The photochemistry of benzophenone,
a paradigmatic organic molecule
for photosensitization, was investigated by means of surface-hopping
ab initio molecular dynamics. Different mechanisms were found to be
relevant within the first 600 fs after excitation; the long-debated
direct (S1 → T1) and indirect (S1 → T2 → T1) mechanisms
for population of the low-lying triplet state are both possible, with
the latter being prevalent. Moreover, we established the existence
of a kinetic equilibrium between the two triplet states, never observed
before. This fact implies that a significant fraction of the overall
population resides in T2, eventually allowing one to revisit
the usual spectroscopic assignment proposed by transient absorption
spectroscopy. This finding is of particular interest for photocatalysis
as well as for DNA damages studies because both T1 and
T2 channels are, in principle, available for benzophenone-mediated
photoinduced energy transfer toward DNA.
Molecular switches based on E/Z photoisomerization have been used in different contexts in order to control a variety of processes in different systems, from peptide conformation control to molecular data storage devices, from catalysis to smart materials. The syntheses, properties and applications of several types of E/Z photochemical switches are presented with special attention paid to azobenzenes, overcrowded alkenes and switches based on the protonated Schiff base chromophore of rhodopsins.
Channelrhodopsin (ChR) is a key protein of the optogenetic toolkit. C1C2, a functional chimeric protein of Chlamydomonas reinhardtii ChR1 and ChR2, is the only ChR whose crystal structure has been solved, and thus uniquely suitable for structure-based analysis. We report C1C2 photoreaction dynamics with ultrafast transient absorption and multi-pulse spectroscopy combined with target analysis and structure-based hybrid quantum mechanics/molecular mechanics calculations. Two relaxation pathways exist on the excited (S1) state through two conical intersections CI1 and CI2, that are reached via clockwise and counter-clockwise rotations: (i) the C13=C14 isomerization path with 450 fs via CI1 and (ii) a relaxation path to the initial ground state with 2.0 ps and 11 ps via CI2, depending on the hydrogen-bonding network, hence indicating active-site structural heterogeneity. The presence of the additional conical intersection CI2 rationalizes the relatively low quantum yield of photoisomerization (30 ± 3%), reported here. Furthermore, we show the photoreaction dynamics from picoseconds to seconds, characterizing the complete photocycle of C1C2.
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