Micro-Solvated DMABN: Excited State Quantum Dynamics and Dual Fluorescence Spectra

Abstract

In this work, we report a complete analysis by theoretical and spectroscopic methods of the short-time behaviour of 4-(dimethylamino)benzonitrile (DMABN) in the gas phase as well as in cyclohexane, tetrahydrofuran, acetonitrile, and water solution, after excitation to the La state. The spectroscopic properties of DMABN were investigated experimentally using UV absorption and fluorescence emission spectroscopy. The computational study was developed at different electronic structure levels and using the Polarisable Continuum Model (PCM) and explicit solvent molecules to reproduce the solvent environment. Additionally, excited state quantum dynamics simulations in the diabatic picture using the direct dynamics variational multiconfigurational Gaussian (DD-vMCG) method were performed, the largest quantum dynamics “on-the-fly” simulations performed with this method until now. The comparison with fully converged multilayer multiconfigurational time-dependent Hartree (ML-MCTDH) dynamics on parametrised linear vibronic coupling (LVC) potentials show very similar population decays and evolution of the nuclear wavepacket. The ring C=C stretching and three methyl tilting modes are identified as the responsible motions for the internal conversion from the La to the Lb states. No major differences are observed in the ultrafast initial decay in different solvents, but we show that this effect depends strongly on the level of electronic structure used.

Benchmarking non-adiabatic quantum dynamics using the molecular Tully models

“…The structures thus cover the configurations needed to fit with the known behaviour of these molecules in the first 100 fs after photo-activation as the torsional motion is strongly activated in ethene 24 and fulvene, 31 while DMABN remains planar. 33 The similarity between the LVC and direct dynamics results for IC2 and IC3 indicate that the potential surfaces are well characterised by the single conical intersection seam of the LVC model. The main addition from the direct dynamics is allowing the molecules to move away from the intersection faster, leading to reduced population transfer due to new pathways being available in the active states.…”

“…The structures thus cover the configurations needed to fit with the known behaviour of these molecules in the first 100 fs after photo-activation as the torsional motion is strongly activated in ethene 24 and fulvene, 31 while DMABN remains planar. 33…”

“…To take the focus away from a comparison to the Tully models, we shall name them the Ibele–Curchod (IC) models and refer to the ethene system as IC1, DMABN as IC2 and fulvene as IC3. The potential surfaces and short-time dynamics of ethene, 24–28 fulvene 29–31 and DMABN 32,33 have all been studied at various levels of theory. The IC models thus need to be defined by a particular level of electronic structure theory and initial starting geometry, which will be detailed below.…”

Ab Initio Investigation of Intramolecular Charge Transfer States in DMABN by Calculation of Excited State X-ray Absorption Spectra

“…Since the discovery of dual fluorescence in 4-(dimethyl­amino)­benzonitrile (DMABN, Figure ), the valence excited electronic states of DMABN and their evolution following photoexcitation have been studied extensively to gain a mechanistic understanding of this phenomenon. − Initially the state reversal model was proposed to explain the dual fluorescence . In the context of this model, it was suggested that initial excitation to the lowest absorption band leads to population of S 1 and S 2 states.…”

Ab Initio Investigation of Intramolecular Charge Transfer in Dmabn by Calculation of Transient X-Ray Absorption Features

“…1), 1 the valence excited electronic states of DMABN and their evolution following photoexcitation have been studied extensively to gain mechanistic understanding of this phenomenon. [2][3][4][5][6][7][8][9] Initially the state reversal model was proposed to explain the dual fluorescence. 10 In the context of this model, it was suggested that initial excitation to the lowest absorption band leads to population of S 1 and S 2 states.…”