nσ* and πσ* excited states in aryl halide photochemistry: a comprehensive study of the UV photodissociation dynamics of iodobenzene

Abstract

A recent review (Ashfold et al., Phys. Chem. Chem. Phys., 2010, 12, 1218) highlighted the important role of dissociative excited states formed by electron promotion to σ* orbitals in establishing the photochemistry of many molecular hydrides. Here we extend such considerations to molecular halides, with a particular focus on iodobenzene. Two experimental techniques (velocity mapped ion imaging (VMI) and time resolved infrared (IR) diode laser absorption) and electronic structure calculations have been employed in a comprehensive study of the near ultraviolet (UV) photodissociation of gas phase iodobenzene molecules. The VMI studies yield the speeds and angular distributions of the I((2)P(3/2)) and I*((2)P(1/2)) photofragments formed by photolysis in the wavelength range 330 ≥λ≥ 206 nm. Four distinct dissociation channels are observed for the I((2)P(3/2)) atom products, and a further three channels for the I*((2)P(1/2)) fragments. The phenyl (Ph) radical partners formed via one particular I* product channel following excitation at wavelengths 305 ≥λ≥ 250 nm are distributed over a sufficiently select sub-set of vibrational (v) states that the images allow resolution of specific I* + Ph(v) channels, identification of the active product mode (ν(10), an in-plane ring breathing mode), and a refined determination of D(0)(Ph-I) = 23,390 ± 50 cm(-1). The time-resolved IR absorption studies allow determination of the spin-orbit branching ratio in the iodine atom products formed at λ = 248 nm (ϕ(I*) = [I*]/([I] + [I*]) = 0.28 ± 0.04) and at 266 nm (ϕ(I*) = 0.32 ± 0.05). The complementary high-level, spin-orbit resolved ab initio calculations of sections (along the C-I bond coordinate) through the ground and first 19 excited state potential energy surfaces (PESs) reveal numerous excited states in the energy range of current interest. Except at the very shortest wavelength, however, all of the observed I and I* products display limiting or near limiting parallel recoil anisotropy. This encourages discussion of the fragmentation dynamics in terms of excitation to states of A(1) total symmetry and dissociation on the 2A(1) and 4A(1) (σ* ← n/π) PESs to yield, respectively, I and I* products, or via non-adiabatic coupling to other σ* ← n/π PESs that correlate to these respective limits. Similarities (and differences) with the available UV photochemical data for the other aryl halides, and with the simpler (and more thoroughly studied) iodides HI and CH(3)I, are summarised.

Ultrafast REMPI in benzene and the monohalobenzenes without the focal volume effect

“…However, ionization from the 3 ns* state during the dissociation explains the four-photon process. Thus, we determine that the slope of four is representative of a three-photon excitation to the singlet 1 pp* state followed by a nearly immediate intersystem crossing to the dissociative triplet 3 ns* state, which then requires four photons to ionize before it can completely dissociate, 22,29 as shown in Fig. 2c.…”

UV Photodissociation Action Spectroscopy of Haloanilinium Ions in a Linear Quadrupole Ion Trap Mass Spectrometer

“…In the case of neutral iodobenzene, these states reside at approximately 4.05, 4.12 and 4.36 eV (306 -284 nm) vertically above the ground state. [52] As the PD action spectra probe photo-induced fragmentation, the bromo-and chloro-species may absorb photons at this wavelength but fragmentation is not forthcoming. REMPI spectra have measured vibrationally-resolved S 1 ←S 0 transitions at ~270 nm for the chlorobenzene [60] and bromobenzene [61] neutrals respectively.…”

“…[45] Neutral aryl halides have been extensively studied and the photodissociation dynamics of many systems have been reported. [46][47][48][49][50][51][52] The photo-induced cleavage of the carbon-halogen bond occurs under UV-excitation as the system accesses dissociative electronic states. These dissociative states are rationalized by the excitation of electrons from non-bonding (n) orbitals on the halogen to anti-bonding σ* orbitals localized around the carbon-halogen bond (σ*←n).…”

Exploring quantum phenomena and vibrational control in σ* mediated photochemistry

“…Moreover, it has been postulated that ultrafast dynamics on 1 ps* and 1 ns* PESs may be key to reducing the lifetimes of excited states that could otherwise lead to toxic processes in biological systems, 3 contributing to the remarkable resilience many heteroatom containing aromatic biomolecules display with respect to UV induced photodegradation. In addition to their proposed role in fundamental photobiology, s* mediated dynamics contribute to wider aspects of excited state photochemistry, and can also be active in both aromatic and aliphatic molecules along X-Y/R bond coordinates (Y ¼ halogen, R ¼ alkyl group), [6][7][8] broadening the signicance of this behavior to encompass a myriad of chemical species. 4 In this work, we turn our attention to understanding more exotic aspects of photochemistry driven by (n/p)s* excited states, with particular focus on: (i) the ways in which specic vibrations can promote/control non-adiabatic processes through CIs; and (ii) the role purely quantum mechanical phenomena play in these dynamics -specically the penetration of vibrational wavefunctions into classically forbidden regions of the potential energy landscape (i.e.…”