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2618
Reversal of Excitation Behavior of Proton-Transfer vs. Charge-Transfer by Dielectric Perturbation
of Electronic Manifolds
Pi-Tai Chou,' Marty L. Martinez, and John H. Clements
Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208
Received: October 23, 1992; In Final Form: December 16, 1992
Reversal of excitation behavior of proton-transfer vs. charge-transfer by dielectric perturbation is reported for
4'-(diethylamino)-3-hydroxyflavone (I). At room temperature I exhibits a dominant proton transfer emission
and a nonnegligible normal emission with maxima at 555 and 440 nm, respectively in alkane solvents, in contrast
to 3-hydroxyflavone in which only tautomer emission is observed. The result may be rationalized by a mechanism
incorporating fast proton tunneling between normal and tautomer forms which are in equilibrium in the excited
singlet state. The normal emission maximum exhibits a drastic solvent dependence with a red shift of >3000
cm-' from n-heptane to methanol. In ethanol a unique normal emission with an unusually high yield (a
0.52 f 0.02) is observed. It is proposed that the SIstate of I may be ascribed to a zwitterionic configuration
-
induced by the charge transfer from the 4'-diethylamino group to the carbonyl oxygen, for which the energy
is even lower than that of the tautomer in strong polar, protic solvents, precluding the proton-transfer reaction.
Introduction
Excited-state intramolecular proton transfer (ESIPT) has
received considerable attention both experimentally and theoretically.14 The ESIPT process generally involves transfer of a
hydroxyl (or amino) proton to an acceptor such as a carbonyl
oxygen or a nitrogen atom in the excited state, resulting in a large
Stokes shifted tautomer emission. This unusual property has led
to several recent practical application^.^-^ As a prominent example
3-hydroxyflavone (3HF) has been widely used as a radiation
hard-scintillator counter5.* based on its large Stokes' shifted
(- 1000 cm-I) tautomer emission and relatively high quantum
yield. Unfortunately, its photochemical reactivity9 limits its
practical long-term application. Thus, one focus of the search
for an ideal radiation-hard scintillator has been the design of
derivatives of 3HF which can overcome these drawbacks. We
proposed that the addition of auxiliary substituents such as a
4'-dialkylamino group on the &phenyl ring of 3HF (see Figure
1) may greatly change the electron density distribution due to its
strong electron donating property. Thus, the photophysical and
photochemical properties may besignificantly altered with respect
to that of 3HF, resulting in an ideal candidate for radiation-hard
scintillators. In this report the photophysicsof the 3HFderivative,
4'-(diethylamino)-3-hydroxyflavone (I, Figure 1) have been
investigated.I0 The non-negligible normal emission in alkane
solvents at room temperature, in contrast to 3HF in which a
unique tautomer emission is observed, may be rationalized by an
excited-stateequilibrium between the normal and tautomer species
associated with a rapid proton tunneling mechanism. The
drastical solvent polarity dependent maximum of the normal
emission is discussed based on the charge transfer property of the
diethylamino substituent (electron donor) coupled with carbonyl
oxygen (electron acceptor). In strong polar, protic solvents such
as ethanol the charge transfer state is proposed to be lower in
energy than the excited singlet tautomer state. Thus, ESIPT is
energetically unfavorable, resulting in a dominant charge transfer
emission. The observation of the reversal of the excitation behavior
of proton-transfer vs. charge transfer by dielectric perturbation
is unique among the class of flavonols.
x AIb
la
-0
H
IC
Figure 1. Structure of a. the normal species and b. the tautomer species
of I, c. the proposed structure for the charge-transfer state.
The product was purified by repeated recrystallization from
n-heptane. The purity of the product was checked by NMR, GC
and the fluorescence excitation spectrum. 4'-(Diethylamino)3methoxyflavone (11) was synthesized by the reaction of I with
dimethyl sulfate under a nitrogen atmosphere. The product was
purified by column chromatography (eluent 4: 1 v/v hexanes/
ethyl acetate) and repeated recrystallization from n-heptane. All
solvents were of spectrograde quality and purified as previously
described.12 The solutions were either saturated with 760 torr
oxygen or degassed by four freeze-pump-thaw cycles under
vigorous stirring conditions on the vacuum line.
Measurements. Steady-state absorption and emission spectra
were recorded by an H P (Model 8452A) spectrophotometer and
Shimadzu (Model RF5000U) fluorometer, respectively. Details
of the setup of the transient absorption, two-step laser-induced
fluorescence and low temperature measurements have been
described elsewhere.12-'4 Quantum yields were measured using
quinine sulfate/ 1.O N H$304 as a reference, assuming a yield of
0.564 with 360 nm excitation.'s
Experimental Section
Materials. I was synthesized by the reaction of o-hydroxyacetophenone and 4-(diethylamino)benzaaldehyde by the standard
Claisen-Schmidt condensation followed by oxidation with H2O2.II
0022-3654/93/2097-261 8S04.00/0
Results
Steady-State Measurements. The UV absorption of I in
n-heptane'is shown in Figure 2a. The SO- S I ( m * )absorption
0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2619
Dielectric Perturbation of Electronic Manifolds
1.01
L
11.0
1
fi.
0
W.\VFl.tB(;TtI
300
400
500
600
Wavlength nm
Figure 2. Absorption and emission spectra of I a h n-heptane and b. in
ethanol (emission is denoted by the prime sign). c. The emission spectrum
of a expanded 30 times in the 400-500 nm region.
1
+o
1.0
400
500
Wavelength
600
nm
Figure 3. Absorption and emission spectra of I1 a. in n-heptane and b.
in ethanol (emission is denoted by the prime sign).
exhibits an onset at -440 nm with a band maximum a t 400 nm.
The absorption maximum of I shows a red shift with increasing
solvent polarity. In addition, the sharp-rise of the absorption
profile becomes broadened when solvated in polar solvents as
opposed to alkane solvents. In ethanol, the absorption (Figure
2b) is structureless with a maximum which is only - 5 nm redshifted with respect to thpt in n-heptane. However, the onset of
the absorption is unusually long extending to -500 nm. The
concentration independent absorption spectra in various solvents
applied indicate that there is no dimer or oligomer formation for
I in therangeof 1W-lWM. Theroomtemperatureluminescence
of I in n-heptane (Figure 2a') is characterized by a predominant
yellow-orange emission maximized at 555 nm with a quantum
efficiency @fof0.25 f 0.02. In addition, a normal Stokes shifted
emission maximum at 440 nm is observed with an emission
intensity which is -2 orders of magnitude weaker than that of
the 555 nm band (Figure 2c). The excitation spectra monitored
at both440and 555 nmareidentical with theabsorptionspectrum.
A similar ratio of 555 nm/440 nm emission intensities was
observed in various alkane solvents, such as 3-methylpentane,
n-pentane, n-hexane, methylcyclohexane and dodecane. On the
other hand, 11, which is considered as a non-proton-transfer model
with an electronic configuration similar to I exhibits a normal
Stokes shifted fluorescence maximum at -420 nm in n-heptane
(Figure 3). No long wavelength emission in the 500-600 nm
region was observed. In conclusion, the 555 nm emission band
observed in I is consistent with a tautomerization (proton transfer)
occurring in the lowest excited singlet state following electronic
excitation. The energy gap between absorption and tautomer
emission maxima is calculated to be -6,400 cm-1. This is believed
to be one of the smallest Stokes shifts among this class of 3HF
derivatives. Accordingly, the 440 nm emission of I observed in
UiI
Figure 4. Room temperature emission spectra of I in (-) ether,
dioxane, (- -) CHCI3,
CH2C12, (- -) CH3CN. It is noted that all
spectra measured are uncorrected. The ratio of longwavelength/
shortwavelength emission intensities may not be correct.
-
(-9)
(e-)
n-heptane, similar to that of 420 nm for 11, is ascribed to normal
emission (vide infra).
In weakly polar, aprotic solvents the ratio of normal/tautomer
emission intensities for I increases as the solvent polarity increases.
The maximum of the tautomer emission is slightly red-shifted
with increasing solvent polarity. However, the red shift of the
normal emission with respect to the solvent polarity is drastic
(Figure 4). In protic solvents such as ethanol a unique, strong
emission maximum at 510 nm was observed. This 510 nm
emission, 1400 cm-l blue shifted with respect to the tautomer
emission measured in n-heptane, does not fit the polarity dependent
tautomer emission maxima, which all show a slight red shift in
polar, aproticsolvents with respect to that in hydrocarbon solvents.
Furthermore, in spite of the 3600 cm-l gap between absorption
and emission maxima the 0-0 onsets closely overlap in ethanol
(Figure 2). These observations lead us to conclude that the 510
nm emission does not originate from the excited tautomer species.
This argument can be supported by the observation of the emission
maximum at 505 nm for I1 in ethanol (Figure 3), for which no
ESIPT takes place. Since alkoxy1 anion formation for I1 is not
possible the possibility that the 510 nm emission for I results
from adiabatic proton dissociation to the surrounding solvent in
the excited state is also eliminated. In contrast to the weak normal
emission (@ < 0.01) of 3HF in ethanol, the fluorescence quantum
yield for I in ethanol is usually high with a value of 0.52 f 0.02.
Laser action was observed for I at 560 nm in alkane solvents and
510 nm in ethanol. Consequently, for I an unusually wide range
of lasing frequencies can be achieved from 535 nm to 575 nm,
depending on the chosen solvents. A strong laser action was also
observed for I1 in alcohol solvents. A detailed study of the lasing
properties of I and I1 will be published elsewhere.16
In comparison to the 440 and 420 nm emission maxima for the
normal forms of I and I1 in n-heptane, the 510 nm and 505 nm
emissions in ethanol are -3,000 cm-' and -2,800 cm-I redshifted, respectively. This drastic solvent dependence is clearly
shown in the continuous spectral changes observed when various
amounts of ethanol are added to solutions of I and I1 in n-heptane.
As shown in Figure 5 a significant red-shift of the normal emission
was observed with the added ethanol concentration varying from
0-4.0 M, while the maximum of the tautomer emission shows
negligible spectral shift. In addition, the ratio of normal/tautomer
emission bands for I increases during the titration. Similar redshifted emission maximum as a function of ethanol concentration
was observed for 11.
Nano-microsecond Time-ResolvedStudies. Figure 6 shows the
photoinduced transient absorption spectra of I in degassed
n-heptane at various delay times. The spectra are characterized
by broad absorption maxima at -430 nm and 520 nm. The
intensities of these two absorption bands, within theexperimental
error, show the same time dependence, indicating that they
originate from the same transient species. Two important
-
2620 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993
a
n
500
450
550
600
nm
Figure 5. Room temperature emission spectra of I in n-heptane with
varying ethanol concentrations: a. no ethanol b. 0.04 M c. 0.06 M d. 0.08
M e. 1.2 M f. 2.0 M g. 4.0 M.
Wavelength
500
460
L1!4\’FLFNCTH
580
540
nm
Figure 6. Time-dependent transient absorption spectra of I in degassed
n-heptane (1.7 X 10.’ M) at various delay times: a . 200 ns; b. 6 p s ; c.
20 ps; d. 100 gs. All spectra are measured at room temperature. The
pump pulse energy is 3.0 mJ cm-?/pulse. Data are taken by an average
of 60 shots for a. and b. and an average of 100 shots for c. and d. Due
to the strong SO Si absorption at < 420 nm the transient absorption
maximum at 430 nm may be even shifted toward higher energy.
-
observations for the decay dynamics of the transient absorption
can be pointed out: (1) The decay is excitation energy dependent
and behaves primarily by second-order kinetics at the highest
excitation energy applied. (2) The time-profiles of these two
transient absorption bands are extremely sensitive to the presence
of oxygen and the pseudo-first-order quenching rate constant for
oxygen is calculated to be -2.0 f 0.2 X lo9 SKI. The results can
be well explained by assigning the long-lived transient species to
a triplet state. Since the triplet-triplet (TI T,) absorption of
I1 exhibits a maximum at -460 nm which is different from that
of I observed in n-heptane we conclude that the triplet-triplet
absorption of I originates from the tautomer triplet state (TtI).
It is noted that although in the two-step laser-induced fluorescence
experiment the tautomer emission was observed when the probe
laser was tuned to the transient absorption maximum of 430 nm,
the tautomer emission was also observed when the probe laser
was tuned to > 600 nm in which no S’O S’l absorption takes
place. Therefore, our previously proposed mechanism for 3HF
and 7-hydroxy-l-indanone, which incorporates TtI T’, absorption followed by T’2 SI intersystem crossing, resulting in
the tautomer emissionl3,14 is also suitable for I.
-
-
-
-
Discussion
A detailed understanding of the photophysics of 3HF in various
solvents has been achieved due to the efforts of many research
groups. In alkane solvents the rate of ESIPT is extremely rapid,
even in inert gas matrices at cryogenic temperatures, indicating
that the ESIPT barrier for 3HF in alkane solvents and inert gas
Chou et al.
matrices is either negligible or small with the ESIPT dynamics
dominated by proton tunneling.”-I9 On the other hand, several
laboratories have reported that in protic solvents the rate of proton
transfer consists of both slow and fast component~.I~J@2~
The
slow proton transfer component, k,~,,,depends on the hydrogenbonding ability of the solvent and follows Arrhenius-type behavior.
Barbara et al.*O concluded that in alcohol solvents the proton
transfer process is not a function of the solvent relaxation
dynamics, but rather is more sensitive to energetic factors of the
solute/solvent interactions. The hydrogen bonding effect of protic
solventson the proton transfer of 3 H F has been recently extended
by Harris et al.17based on femtosecond dynamics. Their results
for the slow component of the ESIPT in alcohol solvents are
consistent with those of previous reports.2G24 In addition, the
fast component is attributed to the rate of excited-state proton
transfer of a 1:l 3HF/monohydroxyl alcohol complex in which
the wavepacket moves along the reaction coordinate even faster
than the intact five-membered ring intramolecularly hydrogen
bonded 3HF due to the higher frequency 0 - H vibration. hast
of
ESIPT is determined to be in the range of 10’3s-l - lOI4 s-I,
depending on thestrengthof the 3HF-solvent hydrogen b0nding.l’
Although Harris et al.3 work failed to consider the spectral
evolution associated with vibrational relaxation the result unambiguously concludes that in alcohol solvents the deactivation
of the electronically excited 3HF normal species is dominated by
the overall rate of proton transfer (including solvent dynamics
and intrinsic proton transfer), resulting in a short lifetime and
low quantum yield for the normal emission in alcohol solvents.
In comparison to the known photophysics of 3HF the results
for I provide interesting new features in proton transfer dynamics
among flavonols. First, the appreciable normal emission in alkane
solvents is unlike 3HF for which only tautomer emission was
observed at room temperature. Since the excitation spectra are
identical when monitored at normal (440 nm) and tautomer ( 5 5 5
nm) emission regions, the possibility that the short wavelength
emission is caused by an impurity is small. In addition, the
intensity of the normal emission was unchanged after further
solvent purification attempts. Thus, perturbation due to trace
polar solvent impurities at room temperature also seems unlikely.
The observation of an intrinsic normal emission in alkane solvents
leads to three postulated proton transfer mechanisms:
(1) It may indicate that the proton transferreactionisassociated
with a significantly large activation barrier crossing in the sense
of transition-state theory.
(2) It may indicate a slow rate of proton transfer due to nontransition-state effects. These include either the dielectric
relaxation of solvent configuration or a modulation of the slow
isomerization rate due to coupling of the large amplitude motion
of the solute to the configurational dynamics of the solvent.
(3) It may imply that there exists an equilibrium between
normal and tautomer excited singlet states. For this case the
rateof forward and reverse ESIFTcan beextremely rapid, possibly
dominated by proton tunneling.
Although our instrument response limits dynamic studies in
t h e excited state, a mechanism may be deduced through steadystate measurements in combination with numerous dynamic
results for 3HF.I7-*4
Since n-alkanes are non-polar and generally treated as “inert
solvents” the proposed slow dielectric relaxation of the solvent
preceding ESIPTcan be eliminated. In additioqdue to thestrong
electron donating property of the diethylamino group in the para
(4’) position theC2-CI’(seeFigure1) bondshould bemoredouble
bond like for I than 3HF, resulting in a coplanar structure for
the y-pyrone and phenyl rings. Thus, ESIPT may take place
with negligible coupling with phenyl torsional motion. Consequently, the possibility of the proposed mechanism (2) in which
the proton-transfer activation energy is a function of large
amplitude motion, such as the torsional motion of the phenyl
Dielectric Perturbation of Electronic Manifolds
X
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2621
2
3
Y
3~
xp
5
7
4.
5.
8
7
8
4~
5~
60
80
7
Figure 7. Frequencies at normal (closed circles) and tautomer (open
circles) emission maxima of I in various polar solvents with respect to
that measured in n-heptane. ‘X” symbol represents the frequencies a t
normal emission maxima of I1 in various polar solvents with respect to
that measured in n-heptane. Solvents used in this experiment are 1.
n-heptane, 2. diethyl ether, 3. dioxane, 4. ethyl acetate, 5 . CHC13, 6 .
CHzCll 7 . acetone 8. acetonitrile.
group is discounted. In fact, studies of 3HF and (8-ring)methylsubstituted 3HF by Barbara and co-workers showed that phenyl
torsion is not important to the ESIPT dynamic^.^^.^^ Theoretically,
mechanism (1) can be verified by examining the temperaturedependent dynamics of ESIPT or the intensity ratio of tautomer/
normal emissions. However, one should note that in a temperature-dependent study of 3HF the dynamics of ESIPT are strongly
perturbed by trace polar solvent i m p ~ r i t y . 2 ~Similar
3 ~ ~ to 3HF,
the emission spectrum of I unfortunately is also highly sensitive
to trace solvent impurities in the temperature dependent study.
The spectrum of I in a 77 K methylcyclohexane glass exhibited
a dominant green emission when the methylcyclohexane was not
purified. This green emission maximum at 500 nm is identical
with the emission of I in a 77 K ethanol glass, indicating that the
500 nm emission of I observed in a 77 K methylcyclohexane glass
originates from the I/(protic solvent impurity) hydrogen bondedcomplex. After extensive purification of methylcyclohexane
according to reported method~l2.2~,*~
the spectrum shows a
dominant yellow-orange tautomer emission maximum at 560
nm. Although our best purified solvents still exhibited 10% of
the normal emission, the result clearly demonstrates that under
-
--
the solvent impurity perturbation-free condition the tautomer
emission is expected to be dominant and thus cannot be explained
by mechanism (1) in which the ratioof normal/tautomeremission
intensities should increase as the temperature decreases. Accordingly, mechanism (3) is favorable with an equilibrium between
normal and tautomer species in the excited singlet state, where
the energy level of the tautomer state is lower than that of the
normal state. Since the Stokes shift (the frequency difference
between absorption and tautomer emission maxima) for the
tautomer emission of I is only -6,000 cm-I, based on the AM1
calculation that the energy difference between normal and
tautomer forms is (24.2 f 5.0 kcal/mol) in the ground state, the
energy levels of SI (normal species) and S’I (tautomer species)
are expected to be very close. This property is different from
3HF for which a highly unsymmetric potential surface along the
reaction coordinate, i.e. large exothermic ESIPT, is generally
accepted, resulting in a negligible degree of reverse proton transfer
during the lifetime of the excited state. It is noted that close
energy levels between normal and tautomer excited singlet states
have been recently reported for several other classes of ESIPT
molec~les.2~J8
The increase in the ratio of normal/tautomer emission
intensities observed in polar solvents is intriguing. For instance,
a 2: 1 ratio of normal/tautomer emission intensities was observed
in CH2C12. Since the proton accepting ability of CHzClz is weak,
the hydrogen bonding between I and the solvent is negligible.
Thus, the intramolecular hydrogen bond in I is expected to remain
intact. This is supported by the dominant tautomer emission
observed for 3HF in CHzC12. In order to achieve a detailed
understanding of the solvent effect, a series of solvents was used
to permit a study of environmental perturbation of the competing
electronic process. Figure 7 shows the solvent polarity dependence
of the normal and tautomer peak frequencies for I and I1 as a
function of solvent polarity ( E ~ ( 3 0 ) ) . *It~ is expected that the
intramolecular proton transfer fluorescence for I should be slightly
affected by the solvent polarity index, consistent with the result
shown in Figure 7. On the other hand, the frequency shift of the
normal fluorescence peak is drastically dependent on the solvent
polarity index. A Av change from 1200 cm-l in CHzClz to 3000
cm-i in acetonitrile is observed. Similar results were obtained
for the normal emission of 11. Since the excitation spectra
monitored at tautomer and normal emissions are identical in
various solvents studied, attributing the dual emission to two
different conformers which equilibrates in the ground state is not
possible. We thus propose that for I and I1 the SI state responsible
for the normal emission has charge transfer properties induced
by the electron donor, the para-diethylamino substituent coupling
with the electron acceptor, the carbonyl oxygen (Figure IC).This
assignment can be supported by an experiment in which I was
dissolved in CHC13 saturated with HC1 gas. The absorption
maximum was significantly blue shifted to 350 with a spectral
-
S;
E
emission
charge transfer
emission
so
so
--
so
in
nonpolar solvents
in polar. aprotic solvents
in polar. proric solvenis
Figure 8. The proposed ESIPT/charge transfer mechanisms incorporating solvent dielectric perturbation.
2622 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993
profile identical with that of 3HF. The tautomer emission was
also analogous to that of 3HF with a emission maximum at 528
nm. The result can be rationalized by the protonation of the
diethylamino substituent, prohibiting the charge transfer reaction.
Although without ultrafast time-resolved studies the relaxation
dynamics cannot be resolved at this stage, a mechanism incorporating the reversal of excitation behavior of proton-transfer vs.
charge-transfer by dielectric perturbation of electronic manifolds
is tentatively proposed in Figure 8. In nonpolar, alkane solvents
thecharge-transfer SI state, due to thelackof solvent stabilization,
is higher in energy than the proton-transfer S’Istate. According
to Figure 7 it is reasonable to assume that the normal (charge
transfer) state is more sensitive to solvent polarization than the
tautomer state. Thus, the relatively small energy difference for
I with respect to that of 3HF in alkane solvents may lead to a
near degeneracy between SI and S’,states in weak polar media.
In relatively strong polar solvents the reversal of the relative
energies of the SIand S’Istates may even take place. Thus, dual
fluorescences were observed corresponding to proton transfer and
charge transfer emissions. Thedeductionof the kinetics involving
forward and reverse proton transfer competing with solvational
relaxation and nonradiative emissions of SIand S’]states may
be complicated and required further timeresolved study. In protic
solvents, such as in ethanol, the unrelaxed charge transfer (SI)
state may even be energetically much lower than the SI’state,
prohibiting the excited-state proton transfer. Our results also
suggest that in polar solvents significant (possibly with ultrafast
dynamics) time-dependent Stokes shifts should be observed for
the charge transfer emission. In contrast, nearly time-independent
Stokes shifts are expected for the tautomer emission.
It is noted that our attempt is to qualitatively derive a
mechanistic picture to explain our results. Detailed studies of
proton transfer versus charge transfer dynamics in various solvents
require pico-femtosecond time-resolved studies with the assistance
of a theoretical approach based on the calculation of the molecular
dynamics. Due to the unusually high fluorescence yield for both
tautomer (proton transfer) and normal (charge transfer) emissions, the proton/charge transfer dynamics of I are believed to
be of interest to a broad spectrum of researchers.
References and Notes
( I ) Kasha, M. J . Chem. Soc., Faraday, Trans. 2 1986, 82, 2379.
Chou et al.
29.
(2) Barbara, P. F.; Walsh. P. K.; Brus, L. E. J . Phys. Chem. 1989, 93,
(3) Special Issue (Spectroscopy and Dynamics of Elementary Proton
Transfer in PolyatomicSystems, ed. by P. F. Barbara and H. D. Trommsdorff),
Chem. Phys. 1989, 136, 153-360.
(4) Special Issue(M. Kasha Festschirift),J. Phys. Chem. 1991,95,l022&
10524.
( 5 ) Renschler, C. L.; Harrah, L. A. Nucl. Inst. Methods Phys. Res.,
A235, Sept., U S .Patent, 1985, 636.
(6) Costela, A.; Munoz, J. M.; Douhal, A,; Figuera, J. M.; Acuna, A.
Appl. Phys. 1989, 49, 545.
(7) Chou, P. T.; Studer, S. L.;Martinez, M. L. Appl. Spec. 1991, 45,
5 13. laser
(8) Radial. Phys. Chem. (special issue), in press.
(9) (a) Studer, S.L.; Brewer, W. E.;Martinez, M. L.; Chou, P. T. J. Am.
Chem. SOC.1989, 7643. (b) Chou, P. T.; Studer, S. L.; Martinez, M. L.;
Orton, E.; Young, M. Photochemistry and Photobiology 1991, 581.
(IO) Itoh, M.; Fujiwara, Y.; Sumitani, M.; Yoshihara, K. J . Phys. Chem.
1986, 90, 5672. In this paper the tautomer emission of the analogue of I,
3-hydroxy-2-(4’-dimethylaminophenyl)-4H1-benzopyran-4-one, in 3-methylpentane has been reported.
( I I ) Smith, M. A.; Neumann, R. M.; Webb, R. A. J . Heterocyclic
Chemistrv 1968. 5. 425.
(12) Brewer,’W. E.; Studer, S. L.; Standiford, M.; Chou, P. T. J. Phys.
Chem. 1989, 93, 6088.
(13) Martinez,M.L.;Studer,S.L.;Chou,P.T.J.Am.Chem.Soc.1990,
112, 2427.
(14) Chou, P. T.; Martinez, M. L.; Studer, S. L. J. Phys. Chem. 1991,95,
10306.
(15) Demas, J. N.; Crosby, G. A. J . Phys. Chem. 1971, 75, 991.
(16) Chou, P. T.; Martinez, M. L.;Clements, J. H. Chem. Phys. Lett, in
press.
(17) Schwartz, B. J.; Peteanu, L. A.; Harris, C. B. J . Phys. Chem. 1992,
96, 3591.
(18) Dick, B.; Ernsting, N. P. J . Phys. Chem. 1987, 91, 4261.
(19) Brucker, G. A.; Kelley, D. F. J. Phys. Chem. 1987, 91, 2856.
(20) Strandjord, A. J. G.;Courtney, S. H.;Friedrich, D. M.; Barbara, P.
F. J . Phys. Chem. 1983, 87, 1125.
(21) Strandjord, A. J. G.; Barbara, P. F. Chem. Phys. Chem. 1983, 98,
21.
(22) Strandjord, A. J. G.; Barbara, P. F. J . Phys. Chem. 1985,89,2355.
(23) Strandjord, A. J. G.; Smith, D. E.; Barbara, P. F. J . Phys. Chem.
1985,89, 2362.
(24) Brucker, G . A.; Swinney, T. C.; Kelley, D. F. J . Phys. Chem. 1991,
95, 3 190.
(25) McMorrow, D.; Kasha, M. J . Phys. Chem. 1984,193, 2235.
(26) McMorrow, D.; Kasha, M. Proc. Natl. Aead. Sci. 1984, 81, 3375.
(27) Smith, T. P.;Zaklika, K. A.; Thakur, K.; Barbara, P. F. J . Am.
Chem.Soc. 1991,IJ3,4035. Smith,T. P.;Zaklika, K. A.;Thakur,K.; Walker,
G. C.; Tominaga, K.; Barbara, P. F. J . Phys. Chem. 1991, 95, 10465.
(28) Mordzinski, A.; Grabowska, A.; Kuhnle, W.; Krowczynski, A. Chem.
Phys. Lett. 1983, 101, 291. Mordzinski, A.; Grabowska, A.; Teuchner, K.
Chem. Phys. Lett. 1984, 1 1 1 , 384.
(29) Christian, R. Ed. Solvent and solvent effects in organic chemistry
VCH publishers, chap. 7 pp. 365.