Condensed phase photophysics

The form of this review follows that in previous volumes although it has been decided this year to cite more of the work published on biochemical systems. The applications of luminescence to biological problems is the most active area in condensed phase photophysics and this survey attempts to reflect this situation. 

Excited-State Relaxation. A further photophysical topic of intense interest is pathways for thermal relaxation of excited states in condensed phases. According to the Franck-Condon principle, photoexcitation occurs with no concurrent relaxation of atomic positions in space, either of the photoexcited chromophore or of the solvating medium. Subsequent to excitation, but typically on the picosecond time scale, atomic positions change to a new equihbrium position, sometimes termed the (28)- Relaxation of the solvating medium is often more dramatic than that of the chromophore... 

Dendrimers have precise compositional and constitutional aspects, but they can exhibit many possible conformations. Thus, they lack long-range order in the condensed phase, which makes it inappropriate to characterize the molecular-level structure of dendrimers by X-ray diffraction analysis. However, there have been many studies performed using indirect spectroscopic methods to characterize dendrimer structures, such as studies using photophysical and photochemical probes by UV-Vis and fluorescence spectroscopy, as well as studies using spin probes by EPR spectroscopy. 

The S2 state fluorescence of metalloporphyrins was first noticed by Bajema et al. ( ) and later the photophysical parameters concerned with the S2 state were determined on several metalloporphyrins (9,10). S2 - Sq emission from large molecules in condensed phases has also been recognized in many other organic compounds, e.g., azulene (11), thiocarbonyl compounds (12), and several polyenes (13). However, in some metalloporphyrins one can observe the S2 state fluorescence even after the excitation to the state (14-17), that is, the blue... 

Photophysics in the condensed phase is much more involved than in solutions. In fact, there is much debate on the subject and the nature of the primary excitations are not easily assigned. Due to new mechanisms not yet elucidated or identified, singlet, triplet and charged states are all possible candidates for observation at short times, and there are experimental claims and evidence for each of them [97-103]. Defects and impurities may mask the identification of molecular states, which can also be substantially modified by intermolecular interactions [61,104,105]. 

In the condensed phase the close proximity of neighbouring molecules can have a considerable effect on the subsequent fate of the excited species. Solvation can reduce molecular energies, affect the selection rules, and greatly increase the number of collisions undergone by each molecule [23]. The last effect can increase the photophysical decay and decrease the photoreaction quantum yield. Moreover, the... 

For years, the capabilities of TDDFT to describe excited states have been limited to isolated molecules, despite the fact that a large part of the spectroscopic experiments probe molecules in liquids. However, in the last few years there have been several extensions of the TDDFT to describe excited state of molecules in solution. These extensions are of particular interest as they will allow to expand the areas of application of the TDDFT to several photophysical and photochemical processes in condensed phase. 

For space limitation, other recent extensions of the PCM-TDDFT scheme aimed at describing other interesting photophysics phenomena of solvated molecules have not been considered. We cite here, as examples, the application of PCM-TDDFT to the study of the Excitation Energy Transfer (EET) between chromophores in different environments [48], and of absorption/emission spectra of chromophores in complex environments such as the interphase between two different media [49], All these new QM computational tools may be of support to the efforts toward an even better understanding and description of the photophysics and of the photochemistry of molecules in condensed phase and in complex environments. 

We have presented nonadiabatic ab initio molecular dynamics simulations of the photophysical properties of a variety of nucleobases and base pairs. In addition to the canonical tautomers a number of rare tautomers have been investigated. Moreover, effects of substitution and solvation have been studied in detail. The simulations of nonradiative decay in aqueous solution, in particular, demonstrate the strength of the na-AIMD technique employed here as it permits the treatment of solute and solvent on an equal footing. Condensed phase calculations can be directly compared with those in the gas phase because the same computational setup can be used. 

We present experimental results on photophysical deactivation pathways of uracil and thymine bases in the gas phase and in solvent/solute complexes. After photoexcitation to the S2 state, a bare molecule is tunneled into and trapped in a dark state with a lifetime of tens to hundreds of nanoseconds. The nature of this dark state is most likely a low lying nn state. Solvent molecules affect the decay pathways by increasing IC from the S2 to the dark state and then further to the ground state, or directly from S2 to S0. The lifetimes of the S2 state and the dark state are both decreased with the addition of only one or two water molecules. When more than four water molecules are attached, the photophysics of these hydrated clusters rapidly approaches that in the condensed phase. This model is now confirmed from other gas phase and liquid phase experiments, as well as from theoretical calculations. This result offers a new interpretation on the origin of the photostability of nucleic acid bases. Although we believe photochemical stability is a major natural selective force, the reason that the nucleic acid bases have been chosen is not because of their intrinsic stability. Rather, it is the stability of the overall system, with a significant contribution from the environment, that has allowed the carriers of the genetic code to survive, accumulate, and eventually evolve into life s complicated form. 

Only condensed-phase systems will be considered in this chapter. General systematic approaches to the spectroscopy and properties of CT excited states were described in detail by Mulliken [1]. Approaches to the CT spectroscopy of transition metal complexes were developed by Jorgensen [2]. Lever developed these approaches further and summarized their application to a range of transition metal complexes [3] Solomon and Hanson [4] considered their application to metalloenzymes and model systems. Endicott [5], Hovarth and Stevenson [6], and Ferraudi [7] discussed the photochemistry and photophysics of transition metal CT systems, including the applications of the various theoretical models. These works may be consulted for... 

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