Issue 10, 2007

Ultrafast relaxation and coherent oscillations in aminobenzonitriles in the gas phase probed by intense-field ionization

Abstract

4-Aminobenzonitrile derivatives have two excited states of similar energy: besides the benzene-like Lb state (also termed “locally excited” or LE state) one with charge-transfer (CT) character that is slightly higher in the isolated molecules. The CT state can be lowered by solvents of suitable polarity, so that dual fluorescence can be observed in them. It is controversial along which coordinate this state is displaced, although the amino-group twist is a wide-spread assumption. We investigated a number of such compounds by transient ionization in the gas phase, initially exciting the higher-lying La state (S2). Here we briefly review the previous results on 4-(dimethylamino)benzonitrile (the prototype of this class of molecules), 4-piperidino-, pyrrolidino- and pyrrolyl-benzonitrile and compare them with new results on 4-aminobenzonitrile and on the bridged derivative N-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6). Although in the latter two molecules the CT state has never been detected before, we find the same relaxation path for all compounds: From S2, the wave packet passes through a conical intersection (CI); from there part of it reaches the S1 (Lb) state directly, whereas another part temporarily populates the CT state (also in NMC6), from where it goes around the CI also to the Lb well. The wave packet directly reaching the Lb well oscillates there along coordinates involving amino-group twist and wagging or molecular arching and a quinoidal distortion. These coordinates must be components of the CI displacement vector. A vibration involving bond-length alternation of the benzene ring is ascribed to a momentum caused by the electronic symmetry change in the CI, i.e., to the nonadiabatic coupling vector. Also the CT state involves amino-group twist, as to conclude from the anisotropy of the corresponding signal. The six-membered aliphatic ring in NMC6 hinders the twist and raises the CT state to an energy that is, however, still below the La state, so that it can be temporarily populated in a barrierless process. Also in aminobenzonitrile the CT state is between La and Lb and is reached from La without a barrier. The twist is rationalized by vibronic interaction with a higher state that is π-antibonding between the amino group and the aromatic ring.

Graphical abstract: Ultrafast relaxation and coherent oscillations in aminobenzonitriles in the gas phase probed by intense-field ionization

Article information

Article type
Invited Article
Submitted
17 Aug 2006
Accepted
30 Nov 2006
First published
22 Dec 2006

Phys. Chem. Chem. Phys., 2007,9, 1151-1169

Ultrafast relaxation and coherent oscillations in aminobenzonitriles in the gas phase probed by intense-field ionization

W. Fuß, W. E. Schmid, K. Kuttan Pushpa, S. A. Trushin and T. Yatsuhashi, Phys. Chem. Chem. Phys., 2007, 9, 1151 DOI: 10.1039/B611877A

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