Photochemistry of Bromofluorobenzenes

Abstract

The photochemistry of low lying excited states of six different fluorinated bromobenzenes has been investigated by means of femtosecond laser spectroscopy and high level ab initio CASSCF/CASPT2 quantum chemical calculations. The objective of the work was to investigate how and to what extent light substituents, position on the benzene ring and number, would influence the dissociation mechanism of bromobenzene. In general, the actual position of a fluorine atom affects the dissociation rate to a less extent than the number of fluorine atoms. A clear connection between a lowering of a repulsive pisigma relative to a bound pipi state and the number of fluorine substituents exists, and the previously suggested model of coupling between dissociation rate and relative location of bound and repulsive state still holds for these molecules. A more elaborate examination of the electronic structure of the excited states in bromobenzenes than previously reported is presented.

Role of the πσ* State in Molecular Photophysics

“…Takemura et al 50 Unlike the fluorinated and chlorinated benzenes, the πσ* state of bromobenzenes is repulsive along the C-Br stretching coordinate. 52 In several of gaseous bromoflorobenzenes, the main dissociation channel following 270 nm excitation is a predissociation involving the bound S 1 (ππ*) state and a repulsive 3 πσ* state, giving rise to the dissociation rates of 20-40 ps. 52 In di-and pentafluorobromobenzenes, where a repulsive 1 πσ* state lies lower in energy, a direct excitation to the state leads to a subpicosecond (<300 fs) dissociation rate.…”

“…52 In di-and pentafluorobromobenzenes, where a repulsive 1 πσ* state lies lower in energy, a direct excitation to the state leads to a subpicosecond (<300 fs) dissociation rate. 52 …”

Photochemistry of aryl halides: Photodissociation dynamics

“…Experimentally, the photodissociation dynamics of aryl halides have widely been studied in the gas phase by photofragment translational spectroscopy (PTS) [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65], in real time by ultrafast pump-probe spectroscopy combined with timeof-flight (TOF) [66][67][68][69][70][71], by time-resolved Fourier-transform spectroscopy [35,71,72], by resonance enhanced multiphoton ionization (REMPI) technique [73][74][75][76], and by multimass ion imaging techniques [36,48,49,[77][78][79][80]. Fox example, Bersohn and coworkers [50,51] were firstly to study the photodissociation dynamics of the aryl halides at 193 and 248 nm.…”

Photodissociation Dynamics of C6F5Br at 234 nm: Fluorination Effects on Br/Br*Formation Pathways

“…The A-band of alkyl bromide arises from the C-Br bond localized transition and consists of three overlapping transitions to repulsive states ( Q 1 PESs via a conical intersection along the C-Br bond coordinate. [4][5][6] Compared to the photodissociation of alkyl bromide, the photodissociation dynamics of aryl halides [7][8][9][10][11][12][13][14][15][16] show more complicated because more electronic states are involved and thus making multiple dissociation pathways probable. Multiple dissociation pathways include the predissociation between excited electronic states, internal conversion processes from vibrational excited states, and dissociation processes from repulsive states.…”

“…It has been unraveled by numerous studies that dissociation pathways are substantially dependent on the type of halogen, 9-11 atomic substituents, 12 and excitation wavelengths. 13 In the early work on the bromobenzene (C 6 H 5 Br) photodissociation near 266 nm, [7][8][9][10][11][12]14 the main dissociation channel is an indirect dissociation involving the bound (π, π * ) and repulsive (π, σ * ) states. In the short wavelength, significant UV absorption of bromobenzene is caused by and transition; the mixing between these states becomes more probable and additional routes can be observed as in the case of photodissociation of iodobezene.…”