~
w
-.J
f-
<!
_J
\
..,,/
W
11::
' ....
,
500
550
z
5
\\
///
f-
~
I
1---
450
7
w
-"--_-"--_L-L---'-_-"--_--'-__ ---'--<_L_~~ _
400
I
tl
6
,
I
1-
8
\
\,\
I
2
o
>-
x 12
1/
I
/
\
I I
I
I
\ ...J:
9
\
!
!
\
\
11::
::J
{
\
g 2~
Z
I
(j)
z
/\
I
\
S
x 4~/
W
Pb (IV)OEP
\
I
8
:
I
it
8I
I
9
o
10
------r-
600
_'____'''''___
650
_'_~ __ L _ _ _L__.L"''"___
700
750
800
WAVELENGTH (NANOMETERS)
FIG. 5. Uncorrected excitation and corrected emission spectra of Pb(IV)OEP in methanol at 77 "K.
Downloaded 10 Oct 2008 to 142.103.92.58. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/jcp/copyright.jsp
683
PORPHYRINS. XXVII
10
Pb (II)OEP
0.9
08
>-
I-
x 0.1
x2
m 07
z
w
0
06
FIG. 6. Absorption spectrum of
Pb(IT)OEP in dichloromethane at
298 oK.
w
>
I-
«
--.J
~
0;:
01
~~_ _- L_ _~~~_ _ _ _L -_ _~_ _~~-L
350
400
450
_ _~._ _ ~----L~~~~~_
550
600
650
WAVELENGH (NANOMETERS)
near IR, an accurate wavelength value was obtained.
In this case the energy gap between Q(O, 0) and
Q(O, 1) and between T(O, 0) and T(O, 1) agreed to 20
cm-I, well within our error of measurement.
Table n gives quantum yields for fluorescence
and phosphorescence, <Pf and <PI>, and observed
phosphorescence lifetimes 71>' It is interesting to
contrast the behavior of these measurable over the
series M(IV)Cl20EP, where M = Si, Ge, Sn and
Sn(IV)X2Etio, where X= Cl, Br, I. The change in
TABLE II.
nuclear charge-14 to 50 in the one series and 17
to 53 in the other-is quite similar. We see along
the metal series that <PI>/<Pf increases a factor of
100, <PI> increases a factor of 4, and 71> decreases
a factor of 3. Along the halide series <PI>/<Pf increases a factor of L 6, <PI> decreases a factor of
20, while 71> decreases a factor d 30. Naively it
would seem that along the metal series there is a
greater enhancement of Tl -SI than of T('VvY-SI while
along the halide series the reverse is true. A fur-
Luminescence yields and lifetimes for Group IV porphyrins a•
TI>(N.T.)
(msec)
Solventb
<I>fCIt. T.)C
MTHF
2.0 x 10-1
2.0
10-1
1,8x10-2
9.0 X 10-2
95
MTHF
7.8x10-2
7.4 X 10-2
4.2 X 10-2
5.5
X 10-1
42
SnCl 20EP
MTHF
7.8x10-3
7.3 x 10-3
6.8X 10-2
9.7
30
Sn(Bz)2Etio
CHCl 3
8.2 x 10-3
[8.2 x 10-3]d
[1. 9 x 10-2]
2.3
31
EPAF
1. 0 x 10-2
[1.0
f2.2 x 10-2]
2.2
39
8.9x 10-3
3.8 x 10-2
SiCl 20EP
GeCl 20EP
Sn(OH)2Etio
<I>f(N.T.)C
X
X
<I>I>(N.T.)
10-2]d
<I>P/<I>f
SnF 2Etio
MTHF
8.9 x 10-3
4.3
50
SnCl 2Etio
MTHF
7.8x10-3
7.3 x 10-3
6.8 X 10-2
9.7
28
SnBr2Etio
CHCl 3
3.1 x 10-3
[3.1 x 10-3]
[3.5 x 10-2]
11.3
6
SnI2Etio
CHCl 3
2.2x10-4
[2.2 x 10-4]
[3.5 x 10-3]
15.8
PbX2OEp·
MeOH
SnCl 2Tppb
MTHF
1.1 x 10-2
2.5x10-2
2.4x10-2
1.0 ± 0.02
4.0
aAlI metals are valence IV.
Ils e e footnotes, Table I.
crt.T., N.T. are room temperature and liquid nitrogen temperatures.
0.96
2.8
12/2
d[ ], absolute <I>f(N. T.) not measured but set equal to
<I>f CIt • T .).
"Phosphorescence decay shows double lifetime.
Downloaded 10 Oct 2008 to 142.103.92.58. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/jcp/copyright.jsp
684
GOUTERMAN, SCHWARZ, SMITH,
ther discussion of these results is given below.
Three other points can be noted: In comparing
Sn(IV)F2Etio and Sn(IV)(OH)2Etio we see that although iJ>p / iI>f is larger for the fluoride, the lifetime is not smaller as expected. Except for
Sn(IV)CI2TPP, all fluorescence quantum yields
measured at liquid N2 temperature were within experimental error of being identical to the yield
measured at room temperature. Finally we might
note that Pb(II)OEP has an altogether different
emission from Pb(IV)OEP. Pb(II)OEP has been reported to show no fluorescence and a very weak
phosphorescence at 790 nm with lifetime under 0.5
msec. 21
IV. THEORY
A. Spin·Vibronic Coupling
Here we shall attempt to conSider the over-all
vibronic states for the lowest triplets of porp.hyrin.
Some previous work along this line has been
done, 23-27,39 but we shall here attempt a more complete theory. On this basis, then, we can attempt
to account for the radiative and radiationless rates
responsible for the luminescence and for the vibronic envelopes of the luminescence spectra.
It is useful to follow Perrin et al., 39 who considered vibronic coupling among porphyrin singlet
states USing a cyclic polyene model. In this model,
porphyrin is considered to be a 16 membered polyene with 18 electrons. The orbitals have quantum
numbers 0, ± 1, ••• ± 7, + 8. The zeroth order excited electronic states will be constructed from
transitions between the top filled and lowest empty
orbitals:
1,30 +1 = 2-1/ 2 ¢-4 ¢-4¢4¢51 'f ¢-4¢-4¢4¢5Il ,
[I
I
1 3
- - I'f I¢4¢4¢-4¢-5
- I] ,
, 0 -1 = 2-112 [ I¢4¢4¢-4¢-S
(1)
1
1,30 +9 = 2- / 2 [ I¢4¢4¢-4¢51 'f I¢4¢4¢-4¢51 ] ,
1,30 -9= 2-112 [I ¢-4¢-4¢4¢-51'F I¢-4¢-4¢4¢-51] •
Here we list the triplet states with Sz = 0. We shall
take up the triplet states with SI! = ± 1 later.
Let us first conSider the vibronic coupling among
these states. This is a spin independent term and
will not couple states with different values of Sand
Sz. Moreover the spin degeneracy cannot be lifted
by vibronic effects. The result is that there is the
same vibronic coupling among the various spin
manifolds as among the singlet states. The latter
were discussed by Perrin et al. , 39 who showed that
the 7T, 7T* states are only vibronically coupled by a
set of normal modesq". (}J.=O, ±1, •.• ±7,8)that
depend on the C-C bond stretches. These modes
couple the excited states as shown in Table III,
where A6 and As are constants evaluated earlier. 39
Let us now consider spin-orbit coupling among
this manifold. We follow earlier work on Zn etio-
AND DOLPHIN
TABLE III. Spin-vibronic coupling in porphyrins. a, b
,@,
,@,
,@.,
'EB
,@.,
'EB
1@9
Asqt
AGfJ(,
[email protected]
AGq~
A,qB
3@,
Z/2
'@8
[email protected]
As,!,
A{jqG
Ac'!t
Agrd
3@,
o
Z/2
-Z/2
' EQ
Z/2
'EQ
-Z/2
AS'!8
[email protected]
-Z/2
3@,
0
ABIJIl
Z/2
-Z/2
[email protected]
A61J~
3EB
A (/llr
A,;r/r:
3EQ
As'!s
AI/il;
A8 rd
3EQ
aFor definition of A 6q6' A6q~, Asqs see text and Ref. 39.
bpor definition of Z see Eq. (4) and Ref. 24.
porphyrin 26 and write
Hso =6b(i)s(i) ,
(2)
i
where b(i) is the one electro'} spin-orbi.t operator
and sri) is the one electron spin operator. Symmetry arguments show that Hso can couple the singlets and triplets of Eq. (1) through the following
one electron terms:
(¢4Ib z l¢4)=- (¢_4I bzl¢_4)"'0,
(3)
(¢5I bzl¢5)=-(¢-slb z l¢-5)=-Z.
(4)
The fact that the term of Eq. (3) is negligible compared to that of Eq. (4) was shown earlier. 24 The
orbitals CP"4 can be related to the porphyrin orbi.tals
2-1/2 [a2U(7T)± ia1U(7T)]. 36 As a result the integral of
Eq. (3) is i(a2u I bz Ia1U), which depends only on small
three-center terms. 24,40 The orbitals ¢"5 can be
related to the porphyrin orbitals 2-1/2 [e gX ± ie gy ], 36
and Z defined in Eq. (4) agrees with an earlier definition (e gX Ibz Ie gy ) =iZ. 24 This integral has one
center contributions that will be evaluated below.
The spin-orbit matrix element Z enters the spinvibronic matrix as shown in Table III.
Let us consider the vibronic pattern for triplet
luminescence expected from the coupling of Table
III. (The vibronic pattern for singlet luminescence
was considered in Ref. 39.) We see that 36±9 couples to 10 ±9 through Z. However the emission
10±9 - 10 GND is forbidden. 39 To gain intensity 30 ±9
must borrow intensity from the allowed transition
10±1 _1(3 GND. The second order borro-.ving path is
diagrammed in Fig. 7 along with the resulting
transitions. We see that transitions from 36 ±9 to
the vibrationally unexcited ground state are forbidden, while transitions to the vibrationally excited
ground state-with one mode of q 8, q 6, or q: -are
allowed in second order. Thus the vibronic envelope for phosphorescence from 38 ±9 will be rather
like that for fluorescence from 10 ±9 or absorption
to this state. These electronic transitions do not
show a normal Franck-Condon envelope but sho',v
a strong Q(l, 0) (absorption) or Q(O, 1) (fluorescence)
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685
PORPHYRINS. XXVII
FIG. 7. Spin-vibronic coupling paths for intensity
borrowing by 3®-9 _1 ®GND transitions. The symbols 8 o,
8f, etc., refer to vibration q8 with 0, 1, etc. quanta of
a vibration. (See text.)
relative to Q(O, 0).36,39 However, the degree of forbiddenness of Q(O, 0) depends on the extent that the
actual molecule is like the 16-membered polyene
with 18 electrons. In the lower symmetry D4h of
porphyrin all the states of Eq. (1) are of symmetry
E u , and there can be some mixing of ± 9 with ± 1.
Thus Q(O, 0) may gain some intensity. 36,39,41
The borrowing of 30 ±1 is altogether different. It
has a direct spin-orbit coupling to 10 ±1 and so will
show a very strong origin band and should have a
vibronic envelope like a B band. Moreover it .
should have a Significantly shorter radiative lifetime than 30 ±9'
Table III then predicts two distinct types of vibronic envelopes depending on whether the lowest
triplet is 30 ±9 or 30 ±1' Now it has been pointed out
earlier24 that there is no direct e2 /riJ coupling between triplet states constructed from transitions
a2u-eg and those constructed from a1"-eg. If such
pure configurations did describe the triplets, they
are equivalent to linear combinations 36 ±1 ± 36 ±9
and would have vibronic patterns intermediate between the two types just described. However, Roos
and Sundbom 42 found that the two pure configurations strongly mix because e2/ri} couples both to a
third state. Thus the question how best to describe
the lowest triplet remains unsettled. As we shall
discuss in Sec. V, the phosphorescence envelopes
suggest that for Group IV TPP compounds 36 ±9 is
the best description, while for Group IV OEP compounds it is 36 ±1'
Finally let us consider triplet states with Sz = ± 1.
These are given in Eqs. (5) along with their energy
including first order spin-orbit effects due to Z:
36'±9,1= 1<P±4¢±4<PT4<P±sl;
E=3EQ 'f.Z/2,
36 ±9,_1= I<P±4¢±4¢l'4iii±sI;
E=3E Q ±Z/2.
(5)
In Eqs. (5) the eigenvalue of Sz is given as the second subscript. These states do not couple by Hao
to the Singlets defined in Eq. (1) because of symmetry.24,26 However they can mix by Hao with singlet transitions UIT* or rru* and gain z polarized intensity in the 0-0 vibronic band. These singlet
transitions can, in turn, mix vibronically with the
intense 16±1 transitions through vibrations perpendicular to the porphyrin ring in a pattern similar to
that of Fig. 7. The vibrations that make the Sz
= ± 1 levels allowed would be different from those
that make 36 ±9 (Sz=O) allowed. The vibrations are
perpendicular to the ring, and the transitions remain rr, rr* polarized.
There is considerable limitation on the type of
urr* or 1TU* states from which the states of Eqs. (5)
can borrow z polarized intensity: (i) The states
must differ from those of Eq. (5) by one orbital;
(ii) there must be a strong one-center spin-orbit
coupling term between the orbitals that differ; (iii)
the state must be of A 2" symmetry and be strongly
allowed. With these limitations, from the top filled
and ION est empty orbitals of Group IV porphyrins
(Fig. 3, Ref. 26) the most likely source of z polarized intensity for the triplet states of Eqs. (5) are
transitions e"(ligand) - eg(rr*). The spin-orbit coupling will have a one-center term (nPx Iby Inpz)
=(npz Ibx Inpy) located on the ligand and may be
strongly affected by changes among ligands F-,
CI-, Br-, C However these U1T* transitions should
not have much z polarized intensity to give to the
triplet. Hence spin-orbit coupling of the 3(rr, rr*)
states to 1A 2"(u, 11"*) cannot provide much intensity
to the 0-0 band.
The coupling with e"(ligand)-eg(rr*) can, however,
provide several important physical effects. The
mixing in of singlet states can provide a mechanism
for radiationless decay for all the Sz =± 1 sublevels.
In D4h the sublevels 3E" (Sz =± 1) have spin-orbital
symmetries A 1", A 2 ", B lu , ~". 26 While only A 2 "
can gain z polarized radiative intensity, the others
can gain singlet components that make for radiationless decay. Moreover, U1I"* singlet states of
any symmetry can serve as an intermediate state
for mixing 10 ±1 into the 3E" (Sz = ± 1) states through
Vibrations perpendicular to the plane. studies of
optically detected magnetic resonance (ODMRj26,28
monitoring different vibronic peaks can show whether such spin-vibronic interactions are occurring.
B. Size of the Spin-Orbit Integral
In an earlier discussion of the spin-orbit coupling integral Z defined in Eq. (4),24 it was pointed
out that there are two types of one-center contribution:
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686
GOUTERMAN, SCHWARZ, SMITH, AND DOLPHIN
I
I
,
(nd xz I~M(r)l: Ind~z) '" ~M/i
(nPx ~L(r)l~ np~) '" ~di
(6)
(7)
•
Here ~L(r)l~ is the spin-orbit interaction on the
ligand atom and ~M(r)l: is that on the metal. The
result is that
(8)
where CM and cLare the coefficients of the eg orbital on metal and ligand. [Since the bra and ket
atomic orbitals of Eqs. (6) and (7) have zero overlap, we neglect the contribution of the overlap
charge density.] The molecular orbital coefficients
have been calculated for the series SHOH)2, SiClz,
GeClz, Sn(OH)2, SnF2, SnCI2, SnBr2, and SnI2 derivatives of porphin by the extended Huckel method. 29 ,43 The ~M and ~L constants were obtained
from Charlotte Moore's tables. 44 (See the Appendix
for details.) In Table IV we present the CM, CL,
~M' and ~L values for this series along with the calculated Z values. The results of the calculation
are that the main contribution to Z arises from the
ligand terms (6) rather than the metal terms (7).
In Sec. V we discuss how well these calculated Z
values compare to experiment.
C. Effect of the Spin-Orbit Coupling on the Transition Rates
The emission pattern of molecules is determined
by three radiationless rates k(Sc""'-"-So), k(Sl~ T l ),
k(Tl~SO) and two radiative rates k(Sl-SO) and
k(Tl-S O)' For the Group IV OEP and Etio molecules, the singlet energy and absorption intensities
are sufficiently similar that it is reasonable to set
(9)
i. e., the same for all cases. [We expect that k f
- (60 nsec)"l. 31] As discussed below, the vibronic
envelopes for the molecules suggests that all are
luminescing from a 30 ±l level. Then according to
Table TIl the luminescing state can be written
(10)
TABLE IV.
Compound
M(1V)X2
Si(OH) 2
SiC1 2
GeC12
GeC12
Sn(OH)2
SnF2
SnCl 2 a
SnCl 2
SnBr2
Sn12
R(M-X)
R(M-N)
(A)
(A)
1.64
2.046
2.10
2.10
2.12
2.10
2.418
2.31
2.55
2.73
1.90
1.95
2.042
1.98
2.101
2.101
2.101
2.06
2.101
2.101
(11)
Radiationless rates are more difficult to calculate. The present accepted theory sets 45
(12)
Here we consider a transition i-I, PE is the density
of final states, and H is the Hamiltonian caUSing the
transition. H is composed of terms omitted from
the Hamiltonian that determines i and I. In the
present case it would be higher order vibronic coupling terms and Born-Oppenheimer correction
terms, for the main spin-orbit terms are already
included. Because all these molecules are so similar in structure and have such similar spectra, it
does not seem unreasonable to set
k(Sl~SO)=Af
,
(13)
k(Sl-vvy..T l ) =Aisc Z2 ,
(14)
k(Tl-'Vv¥-So) = BiscZ2 •
(15)
Thus in Eqs. (11)-(15) we take kj, Aj, A lse , B p ,
B lse as constants for the series of Group IV OEP
and Etio compounds studied. We take this as a
working hypothesis for examining our results, for
it is the simplest way to handle radiationless decay
theory.
V. COMPARISON OF THEORY AND EXPERIMENT
A. Absorption Spectra
Among the Group IV compounds studied, change
in either metal or ligand causes very little spectral
change. Some years ago a correlation between red
shift and a decrease of the intensity of Q(O, 0) was
proposed. 41 It was related to decreasing electronegativity of the metal. 41 From a chemical point
of view one might expect the series M(IV)X20EP to
have increased charge in the ring for M going Si
Contributions to the spin-orbit coupling.
cLx 10 6
2.9
4.8
12
46
46
55
29
19
The transition dipole depends only on the second
term, in which only Z varies substantially in this
series. Thus we set
~M x 10-3
1.8
1.8
3.46
3.46
3.46
3.46
3.46
3.46
C~~M
(cm-!)
c1x 10 2
0.005
0.008
0.041
0.16
0.16
0.19
0.10
0.066
4.9
0.73
0.33
0.58
6.7
0.28
0.27
0.27
2.2
4.8
~L x 10-3
(cm-!)
2~LdL
(cm-!)
(cm-!)
0.15
0.587
0.587
0.587
0.15
0.269
0.587
0.587
2.46
5.07
15
8.6
4.6
6.8
20
1.5
3.2
3.2
108
490
15
8.6
4.6
6.8
20
1.7
3.4
3.4
108
490
Z
aExperimental geometry from R. Scheid (private communication).
Downloaded 10 Oct 2008 to 142.103.92.58. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/jcp/copyright.jsp
687
PORPHYRINS. XXVII
to Pb with X fixed and X going F to I with M fixed.
Table V shows the results of calculations on net
charge distributions for MX2 porphins and a comparison with wavelength for Q(O, 0) and the intensity
ratio Q(O, O)/Q(O, 1). There is a fairly good correlation of red shift and decreaSing intensity ratio
with increaSing negative charge on the ring. Interestingly, the calculated numbers show a clear tendency for charge to shift from the ligands to the
porphin ring with net metal charge remaining more
stable.
The extended Huckel calculations also provide a
rationale for the striking difference between the
absorption spectra of Pb(IV)Cl20EP and Pb(II)OEP~9
The Pb(II) has in addition to the usual top filled orbitals alU(lT) and azu(lT) an orbital al (pz). (We label
this orbital with the C 4V label to highlight the fact
that the metal is most likely out of plane.) There
is considerable mixing between azu(lT) and al (pz)
according to the EH calculations, which gets higher
as the metal moves out of plane. Thus the three
banded Pb(II) spectrum can be attributed to transitions involving five orbitals: azu(lT), alu(lT), al(pz)
- e,(lT*). The transitions azu(lT), alu(lT) - e,(lT*) are
calculated to be red shifted compared to normal
porphyrin bands. It then seems reasonable to attribute the two lower energy bands primarily to
these transitions and the near uv band primarily to
a1 - e, • This is consistent with the Sn(II)TPP spectrum reported by Edwards et al., 33 that shows a
spectrum that suggests a1 - e, is now further red,
as would be expected from a charge transfer transition. The three transitions generated by the five
orbital model would be subject to extensive configuration interaction. A detailed understanding of
their relative role in determining the spectrum requires calculations, such as those based on the
"peel" approximation,42 that include two electron
terms.
The EH calculations show the possibility of low
energy ligand to porphyrin transitions. 29 We have
found no clear peaks attributable to these transitions. However the broadening of the Soret band
TABLE V. Electron densities a for Group IV porphyrins
and spectra.
Compound
M(IV)X2
SiCl,
GeCI,
SnF,
SnCI,
SnEr,
SnI,
M
Net charges (calc)
Porphin
X
0.61
0.61
0.87
0.71
0.71
0.67
-0.29
-0.29
-0.66
-0.39
-0.30
-0.16
-0.03
-0.03
0.45
0.07
-0.11
-0.35
Spectral properties
Q(O, O)/Q(l, O)"A(nm)C
1.04
0.96
1.15
0.93
0.95
0.58
571
571
569
575
578
581
aBased on the extended Huckel method, Refs. 29 and 43.
"'Ratio of peak molar extinction coefficients from Table
I.
'Wavelength of Q(O, 0) from Table I.
along the series SnCl2 , SnBr2 , Snl2 (see Table I
and Results) may be due to the presence of charge
transfer bands in the Soret region.
B. Emission Spectra
The fluorescence bands in Figs. 1-3 and 5 are
essentially mirror image to the absorption bands
except that gap between Q(1, 0) and Q(O, 0) is substantially less than that between Q(O, 0) and Q(O, 1).
This can be explained by the vibronic terms of Table ill, which act to lower the excited state spacing. 39
The phosphorescence spectra have an entirely
different shape from the fluorescence bands showing
a very strong T(O, 0) compared to T(O, 1). Thus
they resemble a mirror image to the B bands. By
the theory given above this suggests they be identified as 3E>±1-1E>GND. We have been studying the
spectra of Group IV tetraphenylporphin (TTP) compounds. While our studies are not complete, it is
clear that the vibronic envelope for the Group IV
TPP compounds is altogether different from Group
IV OEP. We give some data for Sn(IV)ClzTPP in
Fig. 8, Table I, and Table II. The weakness of
T(O, 0) with respect to T(O, 1) suggests, by the theory given above, that these bands be identified as
3 0 ±9 - Ie GND. Moreover, since this transition is
weakly allowed compared to 3e±I-1 0oND we expect
lower phosphorescence yields for the TPP case.
This is, in fact, the case as shown in Table II. We
thus have a fairly good interpretation for the striking difference between the phosphorescence spectra of the two porphyrin compounds. The difference is not, of course, fully understood as we have
no good theoretical reason why the lowest triplet
should differ for the two molecules. Moreover,
the Group IV TPP compounds sho\V double lifetimes
at 77 0 K. (Note added in proof. Double lifetimes
§eem to arise from solvent effects.)
C. Luminescenc Yields Calculated Spin-Orbit Coupling
In Sec. IV above we gave in Eqs. (9), (11), and
(13)-(15) expressions for the five radiative and radiationless decay constants of these systems in
terms of five constants characteristic of all systems and the spin-orbit coupling parameter Z.
This is based on the notion that the energy gaps,
density of states, and coupling terms are all quite
similar. In this way we can derive expressions for
the various experimental observables:
<I>;t -1 = ki1 [Aj + Z 2A iSC ]
7;1 = Z2[Bp+ B
iSC
]
,
,
(16)
(17)
<I>p/<I>j=Z2AiSCBp/kj(Bp+BiSC) ,
(18)
(<I>p/<I>,)7p =Atsc Bp/k j (Bp+B1SC)Z.
(19)
In Table V we have listed values for the experimen-
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688
GOUTERMAN, SCHWARZ, SMITH, AND DOLPHIN
lor
~\
I,
I,
08
II
09
II
~ Olr
I \
-.J
I
~ 05
CL
0
w
I
I
I ,
I I
I I
I I
<[
04
>
I
I
I
I
I
I
01
r-"
/
r-/
400
I
I
\
I~
I
I
I
,
\
I
I
I
\
I
I
7
,
I
\
J
6
-
~----/-
450
5
w
u
w
u
w
4
z
L
~
3
\ f
2
'J
_J
W
2:
~
-l
\
\
550
z
W
I-
(f)
/
500
Qc
>Iiii
z
I
I
0
I0
I
;;
I
I
f
/ '" \.,JI
\
\
8
I
I
I
I
I
: I
I I
~ 03
-.J
w
n: 02
9
I I
I I
I \
I I x66
I I .
o 06
10 U)
z
~
1\
1\
1\
I \
I ,
600
W
n:
\
.....
650
WAVELENGTH (NANOMETERS)
FIG. 8. Sn(IV)CI 2TPP: absorption spectrum in dichloromethane at 298 oK (dashed line). Emission spectrum in 2
Me-THF at 77 oK (solid line).
tal quantities of Eq. (16) and calculated values for
Z2.
Let us first consider the absolute magnitude of
Z. The triplet quantum yield for Sn(IV)Etio has
been measured as <I>t =O. 57 by the flash calorim-
eter. 46 ,47 The ligand was either chloride or hydroxide and from the data of Table II we can estimate that the natural radiative lifetime is - 0.5
sec. From the perturbation expansion of Eq. (10),
the measured molar extinction coefficient of the
Soret band, and the equations relating natural radiative lifetime to absorption we can estimate Z
- 4 cm-1 • This value is roughly that expected for
either SnCl2 or Sn(OH)2' Thus the value calCulated
for SnCl2 in Table IV is reasonably correct while
that for Sn(OH)2 is too high. An examination of the
eg (1T*) orbital [Table 7, Ref. 29] shows an anomalously high delocalization onto OR" ligands. We
then conclude that OH- is not too accurately calculated by the EH method.
What about the experimental trends over this series? As shown in Eqs. (16)-(18) the quantities
<I>i1 -1, 7;1, and <I>p/<I>, should rise with Z2. As can
be seen in Table VI, we can arrange the systems
studied in a series so that all these quantities tend
to rise. However, by Eq. (18), (<I>p/<I>,)Tp shOUld be
constant, but Table VI shows this is not the case.
Therefore, in spite of the great Similarity in these
systems, our simplifying assumptions about radiationless decay do not appear to be valid.
Because the theoretical dependence on Z2 given
in Eqs. (16)-(19) is only qualitatively exhibited by
the data, we have no accurate Z(experimental) with
which to compare Z(calculated). However, from
the trends in the data we can make several general
conclusions. We shall assume, as' stated above,
that Z - 4 cm -1 for SnCI2• The data trends in Table
VI show that Z increases along the series SiCl2 ,
GeCI2, and SnCI2. However our calculated Z values
decrease due to decreasing delocalization of eg (1T*)
into the chlorine. This is clearly in error. Another error would seem to be the very large values
of Z calculated for SnBr2 and SnI2• The trends in
the data suggest these are large by a factor of perhaps as much as 10. The metal d orbitals are calculated to give very little contribution to spin-orbit coupling. While the data trends show that the
ligand gives a larger contribution to spin-orbit
coupling than the metal, in agreement with the calculations, the relative contribution of the metal
may be underestimated. If in SnCl2 the metal contributes about half of the Z value, instead of 5% as
TABLE VI.
Compound
M(IV)X,P
Variation of luminescence parameters with
Z2 (calc).
<!>i-1
,._1p
<!>p/<!>f
(<!>p/<!>f)Tp
SiCl,OEP
4
10.5
GeCl,OEP
12
24
0.55
23
32
Sn(OH),Etio
100
25
2.2
87
580
SnF,Etio
110
20
4.3
215
SnCl,Etio
130
36
9.7
270
SnBr,Etio
SnI,Etio
8.6
Z'(cm-')
0.09
320
167
11.3
68
4540
1000
15.8
15.8
74
2.9
12
12000
240000
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PORPHYRINS. XXVII
shown in Table IV, then an increasing Z value over
the series SiCI2 , GeCI2 , SnClz is understandable.
CONCLUSION
We have reported here the luminescence of Group
IV porphyrins and considered the theory that explains this data. We find that the shifts in spectra
with change in ligand F, CI, Br, I can be nicely explained by a shift of electron density from the halogen to the ring. We find that the phosphorescence
band envelope for the etioporphyrins can be explained as emission from 3 0 ±1 _ l e GND while that
for the tetraphenylporphins is 30 ±9 - 10 GND. For
the former, theory predicts that there is direct
spin-orbit coupling to the strong Soret band while
for the latter intensity borrowing requires both
spin and vibronic coupling. The spin-orbit coupling both theoretically and experimentally depends
much more on the ligand than on the metal. The
ligand effect arises through the mixing of the ring
eg (1f*) with ligand nPx, np y orbitals; the metal effect
arises through the mixing of eg {1T*) with metal nd,w
ndyz • The size of the spin-orbit coupling can be
calculated by the extended Huckel model. The results are none too accurate: The calculations seem
to underestimate the metal contribution and often
overestimate the ligand contribution. A simplified
model for radiationless decay is developed according to only which spin-orbit coupling varies among
the molecules in this series. The model only qualitatively accounts for the luminescence parameters.
Measurement of triplet yield will be needed to establish what assumptions of the Simplified theory
are invalid.
ACKNOWLEDGMENTS
The extended Huckel program used for some of
our calculations was written by Professor Ernest
R. Davidson. Some of the runs were carried out
by Mr. Donald Silver. Advice and help were given
by Drs. James B. Callis and Arnold M. Schaffer.
Ms. Louise Karle Hanson supplied some
Sn(IV)ClzEtio.
APPENDIX
The constants
~L
and
~M
defined in Eqs. (6) and
(7) turn out to be identical to
(AI)
defined by Condon and Shortley. 46 It follows from
their analysis that for an (np)5 2p term the magnitude of the J = ! to J = ! splitting is ! t n ,l; for an
{nd)9 2 D term the magnitude of the J = ~ to J = ! splitting is (~)bn,2; for an (np)4 3 P term the magnitudes
689
of two splitting J = 2 to J = 1 and J = 1 to J =0 is l:n,l
and !l:n,l' For the halogens F, CI, Br, I the 2p
splittings are 404, 881, 3685, and 7603 cm-1 , respectively. For the Group IV atoms we used the
d 9 s 2 configuration for the trications; the 2D splittings for Ge+ 3 , Sn+ 3 , Pb+ 3 are 4503, 8655, 21316
cm -t, respectively. For oxygen the two splittings
of 3p are 158 and 68 cm-1 • All values from Ref.
44.
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