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A Quantum Dynamics Study of Photoexcited Aniline Fang Wang, Simon Neville, Rong-Shun Wang, and Graham A. Worth J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp401116c • Publication Date (Web): 30 May 2013 Downloaded from http://pubs.acs.org on May 31, 2013
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The Journal of Physical Chemistry
A Quantum Dynamics Study of Photoexcited Aniline F. Wang,† S. P. Neville,‡ R. Wang,∗,† and G. A. Worth∗,‡ Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Renmin Road 5268, Changchun, Jilin 130024 P.R. China, and School of Chemistry, University of Birmingham, Edgbaston, B15 2TT, UK E-mail:
[email protected];
[email protected] ∗ To
whom correspondence should be addressed Normal University ‡ University of Birmingham † Northeast
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Abstract A model Hamiltonian based on the quadratic vibronic coupling model is developed to describe the photoinduced dynamics of aniline excited to the manifold of states comprising it’s first six singlet electronic states. The model Hamiltonian is parameterised by fitting to the results of extensive EOM-CCSD calculations, and it’s validity tested through the calculation of the first two bands in the electronic absorption spectrum of aniline. It is found that two previously neglected 3p Rydberg states play an important role in the dynamics of aniline following excitation into the first two 1 ππ ∗ states. Assignments of the vibrational structure seen in the experimental spectrum is made and the role played by the Herzberg-Teller effect in excitation to the first 1 ππ ∗ state is analysed.
Keywords: Aniline absorption spectrum; non-adiabatic phenomena; vibronic coupling model; ultrafast photochemistry; quantum dynamics; MCTDH
Introduction Much attention has been paid recently to the photochemistry and photophysics of small heteroaromatic molecules, such as phenols, indoles and pyrroles. Such interest has been invoked partly due to their being analogues of the chromophores of biologically important molecules, including the DNA bases and aromatic amino acids. This class of molecules is remarkable for their low fluorescence quantum yields, a property that has it’s origins in efficient non-radiative relaxation pathways that connect the ground and electronically excited states. Indeed, this property is believed to endow photostability to the fundamental building blocks of life. 1 Much of the understanding of the mechanisms by which these heteroaromatic species are rendered photoresistive has come from the theoretical studies performed by Sobolewski and coworkers. 2 In this pioneering work, the presence of low-lying singlet 3s/πσ ∗ states that are dissociative with respect to a heteroatom-hydride bond was suggested as providing highly efficient pathways to conical intersections with the ground state that could constitute a universal mechanism for ultrafast electronic relaxation. Consequent experimental and theoretical studies have 2 ACS Paragon Plus Environment
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served to both confirm the important role played by 3s/πσ ∗ states in the excited state dynamics of heteroaromatic species and the wide-range of systems in which this mechanism is prominent. 3,4 The focus of this work is the photoinduced dynamics of the prototypical aromatic amine aniline. Through a number of experimental 5,6 and theoretical 7 studies, the electronic spectrum of aniline in the region 4.0 to 6.0 eV is known to be dominated by two bands centred at 4.35 eV and 5.39 eV, corresponding to excitation to the first two 1 ππ ∗ states. Further, a single low-lying 1 3s/πσ ∗ state has been identified to exist between these two 1 ππ ∗ states at an energy of ∼4.6 eV by the (2 + 2) resonance enhanced multiphoton ionisation measurements of Ebata et al. 8 Recent experimental studies of the excited state dynamics of aniline have served to reveal the rich and complex photochemistry of this molecule. Through the use of energy-resolved H (Rydberg) atom photofragment translational spectroscopy measurements, Ashfold et al. 9 reasoned that for excitation energies in excess of 4.60 eV, N-H dissociation via the S2 (3s/πσ ∗ ) state occurs, with the dissociation proceeding diabatically to produce ground state anilino radicals. The timeresolved ion yield studies of Montero and co-workers 10 furnished both short (165 fs) and long (tens of picoseconds to nanoseconds) timescales for relaxation following excitation in the range 4.60 to 5.17 eV. These timescales were attributed, respectively, to dissociation on the S2 (3s/πσ ∗ ) surface, and sequential transfer of population to the S2 (ππ ∗ ) and S0 surfaces. Using femtosecond pump-probe velocity map imaging, Stavros et al. 11 reported a timescale of 155 fs for the formation of both high and low kinetic energy H-atoms following excitation at 5.17 eV. Further, excitation to the S1 (ππ ∗ ) state was found not to result in direct N-H dissociation. Employing time-resolved photoelectron imaging, Fielding and co-workers find that excitation at energies between 5.17 and 5.21 eV results in a time-scale of decay of
