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1 GRANT GR/M93864/01: REVIEW REPORT ‘TRANSITION METAL COMPLEXES POSSESSING TUNABLE AND SWITCHABLE NONLINEAR OPTICAL PROPERTIES’ BACKGROUND/CONTEXT Because optoelectronic or all-optical (photonic) data processing offer immense benefits over current microelectronics technologies in terms of speed and scale of operation, extensive research has been carried out in recent years on materials with nonlinear optical (NLO) properties. Organic molecular materials are especially promising in this area because they may be readily processed for incorporation into optical devices, e.g. waveguides, and their often very large NLO responses can be tuned via synthetic chemistry.1 As an interesting sub-class of molecular NLO materials, organotransition metal complexes offer extensive possibilities for combining a range of optical effects with other molecular electronic properties (e.g. redox or magnetic behaviour).2 More recent research involving such compounds has generally focused on the establishment of molecular structureactivity relationships, which are currently considerably better developed for purely organic NLO chromophores. Quadratic (second-order) NLO properties derive at the molecular level from first hyperpolarizability coefficients β, which translate into bulk susceptibilities χ(2) in appropriate macroscopic structures. Most known molecules with large β responses contain a powerful electron donor (D) group and an electron acceptor (A) connected via a conjugated π-bridge. Such D-π-A dipolar chromophores also show intense, low-energy absorption bands due to D(π)→A(π*) intramolecular charge-transfer (ICT) transitions. Therefore, ICT resonances can have pronounced enhancing effects on experimentally measured β values, and it is necessary to derive static first hyperpolarizabilities β0, which can be estimated by application of a widely used two-state model (TSM) for dipolar chromophores.3 β0 values are a measure of the intrinsic quadratic NLO response under non-resonant (i.e. working) conditions, and are of particular relevance to device applications where any actual absorption of light must be avoided. NH3 H 3N Me2N Ru N O + N N 3+ [PF6– ]3 NH3 H 3N N Ru N Me H 3N H 3N NH3 1 (λmax[MLCT] = 688 nm; β0[HRS] = 410 × 10–30 esu)6c NH3 H 3N H 3N H 3N Ru N + N Me2N NH3 3 (λmax[MLC T] = 628 nm; β0[HRS] = 220 × β0[Stark] = 93 × 10–30 esu)10 NH3 2 (λmax[MLCT] = 591 nm; β0[HRS] = 151 × 10–30 esu)6d 3+ [PF6– ]3 10–30 3+ [PF6– ]3 + N esu;6c + N PF6– 4 (λmax[ICT] = 448 nm; β0[HRS] = 45 × 10–30 esu;10 β0[Stark] = 75 × 10–30 esu)10 By exploiting a combination of hyper-Rayleigh scattering (HRS) measurements4 and electronic Stark effect (electroabsorption) spectroscopy,5 we have discovered that ruthenium pyridyl ammine complex salts containing pyridinium A groups (e.g. 1 and 2) can have very large β0 values which are associated with intense, low energy metal-to-ligand charge-transfer (MLCT) excitations.6 For comparison, the benchmark dipolar organic chromophore 4-dimethylamino-4′-nitrostilbene has a β0 value of 55 × 10–30 esu.7 Both the MLCT absorption and NLO properties of our complexes are readily tunable by ligand-based changes, in general accordance with the TSM, and can also be reversibly switched by using the RuIII/II redox couple. 8 The latter observation was a world first which has been followed by similar reports by several reknowned international groups in the NLO field.9 In addition, the first detailed and quantitative comparisons involving metal complexes and related purely organic chromophores have led to the conclusion that a pyridyl-coordinated {RuII (NH 3 ) 5 }2+ centre is a more effective πelectron donor than the widely used -NMe2 group.10 This result can be traced to the fact that the higher HOMO energy of the RuII centre more than offsets the stronger π-orbital overlap in analogous organic chromophores (e.g. compare 3 and 4).10 2 KEY ADVANCES AND SUPPORTING METHODOLOGY Synthetic and Characterization Studies (mapped on to original proposal) (i) Mixed-valent Complexes Inspired by previous studies which showed that cyanide-bridged, mixed-valent ruthenium complexes can exhibit large NLO responses,11 we sought to prepare dinuclear complex salts such as 5. In such species it was envisaged that the electron-rich pentaammine centres (E1/2[RuIII/II ] ≈ 0.4 V vs. SCE) should be readily and selectively oxidized to afford mixed-valent complexes in which both MLCT and RuII →RuIII inter-valence chargetransfer excitations should combine to produce very large β0 values. Although we have successfully prepared and partially characterized 5 and also the related pyrazine and 4,4′-bipyridine containing species, these compounds have proven to be inexplicably unstable. After repeated attempts to isolate sufficient samples for further oxidation, this work was discontinued in favour of other more promising avenues of investigation. (ii) Complexes of Pyridyl Polyene Ligands For strategic reasons, these synthetic studies were actually carried out by another PDRA supported by different EPSRC grants (Dr Lathe A Jones; grant refs GR/L56213/01 and GR/R54293/01). A number of complexes containing RuII ammine centres connected to pyridinium A groups via extended polyenyl bridges were prepared and fully characterized, and have been described in the previous IGRs. However, the important Stark spectroscopic studies on these compounds were funded by the present grant and carried out by Dr Harris (see later). [PF6– ]9 Ru(NH3)5 9+ N AsMe2 Me2As Cl Ru H 3N 4+ [PF6– ]4 N N Me2As NH3 Ru AsMe2 H 3N 6 N+ NH3 N NH3 + N 5 (NH3)5Ru N N N + N N Ru(NH3)5 (iii) Octopolar Complexes Octopolar compounds are an important class of quadratic NLO materials,12 and we therefore planned to prepare and study D3h complex salts such as 6 in which three powerful RuII ammine donors are connected via a central electron-deficient 2,4,6-triazenyl core. The proposed synthetic approach, in which mononuclear complexes of ligands such as 4,4′-bipyridine are reacted with cyanuric chloride, proved to be successful. However, the products were found to be unstable and steadily decomposed on attempts at purification. We have not been able to identify the cause of this unfortunate instability, but suspect that it may arise from the extreme electron-deficiency of the central units. Although 1,3,5-triaryl-2,4,6-triazenyl derivatives have been isolated previously,12b systems with three pyridinium rings attached are not known. We have carried out some preliminary experiments working towards species related to 6 with central benzene rings, but limited progress was possible under the present grant. A number of previous NLO studies have been carried out with derivatives of RuII tris(2,2′-bipyridine) (and related complexes of other metals), 13 which although often described as octopolar,13a,b,d-f may more realistically be viewed as a multi-dipolar chromophores.13c However, all of these studies have involved ligands bearing electrondonating groups, and the MLCT transitions in such compounds are necessarily opposed to the intraligand chargetransfer (ILCT) excitations which dominate the β responses. We therefore decided that it would be interesting to prepare related chromophores in which the ligands are subsituted with electron-withdrawing pyridinium groups. Such species can be expected to have more truly octopolar electronic structures in which only MLCT transitions determine the NLO properties. We have prepared the new dications N′′,N′′′-di-R-2,2′:4,4′′:4′,4′′′-quaterpyridinium (R = phenyl, Ph2 Qpy2+; 2,4-dinitrophenyl, (2,4-DNPh)2 Qpy2+; 4-acetylphenyl, (4-AcPh) 2 Qpy2+; 3,5-bis-methoxycarbonylphenyl, (3,5MCPh)2Qpy2+) (Scheme 1) and isolated them as their PF6 – salts. These compounds and the known pro-ligand salt [Me2Qpy2+][PF6] 2 14 have been used to prepare a series of seven new complex salts [MII(R2 Qpy2+)3 ][PF 6 ] 8 (M = Ru, R = Me 7, Ph 8, 4-AcPh 9, 3,5-MCPh 10; M = Fe, R = Me 11, Ph 12, 4-AcPh 13). 7–13 have been fully 3 characterized by using the usual techniques including cyclic voltammetry and UV-visible absorption spectroscopy. The absorption spectra of these compounds display intense, visible MLCT bands, the energies (Emax) of which decrease in the order R = Me > Ph > 4-AcPh = 3,5-MCPh, as the electron-accepting ability of the pyridinium groups increases. Each of the RuII complexes in 7–10 shows two overlapping MLCT bands, whilst the FeII complexes in 11–13 display two well-separated bands. The cyclic voltammograms show quasi-reversible MIII/II waves which are found at lower potentials for the FeII complexes when compared with their RuII analogues, due to the greater electron-richness of the FeII centres. 10 eq N N NO2 O2N O2 N O2N NO2 NO2 Cl N 2+ N Scheme 1 + N + N EtOH ∆ 48 h (2,4-DNPh)2Qpy2+ O N N OMe 11 eq EtOH ∆ 50 h 10 eq H2N O H 2N OMe Me O OMe MeO O O OMe MeO O + N + N N O EtOH ∆ 50 h DMSO 70 °C 3 h 11 eq H2N Me Me O O 2+ 2+ 2+ + N + N + N N N (3,5-MCPh)2Qpy2+ + N N N Ph2Qpy2+ N (4-AcPh)2Qpy2+ (iv) Complex Salts for Redox-switchable NLO-active Thin Films Macroscopic quadratic NLO activity requires non-centrosymmetric bulk structures,1 and these can be created on various substrates by using the Langmuir-Blodgett (LB) film deposition technique.15 Several reports of RuII complexes in LB films with NLO activity have already appeared. 16 We have synthesized several new dipolar complex salts designed to possess the amphiphilic properties necessary for LB film formation. The pro-ligands 14–16 were prepared by adapting existing chemistry6c and used to synthesize their {RuII (NH3)}2+ complexes which were isolated and fully characterised as both their PF6– and n-dodecylsulfate salts. LB-deposition requires compounds that are completely insoluble in water, but soluble in water-immiscible organic solvents (e.g. chloroform, hexane, toluene, etc.). Unfortunately, the salts which we have prepared show considerable water solubility and only limited solubility in non-polar organic solvents. We have attempted to circumvent this problem by using other counterions such as BPh4 – , but have not succeeded in isolating any water-insoluble materials. It appears that the ammine ligands are especially good at engendering aqueous solubility, and our conclusion is that they will have to be replaced by more hydrophobic ligands if salts with the necessary solubility properties are to be prepared. Unfortunately, such a broadening of scope was considered to be beyond the present project, but it is intended to pursue studies along these lines in the future now that the pro-ligands 14–16 are available. O OC6H13-n + N C6H13-n N 14 N + N 15 C6H13-n N + N 16 OC6H13-n O (v) V-Shaped Complexes (New Targets) A variety of organic multidimensional NLO compounds have recently emerged as candidates for quadratic applications,12,17 offering potential advantages over 1D chromophores such as increased β responses without undesirable losses of transparency in the visible region. Dipolar 2D C 2 v molecules, such as (dicyanomethylene)pyran derivatives17a,e or asymmetrically substituted 1,3,5-tris(phenylethynyl)benzenes,17b,d with 4 large off-diagonal β components are attractive for electro-optic applications and also offer new possibilities for achieving phase-matched seond harmonic generation. A relatively small number of neutral dipolar 2D metal complexes have also been studied,2k,18 in particular Schiff base derivatives. We therefore sought to prepare and study complexes relate to our existing 1D dipolar systems, but with V-shaped 2D structures based on cis coordination geometries, together with some related trans complexes for comparison purposes. + N NH3 H 3N – R 4+ [PF6 ]4 NH3 H 3N N Ru H 3N N N R (20) (19), Me (4-AcPh) N + R N O R = Me (17), Ph (18), N NH3 + – R 4+ [PF6 ]4 Ru H 3N N NH3 + N R = Me (28), Ph (29), 4-AcPh (30), 2-Pym (31) N (2-Pym) The existing pro-ligand salts N-R-4,4′-bipyridinium hexafluorophosphate (R = Me, MeQ +; Ph, PhQ +; 4acetylphenyl, 4-AcPhQ+; 2-pyrimidyl, 2-PymQ+ ) were used to prepare eleven new complex salts c i s [RuII (NH 3 ) 4 L2 ][PF6] 4 (L = MeQ + 17, PhQ + 18, 4-AcPhQ+ 19, 2-PymQ+ 20), trans-[RuII (NH 3 ) 4 L2 ][PF6] 4 (L = MeQ + 21, PhQ + 22, 4-AcPhQ+ 23, 2-PymQ + 24) and trans-[RuII (NH 3 ) 4 (MeQ +)L][PF6 ] 4 (L = PhQ + 25, 4-AcPhQ+ 26, 2PymQ+ 27). In addition, the new dicationic pro-ligand salt [(2-Pym)2 Qpy2+][PF 6 ] 2 and the related [R2 Qpy2+][PF6 ] 2 (R = Me, Ph, 4-AcPh) (see (iii) above) were used to synthesize four new complex salts c i s [RuII (NH 3 ) 4 (R2 Qpy2+)][PF6] 4 (R = Me 28, Ph 29, 4-AcPh 30, 2-Pym 31). The electronic absorption spectra of 17–31 display intense, visible MLCT bands, the energies of which decrease in the order R = Me > Ph > 4-AcPh > 2-Pym, as the electron-accepting ability of the pyridinium groups increases. Each of 17–20 shows two overlapping MLCT bands, whilst 21–24 display single bands and 25–31 also show apparently single bands which are composed of multiple transitions. Cyclic voltammetric studies show that the Ru-based HOMO energy is only slightly sensitive to changes in the structure of the pyridinium ligands, but the LUMO energies exhibit a large variation. The trends in the first ligand reduction potentials correlate with the Emax values. Furthermore, the extent of electronic coupling between these ligands increases as reduction proceeds, and is also greater in the Qpy-based complexes in 28 and 29 than in their non-chelated analogues in 17 and 18. A single crystal X-ray structure for the pro-ligand salt [Ph2Qpy2+][PF6] 2 •Me 2 CO shows that the 2,2′-bipyridine unit adopts a planar, transoid geometry with large dihedral angles between the two 4-pyridyl rings and between the pyridyl and attached phenyl rings. Hyper-Rayleigh Scattering and Stark Spectroscopic Studies The octopolar complexes described in (iii) above have been studied by HRS measurements, but so far only with a 1300 nm laser fundamental. Unfortunately, the harmonic signals at 650 nm were too weak to allow β determinations, but it is hoped that data will soon become available using a 800 nm laser. Stark spectroscopic studies with 7−13 in butyronitrile glasses at 77 K, incorporating a Gaussian fitting approach, have afforded dipole moment changes ∆µ12 for the MLCT transitions. These data have been used to calculate β0 values according to the two-state equation β0 = 3∆µ12(µ12)2 /(Emax)2 (µ12 = transition dipole moment). In each case, the use of two dominant Gaussian curves affords two components of the β response. For 7–10, Stark signals associated with the triplet excited states are observed to the low energy side of the lowest energy absorption bands. The β0 values indicate that the molecular quadratic NLO responses of 7–13 are highly 2D in nature, and some evidence for enhancement of β0 on changing R from a Me to an aryl substituent is found. The β values of the V-shaped complexes in 17–20 and 28–31 described in (v) above have been measured by using the HRS technique with acetone or acetonitrile solutions and a 800 nm laser. Stark spectroscopic studies with 17–20 and 28–31 in butyronitrile at 77 K, again using Gaussian fitting, have yielded ∆µ12 and therefore β0 values. In the cases of 17–20 and 28, the use of two dominant Gaussian curves affords two orthogonal components of the β responses which lie in the range ca 40–160 × 10–30 esu. The HRS β800 values do not reveal any clear trends within the two series, but the values for 28–31 are substantially smaller than those of their non-chelated counterparts 17–20. As for 7–13, the β0 responses of 17–20 and 28 are strongly 2D, and these NLO responses also appear to be greater in the N-arylated compounds, in keeping with the behaviour of related 1D dipolar systems.6b–e This grant has also been used to fund Stark spectroscopic studies on a number of other complex salts which were synthesized under closely related EPSRC grants (GR/L56213/01 and GR/R54293/01) and by an EPSRCsupported DTA student. The results of these studies involving RuII pyridyl polyene systems (see (iii) above) have 5 been described to some extent in a previous IGR report, but further data have been obtained and analysed more recently. These polyene complexes have very large β0 values which cover a range of ca 100–600 × 10–30 esu and are maximized at n = 2 (where n is the number of E-CH=CH units), in marked contrast to other known D-π-A polyene chromophores in which β0 increases steadily with n. The Stark results indicate that π →π* ILCT transitions in the UV region also contribute ca 10% of the overall β0 responses of the n = 2 compounds and ca 20% for their n = 3 analogues. However, inclusion of these additional contributions to the NLO responses does not alter the overall conclusion that the β0 values effectively level off or decrease after n = 2. These results are particularly interesting because they contradict the accepted wisdom that π-conjugation extension enhances NLO responses.1,2 MLCT absorption and electrochemical data clearly show that a {RuII (NH 3 ) 5 } 2+ or related centre is more electron-rich than a trans-{RuII Cl(pdma)2 }+ [pdma = 1,2-phenylenebis(dimethylarsine)] unit. However, despite initial assumptions, the β0 values of a series of such arsine complexes determined by using Stark data are only a little smaller then those of their {RuII (NH 3 ) 5 }2+ analogues. Large molecular quadratic NLO responses are hence maintained, with considerable gains in visible transparency (the MLCT bands of the arsine complexes are blueshifted by ca 0.6 eV when compared with those of their pentaammine analogues at 77 K), thermal stability and also crystallising ability. The unusual linear optical behaviour observed with the pyridyl polyene complexes of RuII ammine centres is also found with the analogous arsine species, but no clear evidence for a corresponding decrease in β0 on chain extension is observed. Theoretical Calculations Calculations on selected RuII ammine pyridyl polyene cations involving time-dependant density-functional theory (TD-DFT) and the finite field method generally predict the experimentally observed trends, both in terms of the hypsochromic shifting of the MLCT bands and the maximization of β0 with E,E-buta-1,3-dienyl linkages. The unexpected optical properties can be rationalised in terms of the extent of orbital overlap between the RuII electron donor and N-methylpyridinium electron acceptor groups. The TD-DFT results indicate that the HOMO gains in π character along the two series of complexes and consequently the lowest energy transition usually considered as MLCT in character has some ILCT contribution that increases with the conjugation pathlength. RESEARCH IMPACT AND BENEFITS TO SOCIETY It is not yet possible to assess the impact of the studies supported by this grant because most of the results have yet to be published (see below). The grant has certainly provided valuable training and experience for Dr Harris, including visits to carry out Stark spectroscopic measurements at Brookhaven National Laboratory and California Institute of Technology (CalTech). The broader benefits to society of fundamental research into novel NLO materials can be expected to become apparent as the technologies for device fabrication and exploitation mature. EXPLANATION OF EXPENDITURE Overall spending was within budget, the only minor changes being that a number of small items of equipment were inadvertantly charged to equipment rather than consumables, and some of the consumables funds were spent on travel to conferences. FURTHER RESEARCH OR DISSEMINATION ACTIVITIES A further full paper (‘Syntheses, Spectroscopic and Quadratic Nonlinear Optical Properties of Extended Dipolar Complexes with Ruthenium(II) Ammine Electron Donor and N-Methylpyridinium Acceptor Groups’, B J Coe, L A Jones, J A Harris, B S Brunschwig, I Asselberghs, K Clays, A Persoons, J Garín and J Orduna) has just been submitted to J. Am. Chem. Soc., and three other full articles are presently in the advanced stages of preparation, details as follows: (i) ‘Molecular Quadratic Nonlinear Optical Properties of cis-Tetraammineruthenium(II) Complexes Bearing Two Pyridinium Electron Acceptor Groups’, B J Coe, L A Jones, J A Harris, B S Brunschwig, K Wostyn, K Clays, A Persoons, S J Coles and M B Hursthouse, for Dalton Trans. (ii) ‘Contrasting Linear and Quadratic Nonlinear Optical Behaviour in Ruthenium(II) Ammine and Related Organic Conjugated Donor-Acceptor Chromophores’, B J Coe, J A Harris, B S Brunschwig, J Garín, J Orduna, S J Coles and M B Hursthouse, for Chem. Eur. J. (iii) ‘Syntheses and Optical and Electronic Properties of Iron(II) and Ruthenium(II) Tris-Chelate Complexes of 2,2′:4,4′′:4′,4′′′-Quaterpyridinium Ligands’, B J Coe, J A Harris and B S Brunschwig, for Dalton Trans. 6 Papers (i) and (ii) may be submitted before the end of 2003, whilst (iii) will be submitted within the next few months. A further full paper describing RuII arsine complexes, the Stark spectroscopic studies on which were carried out by Dr Harris, will be submitted to Dalton Trans. at some point later in 2004. Two other conference poster presentations were given at the Coordination Chemistry Discussion Group Meeting in 2003, but these were the same as those given at the SPIE Annual Conference and are therefore not listed on page 5 of the IGR form. The total published output from this grant will comprise of 6 full articles, 2 communications, 3 conference proceedings and 9 conference oral/poster presentations. Parts of Dr Harris’s studies are now being continued by an EPSRC-funded DTA student, and an application for continuation funding has just been submitted to EPSRC. SUMMARY This grant has allowed us to make considerable progress in our investigations into the NLO properties of transition metal complexes. Although a number of the original synthetic objectives were not achieved, the available funds have been put to good use, particularly in making Stark spectroscopic measurements. The highly unusual optical behaviour observed in the RuII ammine pyridyl polyenes is especially noteworthy, and a number of novel multidimensional NLO chromophores have also been prepared and studied. Our international collaborations with Profs Clays and Persoons at Leuven University and Prof Brunschwig at CalTech continue to gain momentum, and a new collaboration with Prof Garín at Zaragoza University is already producing informative and high-quality results. Future significant developments from the related work of a DTA student can also be anticipated. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. Nonlinear Optical Properties of Organic Molecules and Crystals, eds. D. S. Chemla and J. Zyss, Academic Press, Orlando, 1987, vols. 1 and 2; J. 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