A comprehensive photophysical investigation has been carried out on a series of eight complexes of the type (diimine)Pt(-C=C-Ar)(2), where diimine is a series of 2,2'-bipyridine (bpy) ligands and -C=C-Ar is a series of substituted aryl acetylide ligands. In one series of complexes, the energy of the Pt --> bpy metal-to-ligand charge transfer (MLCT) excited state is varied by changing the substituents on the 4,4'- and/or the 5,5'-positions of the bpy ligand. In a second series of complexes the electronic demand of the aryl acetylide ligand is varied by changing the para substituent (X) on the aryl ring (X = -CF(3), -CH(3), -OCH(3), and -N(CH(3))(2)). The effect of variation of the substituents on the excited states of the complexes has been assessed by examining their UV-visible absorption, variable-temperature photoluminescence, transient absorption, and time-resolved infrared spectroscopy. In addition, the nonradiative decay rates of the series of complexes are subjected to a quantitative energy gap law analysis. The results of this study reveal that in most cases the photophysics of the complexes is dominated by the energetically low lying Pt --> bpy (3)MLCT state. Some of the complexes also feature a low-lying intraligand (IL) (3)pi,pi excited state that is derived from transitions between pi- and pi-type orbitals localized largely on the aryl acetylide ligands. The involvement of the IL (3)pi,pi state in the photophysics of some of the complexes is signaled by unusual features in the transient absorption, time-resolved infrared, and photoluminescence spectra and in the excited-state decay kinetics. The time-resolved infrared difference spectroscopy indicates that Pt --> bpy MLCT excitation induces a +25 to + 35 cm(-)(1) shift in the frequency of the C=C stretching band. This is the first study to report the effect of MLCT excitation on the vibrational frequency of an acetylide ligand.

This article describes the results of an IR spectroelectrochemical study of the electrocatalytic reduction of carbon dioxide using the complexes [Re(CO) 3 (bpy)L] n (bpy ) 2,2′-bipyridine; n ) 0, L ) Cl -, CF 3 SO 3 -; n ) +1, L ) CH 3 CN, P(OEt) 3 ) as catalyst precursors. The study was performed for the first time with an optically transparent thin-layer electrochemical (OTTLE) cell. The results confirm unambiguously the catalytic activity of the reduced fivecoordinate complexes, the radical [Re(CO) 3 (bpy)] • and the anion [Re(CO) 3 (bpy)] -. The catalytic behavior of these species could be investigated separately for the first time due to the application of complexes other than those with L ) halide, whose catalytic routes may involve simultaneously both radical and anionic catalysis depending on the solvent used. The complex [Re(CO) 3 (bpy)Cl], so far the most studied catalyst precursor, upon one-electron reduction gives the corresponding radical-anion [Re(CO) 3 (bpy)Cl] •-, which was previously believed to react directly with CO 2 . By contrast, this study demonstrates its stability toward attack by CO 2 , which may only take place after dissociation of the chloride ligand. This conclusion also applies to other six-coordinate radicals [Re(CO) 3 (bpy)L] • (L ) CH 3 CN (in CH 3 CN) and P(OEt) 3 ) whose catalytic route requires subsequent one-electron reduction to produce the anionic catalyst [Re(CO) 3 (bpy)] -(the 2e pathway). The catalytic route of [Re(CO) 3 (bpy)Cl] in CH 3 CN therefore deviates from that of the related [Re(CO) 3 (dmbpy)Cl], the other complex studied by IR (reflectance) spectroelectrochemistry, with the more basic ligand, 4,4′-dimethyl-2,2′-bipyridine (dmbpy). The latter complex tends to form the fivecoordinate radicals [Re(CO) 3 (dmbpy)] • , capable of CO 2 reduction (the 1e pathway), even in CH 3 CN, hence eliminating the possibility of the 2e pathway via the anion [Re(CO) 3 (dmbpy)] -, which operates in the case of the 2,2′-bipyridine complex. For [Re(CO) , the 1e catalytic route becomes possible in weakly coordinating THF, due to the instability of the radical [Re(CO) 3 (bpy)(THF)] • . The inherent stability of the radical [Re(CO) 3 (bpy){P(OEt) 3 }] • was found convenient for the investigation of the 2e pathway via [Re(CO) 3 (bpy)] -. The main, spectroscopically observed products of the CO 2 reduction are, independent of the 1e and 2e catalytic routes, CO, CO 3 2-, and free CO 2 H -. The latter product is formed via one-electron reduction of the radical anion [Re(CO) 3 (bpy)(CO 2 H)] •-, which is the main byproduct in the catalytic cycle.

Charge-transfer excited states have frequently been studied by using 4-dimethylaminobenzonitrile (DMABN) as a model. In nonpolar solvents, a single fluorescence band is observed from a locally excited (LE) state. In polar solvents, the initially populated LE state reacts further to produce a stable intramolecular charge-transfer (ICT) state, which gives rise to a second fluorescence band that overlaps with, but is abnormally red-shifted from, the LE emission.[1] Results of experiments using aprotic solvents are well described by models in which polarity is the only solvent property that affects the charge transfer reaction activation energy and the relative stabilization of the ICT and LE states.[2] Whilst much work continues to concentrate on determining the structures of the LE and ICT states, [3][4][5][6][7] the precise nature of the difference between the properties of the excited state in protic and aprotic solvents is little understood. For example, the fluorescence quantum yield of DMABN in protic solvents is lower and the fluorescence spectrum is further red-shifted and broadened, relative to measurements in aprotic solvents of the same polarity, [8,9] and the fluorescence decay kinetics are difficult to interpret.[2] Hydrogen bonding in protic solvents can lead to complicated interactions [10] but although specific solute-solvent and solute-solute interactions have been discussed, [8,[11][12][13][14] there is no generally accepted explanation. There are similar problems in other cases of dual fluorescence.[15]The time-resolved infrared (TRIR) absorption spectra presented here demonstrate and monitor the formation of a hydrogen-bonded charge-transfer state of photoexcited DMABN in the protic solvent methanol (MeOH), through the development of the CN IR absorption band from an initial singlet into a doublet. The initial single band is interpreted as belonging to an ICT state like that created in aprotic acetonitrile (MeCN), where only one absorption band is observed at all delay times. The second component is interpreted as being due to the hydrogen-bonded chargetransfer state; the kinetics show the populations of the free and hydrogen-bonded species coming to dynamic equilibrium. We designate the hydrogen-bonded state as HICT. This is the first direct observation of hydrogen bonding in an excited state. Since the populations in the LE state and the two charge-transfer states coexist, the fluorescence will be triple, not dual in character. Neglect of this major factor is considered to account for much of the difficulty in interpreting the fluorescence results. [2,8,[11][12][13] A mechanism of this kind has not to our knowledge been proposed before. We believe this interpretation is applicable to other molecules with solvent-dependent dual fluorescence. Figure 1 shows TRIR spectra of DMABN in MeCN (a) and MeOH (b) recorded with sub-picosecond time resolution at pump-probe delays from 2 to 3000 ps after excitation; Figure 2 gives the time-dependence of the absorption band areas. Kinetics parameters were dete...