Excited state Isomerization

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

Although FMO theory shows the reaction to be a ground-state process, photochemical reactions with a trans-ring juncture product have been observed. This has been attributed to an excited state isomerization of the ene-portion followed by a ground state Diels-Alder reaction ... 

Another interesting class of molecules are stilbene derivatives with charge donating groups. These compounds offer the opportunity to explore the role of polar solvation dynamics (dielectric friction) in cis/trans isomerization. Interesting papers on this subject have been published by Waldeck et al. [145] and Rulliere et al. [146]. Other well-studied polar excited state isomerization examples include pinacyanol, l,l -diethyl-4,4 -cyanine, and crystal violet, which have been studied by Sundstrom, Gilbro and their coworkers [148] and Ben-Amotz and Harris [148] and others who are referenced in these papers [148,149],... 

Return now to the questions surrounding the actual sequence of events leading to substitution following population of the reactive state. As in thermal substitution mechanisms it is appropriate to determine whether a dissociative or an associative mechanism obtains. Certainly, this point is the one most often clarified, but other aspects also deserve some scrutiny. These include the possibility of acid-base equilibria in the excited state, isomerization of potentially ambidentate ligands, the extent to which the extruded ligand is electronically or vibrationally excited, the degree of molecular distortion upon population of the reactive state and the possibility of competing chemical processes which may be influenced by the environment or by structural modifications of the molecule. 

Figure 3.39 The minimal PSB model, cis-C5/75M7)7 for the PSB photoisomerization. The vertical line indicates the carbon-carbon bond about which the excited state isomerization occurs and defines the two sides used in the definition of the valence bond states.

Dasgupta J, Frontiera RR, Taylor KC, Lagarias JC, Mathies RA (2009) Ultrafast excited-state isomerization in phytochrome revealed by femtosecond stimulated Raman spectroscopy. Proc Nad Acad Sci USA 106 1784-1789... 

Many features of the emission spectrum can show time dependence, including the spectral shape (l,3 (v-9), the peak intensity, the linear polarization (10) and, in principle, the circular polarization (11). In extreme cases, the emission spectrum can actually have two separate fluorescence bands from two different isomers of the electronically excited molecules (12-15). For molecules with this behavior, it is possible to determine the kinetics of excited state isomerization by transient emission spectroscopy. 

Method A is particularly useful for studying the dynamics of dual fluorescent systems, including excimer formation and excited state isomerization. For example, we have studied the dynamics of the excited state isomerization of 3-hydroxyflavone (3HF). The stable ground-state form of 3HF is the "normal"... 

In a recent paper (12) we showed that the species responsible for the green (543 nm) fluorescence of 3HF was formed kinetically from blue (413 nm) emitting species by two kinetic components. We rationalized this complex behavior by a model (Scheme 1) in which there are two pathways for excited state isomerization, i.e., a slow and a fast component. 

Fantacci, S Migani, A., Olivucci, M., CASPT2/CASSCF and TDDFT/CASSCF Mapping of the Excited State Isomerization Path of a Minimal Model of the Retinal Chromophore, J. Phys. Chem. A 2004, 108, 1208 1213. 

Kara, K., H. Kiyotani, and O. Kajimoto 1995, High-pressure studies on the excited-state isomerization of 2-vinylanthracene - Experimental investigation of Kramers turnover . J. Chem. Phys. 103, 5548. 

Yartsev A, Alvarez JL, Abetg U, Sundstrom V (1995) Overdamped wavepacket motionalong a barrierless potential-energy surface in excited-state isomerization. Chem Phys Lett 243 281... 

Lin, S.W., Groesbeek, M., van der Hoef, I., Verdegem, P., Lugtenburg, J., et al. Vibrational assignment of torsional normal modes of rhodopsin probing excited-state isomerization dynamics along the reactive C-11=C-12 torsion coordinate. J. Phys. Chem. B 102, 2787-2806 (1998)... 

When the photolysis of pyridine-N-oxide is carried out in a polar solvent via the singlet excited state, isomerization to 2-pyridone is observed as intermediate an oxaziridine 120 is postulated ... 

K Kara, N Ito, O Kajimoto. High pressure studies of the Kramers turnover behavior for the excited-state isomerization of 2-alkenylanthracene in alkene. J Chem Phys 110 1662, 1999. 

Top Observed A° values and excited state lifetimes associated with the initial steps of the photoisomerization of Rh. Bottom Schematic representation of the excited state isomerization motion of PSB11 dominated by an asynchronous crankshaft structure deformation (Frutos et al. 2007) and documented for a CASSCF/AMBER model of Rh... 

Fantacci, S., Migani, A., 8c Olivucci, M. (2004). CASPT2//CASSCF and TDDFT//CASSCF mapping of the excited state isomerization path of a minimal model of the retinal chromophore. The Journal of Physical Chemistry B, 108(7), 1208-1213. doi 10.1021/jp0362335. 

A. I. Krylov and R. B. Gerber,/. Chem. Phys., 100, 4242 (1994). Photodissociation of ICN in Solid and Liquid Ar" Dynamics of the Cage Effect and of Excited-State Isomerization. 

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