Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation

Abstract

We report a joint experimental and theoretical study on the photoinitiated ultrafast dynamics of acrylonitrile (AN) and two methylated analogs: crotonitrile (CrN) and methacrylonitrile (MeAN). Time-resolved photoelectron spectroscopy (TRPES) and ab initio simulation are employed to discern the conical intersection mediated vibronic dynamics leading to relaxation to the ground electronic state. Each molecule is pumped with a femtosecond pulse at 200 nm and the ensuing wavepackets are probed by means of one and two photon ionization at 267 nm. The predominant vibrational motions involved in the de-excitation process, determined by ab initio trajectory simulations, are an initial twisting about the C==C axis followed by pyramidalization at a carbon atom. The decay of the time-resolved photoelectron signal for each molecule is characterized by exponential decay lifetimes for the passage back to the ground state of 60 ± 10, 86 ± 11, and 97 ± 9 fs for AN, CrN, and MeAN, respectively. As these results show, the excited state dynamics are sensitive to the choice of methylation site and the explanation for the observed trend may be found in the trajectory simulations. Specifically, since the pyramidalization motion leading to the conical intersection with the ground state is accompanied by the development of a partial negative charge at the central atom of the pyramidal group, the electron donation of the cyano group ensures that this occurs exclusively at the medial carbon atom. In this way, the donated electron density from the cyano group "directs" the wavepacket to a particular region of the intersection seam. The excellent agreement between the experimental and simulated TRPES spectra, the latter determined by employing trajectory simulations, demonstrates that this mechanistic picture is consistent with the spectroscopic results. [http://dx.

Spectroscopic Signature of Chemical Bond Dissociation Revealed by Calculated Core-Electron Spectra

“…Such combined approaches work best, if the experimental and calculated observables can be directly compared as demonstrated for time-resolved valence photoelectron spectroscopy. [3][4][5][6][7][8] Time-resolved spectroscopy of the valence electrons, however, is not able to produce site-specific information about photochemistry, e.g. the dissociation of a specific chemical bond.…”

“…Some of the most important photochemical processes in nature take place in the excited states of organic molecules on ultrafast time scales. , Thorough understanding of ultrafast photochemistry can be achieved by gas-phase spectroscopy in combination with high-level ab initio excited-state dynamics simulations. Such combined approaches work best if the experimental and calculated observables can be directly compared, as demonstrated for time-resolved valence photoelectron spectroscopy. − Time-resolved spectroscopy of the valence electrons, however, is not able to produce site-specific information about photochemistry, e.g., the dissociation of a specific chemical bond. This can be achieved by soft X-ray spectroscopy of the strongly localized inner electrons.…”

Resolving Competing Conical Intersection Pathways: Time-Resolved X-ray Absorption Spectroscopy of trans-1,3-Butadiene

“…In fact, there may exist competition between multiple conical intersections and/or multiple seams, with a branching ratio determined by both nuclear (inertial) and electronic effects inherent in the prepared wave packet [5,11,[17][18][19][20].…”

On the Nature of Interplay among Major Flexibility Channels in Molecular Rotors

“…Interest in the photochemistry of twisted ICT molecules, here exemplified by 2-[4-(dimethylamino)benzylidene] malononitrile (BMN) (Figure 1), stems in part from their profitable potential applications in molecular optoelectronics, organic light-emitting diodes (OLEDs), and bright sensor materials, NLO material, 3D optical data storage, and advanced biological and molecular imaging [1]. Photoinduced dynamics of simple CT molecules such as p-(N, N-dimethylamino)benzonitrile (DMABN) [2][3][4][5][6][7][8], some α, β-enenitriles [9,10], ethylene and derivatives [11], and many other types of molecules [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] have been extensively studied. Flexible chromogens exhibit twisted intramolecular charge transfer (TICT), which is an electron transfer process that occurs upon photoexcitation in molecules that usually consist of a donor and acceptor part linked by single bond(s) and/or double bonds.…”