Ultraviolet Excitation Dynamics of Nitrobenzenes

Abstract

Time-resolved photoelectron imaging was used to investigate nonadiabatic processes operating in the excited electronic states of nitrobenzene and three methyl-substituted derivatives: 3,5-, 2,6-, and 2,4-dimethylnitrobenzene. The primary goal was evaluating the dynamical impact of the torsional angle between the NO2 group and the benzene ring planesomething previously implicated in mediating the propensity for branching into different photodissociation pathways (NO vs NO2 elimination). Targeted, photoinitiated release of NO radicals is of interest for clinical medicine applications, and there is a need to establish basic structure–dynamics–function principles in systematically varied model systems following photoexcitation. Within our 200 ps experimental detection window, we observed no significant differences in the excited-state lifetimes exhibited by all species under study using a 267 nm pump and ionization with an intense 400 nm probe. In agreement with previous theoretical predictions, this suggests that the initial energy redistribution dynamics within the singlet and triplet manifolds are driven by motions localized predominantly on the NO2 group. Our findings also imply that both NO and NO2 elimination occur from a vibrationally hot ground state on extended (nanosecond) timescales, and any variations in NO vs NO2 branching upon site-selective methylation are due to steric effects influencing isomerization prior to dissociation.

Modulating the Roaming Dynamics for the NO Release in ortho-Nitrobenzenes

“…5,14 In contrast, many theoretical reports suggest both the fast and slow translational energy components are due to a second intersystem crossing from T 1 to S 0 surface, 8,9 which is in accord with the time-resolved photoelectron measurements. 12 The dynamophoric nature of the nitro group can be influenced by appropriate substitution in the ortho position, which can affect the branching ratio of dissociation pathways that result in the formation of NO radicals. 15−17 In the case of o-nitrotoluene, experimental and theoretical investigations affirm the presence of additional dissociation channels leading to OH elimination as a result of intramolecular proton transfer from the benzylic carbon to the oxygen atom of the nitro group in the ortho position.…”

“…Moreover, these calculations do not consider direct dissociation from the triplet state, and additionally, the assignment of translational energy components based on a single classical trajectory may not be statistically valid . Theoretical and time-resolved experimental studies show that the motions localized on the nitro group are primarily responsible for the nonadiabatic dynamics in the excited electronic states of nitrobenzene and reveal that irrespective of the initial excitation energy, an ultrafast internal conversion followed by an intersystem crossing results in populating the T 1 state, which is invariant to the nature of substitution on the nitrobenzene. − Lin and co-workers, based on the excitation energy dependence of the slow-to-fast ( s / f ) branching ratio, have surmised that the fast components appear due to direct dissociation from the T 1 state, while the slow component from the S 0 state, in concurrence with other experimental investigations. , In contrast, many theoretical reports suggest both the fast and slow translational energy components are due to a second intersystem crossing from T 1 to S 0 surface, , which is in accord with the time-resolved photoelectron measurements …”

“…In this work, we examine, holistically, the trends in the branching ratio of slow-to-fast ( s / f ) translational energy components for the NO release obtained by a velocity map imaging technique following 266 nm photolysis of nitrobenzene and a series of ortho -substituted nitrobenzenes, viz., o -nitrotoluene, o -nitroanisole, o -nitroaniline, and o -nitrophenol. It has been reported that for nitrobenzene, o -nitrotoluene, o -nitroaniline, and o -nitrophenol time-resolved experiments following 266 nm excitation reveal ultrafast decay of the initially excited state to the T 1 state; however, the corresponding NO release experiments following 266 nm excitation for various ortho -nitrobenzenes, with the exception of nitrobenzene, are conspicuously missing. The experimental results are interpreted using DFT [B3LYP/6-311++g (d,p)] calculations, wherein it is proposed that the dynamics on the T 1 surface along the minimum energy path dictates the s / f branching ratio.…”

“…While the slow component has always been associated with the NO release in the S 0 state, there is a debate over the appearance of the fast component, which could arise either from the direct dissociation from the triplet state ,, or via the T 1 /S 0 crossing points. − , However, it must be pointed out that, with the exception of Lin and co-workers, the dynamics for the NO release on the T 1 surface is not reported . The ultrafast dynamics of the NO elimination reaction in nitrobenzene and substituted nitrobenzenes originates from the S 4 /S 3 excited state(s) following 266 nm excitation, which undergoes ultrafast relaxation to the T 1 excited state in the time scale of a couple of hundred femtoseconds, which is followed by relatively slower dynamics (about 100 ps time scale) on the T 1 surface . While there is no consensus on the exact mechanism of the fast component, the dynamics on the T 1 surface will influence the outcome which could be from either direct dissociation on the T 1 surface or the T 1 to S 0 intersystem crossing.…”

“…4 The ultrafast dynamics of the NO elimination reaction in nitrobenzene and substituted nitrobenzenes originates from the S 4 /S 3 excited state(s) following 266 nm excitation, which undergoes ultrafast relaxation to the T 1 excited state in the time scale of a couple of hundred femtoseconds, which is followed by relatively slower dynamics (about 100 ps time scale) on the T 1 surface. 12 While there is no consensus on the exact mechanism of the fast component, the dynamics on the T 1 surface will influence the outcome which could be from either direct dissociation on the T 1 surface or the T 1 to S 0 intersystem crossing. Additionally, the slow component arises exclusively due to a different T 1 to S 0 intersystem crossing.…”