Martial Boggio-Pasqua, Marcella Ravaglia, Michael J. Bearpark, Marco Garavelli, and Michael A. Robb (J. Phys. Chem. A; 2003; 107(50) pp 11139 - 11152)
DOI: 10.1021/jp036862e

The origin of the photochromic properties of diarylethenes is a conical intersection (which we have located computationally), but we show that dynamics calculations are necessary to explain why the conical intersection is accessible, because the excited state reaction path is not contained in the branching space defining the intersection. Four different systems have been studied: 1,2-di (3-furyl) ethene, 1,2-di (3-thienyl) ethene, 1,2-bis (2-methyl-5-phenyl-3-thienyl) perfluorocyclopentene, and a model hydrocarbon system. Critical points on the ground- and excited-state potential energy surfaces were calculated using Complete Active Space Self Consistent Field (CASSCF) theory; dynamics calculations were carried out using the Molecular Mechanics/Valence Bond (MMVB) method. The main experimental observations (i.e., picosecond time domain, quantum yield, temperature dependence, and fluorescence) can be interpreted on the basis of our results.

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"Intramolecular Charge Transfer in 4-Aminobenzonitriles Does Not Necessarily Need the Twist"

Gomez, I.; Reguero, M.; Boggio-Pasqua, M.; Robb, M. A. (J. Am. Chem. Soc.; (Article); 2005; 127(19); 7119-7129.)
DOI: 10.1021/ja042413w

In electron donor/acceptor species such as 4-(dimethylamino)benzonitrile (DMABN), the excitation to the S2 state is followed by internal conversion to the locally excited (LE) state. Dual fluorescence then becomes possible from both the LE and the twisted intramolecular charge transfer (TICT) states. A detailed mechanism for the ICT of DMABN and 4-aminobenzonitrile (ABN) is presented in this work. The two emitting S1 species are adiabatically linked along the amino torsion reaction coordinate. However, the S2/S1 CT-LE radiationless decay occurs via an extended conical intersection "seam" that runs almost parallel to this torsional coordinate. At the lowest energy point on this conical intersection seam the amino group is untwisted; however, the seam is accessible for a large range of torsional angles. Thus, S1 LE-TICT equilibration and dual fluorescence will be controlled by a) the S1 torsional reaction path and b) the position along the amino group twist coordinate where the S2/S1 CT-LE radiationless decay occurs. For DMABN, population of LE and TICT can occur since the two species have similar stabilities. However, in ABN, the equilibrium lies in favor of LE, as a TICT state was found at much higher energy with a low reaction barrier towards LE. This explains why dual fluorescence cannot be observed in ABN. The S1-->S0 deactivation channel accessible from the LE state was also studied.

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"Mechanism of an Exceptional Class of Photostabilizers: A Seam of Conical Intersection Parallel to Excited State Intramolecular Proton Transfer (ESIPT) in o-Hydroxyphenyl-(1,3,5)-triazine"

Martin J. Paterson, Michael A. Robb, Lluís Blancafort, and Anthony D. DeBellis (J. Phys. Chem. A; 2005; 109(33) pp 7527 - 7537.)
DOI: 10.1021/jp051108+

We present a detailed CASSCF study of the mechanism of excited state intramolecular proton transfer (ESIPT) in the o-hydroxyphenyl triazine class of photostabilizers. The valence-bond analysis of the ground state and the two ππ* excited states permits a simple chemical interpretation of the mechanistic information. Our results show that the barrier to enol Ð keto tautomerism on the ground state adiabatic surface is high. Following photo-excitation to the charge transfer state, the ESIPT is predicted to take place without barrier. Radiationless decay to the ground state is associated with an extended seam of conical intersection, with a sloped topology lying parallel to the ESIPT path, which can be accessed at any point along the reaction path. Our results show that the triazine class of photostabilizers have the photochemical and photophysical qualities associated with exceptional photostability.

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