New insight into the potential energy landscape and relaxation pathways of photoexcited aniline from CASSCF and XMCQDPT2 electronic structure calculations

Abstract

There have been a number of recent experimental investigations of the nonadiabatic relaxation dynamics of aniline following excitation to the first three singlet excited states, 1(1)ππ*, 1(1)π3s/πσ* and 2(1)ππ*. Motivated by differences between the interpretations of experimental observations, we have employed CASSCF and XMCQDPT2 calculations to explore the potential energy landscape and relaxation pathways of photoexcited aniline. We find a new prefulvene-like MECI connecting the 1(1)ππ* state with the GS in which the carbon-atom carrying the amino group is distorted out-of-plane. This suggests that excitation above the 1(1)π3s/πσ* vertical excitation energy could be followed by electronic relaxation from the 1(1)ππ* state to the ground-electronic state through this MECI. We find a MECI connecting the 1(1)π3s/πσ* and 1(1)ππ* states close to the local minimum on 1(1)π3s/πσ* which suggests that photoexcitation to the 1(1)π3s/πσ* state could be followed by relaxation to the 1(1)ππ* state and to the dissociative component of the 1(1)π3s/πσ* state. We also find evidence for a new pathway from the 2(1)ππ* state to the ground electronic state that is likely to pass through a three-state conical intersection involving the 2(1)ππ*, 1(1)π3s/πσ* and 1(1)ππ* states.

Ultraviolet relaxation dynamics of aniline, N, N-dimethylaniline and 3,5-dimethylaniline at 250 nm

“…2 At ∼240 nm, there is clear onset of strong absorption to a higher-lying state, assigned as having singlet ππ * character. 12,14,18,19 However, we note that recent theoretical work by Worth and coworkers suggests the existence of two 3p Rydberg states sitting energetically slightly below this ππ * state. 20 For the purposes of the discussion outlined here, we will therefore simply use the generalized label S 3+ to denote any feature (or set of features) lying energetically above S 2 (3s/πσ * ).…”

“…47,48 The ultimate fate of the S 1 (ππ * ) state is not resolved in our measurements but several possibilities suggested on the basis of previous work in aniline include fluorescence, 49 intersystem crossing to lower-lying triplet states, 50 and internal conversion back to the S 0 ground state, where the role played by prefulvene-like structures has attracted some recent attention. 12,14 Theoretical work by Fielding and co-workers predicts aniline has four CIs connecting the S 1 (ππ * ) and S 0 states, the lowest lying of which has a transition state located at ≥4.96 eV above the S 0 minimum energy. 14 The relatively long S 1 (ππ * ) state lifetime seen in our data (180-230 ps) suggests, however, that this transition state is not directly accessible following excitation at 250 nm (also 4.96 eV) in any of the three systems under investigation.…”

“…This assertion also appears to be supported by recent theoretical calculations. 14 Our own previous TRPEI studies of aniline were restricted by the fact that we did not, at the time, have access to full UV wavelength tunability for both the pump and probe laser beams required for the experiment. It was therefore not possible to undertake "clean" experiments (i.e., measurements free from probe-pump dynamics evolving to negative time delays) using 250 nm excitation in conjunction with single-photon probe ionization (which is desirable as multi-photon schemes can convolute TRPEI data significantly, making data analysis considerably more challenging).…”

New insights into the dissociation dynamics of methylated anilines

“…17 Theoretical explorations have found several conical intersections (CIs) between the 2 1 pp*, 1 1 ps*, 1 1 pp*, and S 0 states, 17,21 including an apparent 2 1 pp*/1 1 ps*/1 1 pp* threestate CI which lies along the minimum energy 2 1 pp* -S 0 relaxation pathway. 21 Recent time-resolved photoelectron spectroscopy (TR-PES) by Fielding and co-workers 14 has yielded considerable insight into the 2 1 pp* decay mechanisms, concluding that photoexcitation at 238 nm populates the 2 1 pp* which decays almost exclusively to S 0 with some IC to both the 1 1 ps* and 1 pp* states via the aforementioned multi-state CI. 21 The latter process is clearly revealed through a delayed rise in photoelectron signal due to the sequential population of the 1 1 pp* state.…”

“…21 Recent time-resolved photoelectron spectroscopy (TR-PES) by Fielding and co-workers 14 has yielded considerable insight into the 2 1 pp* decay mechanisms, concluding that photoexcitation at 238 nm populates the 2 1 pp* which decays almost exclusively to S 0 with some IC to both the 1 1 ps* and 1 pp* states via the aforementioned multi-state CI. 21 The latter process is clearly revealed through a delayed rise in photoelectron signal due to the sequential population of the 1 1 pp* state. 11,12 To help elucidate and explore the relaxation mechanisms of photoexcited aniline, recent TR-PES work by Townsend and co-workers 19,22 has explored the excited state dynamics of N,Ndimethylaniline (N,N-DMA) and 3,5-dimethylaniline (3, at 250 and 240 nm.…”