Exploring the time-scales of H-atom detachment from photoexcited phenol-h(6) and phenol-d(5): statistical vs nonstatistical decay

The prevalence of 1 pi sigma* states in the photochemistry of heteroaromatics is becoming increasingly clear from the recent literature. Photodissociation measurements have shown that following excitation of phenol molecules above the S-1/S-2 conical intersection, H-atoms are eliminated with two distinct ranges of kinetic energy release. Those with high kinetic energy are attributed to direct dissociation while those with low kinetic energy are traditionally attributed to indirect dissociation or statistical unimolecular decay, both pathways giving electronic ground-state phenoxyl fragments. Using a combination of femtosecond pump/probe spectroscopy and velocity map ion imaging techniques, the time and energy resolved H-atom elimination in phenol-h(6) and phenol-d(5), following excitation at 200 nm has been measured. At the lowest kinetic energies, the H-atom elimination from phenol-d(5) occurs in < 150 fs, in sharp contrast to what one expects from a statistical decay process. This implies that these H-atoms are formed through a direct dissociation process yielding electronically excited phenoxyl fragments

Active participation of 1πσ* states in the photodissociation of tyrosine and its subunits

The ubiquitous nature of (1)pi sigma* states in biological molecules has been a prime focus in recent years. Understanding the role of this dissociative state in the amino acids and their respective chromophores following UV excitation would enable a step change toward establishing a better understanding of the mechanisms of photostability of larger peptides in the gas-phase. This letter presents the first evidence of the active involvement of (1)pi sigma* states in the H-atom elimination of the amino acid tyrosine and its subunit tyramine, following excitation at 200 nm

Theoretical Investigation of Phosphinidene Oxide Polypyridine Ruthenium(II) Complexes: Toward the Design of a New Class of Photochromic Compounds.

International audienceA DFT-based computational study performed in the gas phase and in acetonitrile on polypyridine ruthenium isomer complexes [Ru(tpy)(bpy)(POPh)](2+) and [Ru(tpy)(bpy)(OPPh)](2+) (bpy = 2,2'-bipyridine, tpy = 2,2':6',2″-terpyridine, Ph = phenyl) predicts that they constitute a prototype for a new family of inorganic photochromic systems. The two isomers are found to absorb in different spectral regions to excited states that are connected adiabatically through a thermodynamically and kinetically favorable triplet potential energy profile. Nonadiabatic routes were identified and shown to be preferable over the adiabatic mechanism. The reverse isomerization reaction is found to be achievable only thermally. The current predictive work will be of prime importance to experimentalists for the design of new inorganic phosphorus-based compounds with attractive photochromic properties

Unraveling ultrafast dynamics in photoexcited aniline

A combination of ultrafast time-resolved velocity map imaging (TR-VMI) methods and complete active space self-consistent field (CASSCF) ab initio calculations are implemented to investigate the electronic excited-state dynamics in aniline (aminobenzene), with a perspective for modeling 1πσ* mediated dynamics along the amino moiety in the purine derived DNA bases. This synergy between experiment and theory has enabled a comprehensive picture of the photochemical pathways/conical intersections (CIs), which govern the dynamics in aniline, to be established over a wide range of excitation wavelengths. TR-VMI studies following excitation to the lowest-lying 1ππ* state (11ππ*) with a broadband femtosecond laser pulse, centered at wavelengths longer than 250 nm (4.97 eV), do not generate any measurable signature for 1πσ* driven N–H bond fission on the amino group. Between wavelengths of 250 and >240 nm (<5.17 eV), coupling from 11ππ* onto the 1πσ* state at a 11ππ*/1πσ* CI facilitates ultrafast nonadiabatic N–H bond fission through a 1πσ*/S0 CI in <1 ps, a notion supported by CASSCF results. For excitation to the higher lying 21ππ* state, calculations reveal a near barrierless pathway for CI coupling between the 21ππ* and 11ππ* states, enabling the excited-state population to evolve through a rapid sequential 21ππ* → 11ππ* → 1πσ* → N–H fission mechanism, which we observe to take place in 155 ± 30 fs at 240 nm. We also postulate that an analogous cascade of CI couplings facilitates N–H bond scission along the 1πσ* state in 170 ± 20 fs, following 200 nm (6.21 eV) excitation to the 31ππ* surface. Particularly illuminating is the fact that a number of the CASSCF calculated CI geometries in aniline bear an exceptional resemblance with previously calculated CIs and potential energy profiles along the amino moiety in guanine, strongly suggesting that the results here may act as an excellent grounding for better understanding 1πσ* driven dynamics in this ubiquitous genetic building block