The Electronic Origin of the Dual Fluorescence in Donor-Acceptor Substituted Benzene Derivatives Semyon Cogan, Shmuel Zilberg,* and Yehuda Haas* Contribution from the Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem, Israel Received July 21, 2005; E-mail: yehuda@chem.ch.huji.ac.il Abstract: The origin of the dual fluorescence of DMABN (dimethylaminobenzonitrile) and other benzene derivatives is explained by a charge transfer model based on the properties of the benzene anion radical. It is shown that, in general, three low-lying electronically excited states are expected for these molecules, two of which are of charge transfer (CT) character, whereas the third is a locally excited (LE) state. Dual fluorescence may arise from any two of these states, as each has a different geometry at which it attains a minimum. The Jahn-Teller induced distortion of the benzene anion radical ground state helps to classify the CT states as having quinoid (Q) and antiquinoid (AQ) forms. The intramolecular charge transfer (ICT) state is formed by the transfer of an electron from a covalently linked donor group to an anti-bonding orbital of the π-electron system of benzene. The change in charge distribution of the molecule in the CT states leads to the most significant geometry change undergone by the molecule which is the distortion of the benzene ring to a Q or AQ structure. As the dipole moment is larger in the perpendicular geometry than in the planar one, this geometry is preferred in polar solvents, supporting the twisted intramolecular charge transfer (TICT) model. However, in many cases the planar conformation of CT excited states is lower in energy than that of the LE state, and dual fluorescence can be observed also from planar structures. I. Introduction The origin of the dual fluorescence (DF) observed first in DMABN (the structures and atom numbering convention for the molecules discussed in this paper are shown in Scheme 1) and later in a large number of aniline derivatives continues to puzzle workers in the field. 1 Several models have been proposed, initially based on chemical intuition and more recently supported by quantum chemical calculations. Apart from early models that have since been discounted, the current accepted consensus is that the red-shifted band observed in polar solvents is due to an intramolecular charge transfer state. Time-resolved experi- ments 2 indicate that fluorescence arises almost instantaneously from a locally excited state (usually referred to as the B state) which is correlated with the 1 1 B 2u state of benzene (L b in Platt’s notation 3 ). Local excitation means in this context the transfer of an electron from an occupied π MO to an unoccupied one, both localized primarily in the benzene ring. Red-shifted emission, assigned to a charge transfer state, is also observed, usually after some time delay (in the ps range). In this state an electron was transferred from the donor part of the system (the amino or pyrrolo group) to the acceptor part (the phenyl or cyanophenyl group). 1 The nature of the CT state, in particular its geometry, has been a matter of dispute. It was suggested by some authors 1,4 that the state is correlated with the L a state, which is the second locally excited state in the Franck-Condon region. Two leading models appear to prevail at this time. In the twisted intramolecular charge transfer (TICT) model 5 the emitting species is assumed to have a twisted geometry, with the torsion angle φ between the benzene ring and the amino (or pyrrolo) group close to 90°. The planar intramolecular charge transfer (PICT) model 6 proposes a planar structure (φ ) 0°) for the emitting state. In this paper the term torsion angle will refer to φ unless otherwise stated. Experimental evidence advanced for both models has been largely circumstantial, as direct measurement of the structure of the emitting species turns out to be experimentally difficult. (1) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. ReV. 2003, 103, 3899. (2) Leinhos, U.; Ku ¨ hnle, W.; Zachariasse, K. J. Phys. Chem. A 1991, 95, 2013. (3) Platt, J. R. J. Chem. Phys. 1949, 17, 484. (4) Rettig, W.; Wermuth, G.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 692. (5) (a) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, Z. R. Chem. Phys. Lett. 1973, 19, 315. (b) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. NouV. J. Chim. 1979, 3, 443. (6) Schuddeboom, W.; Jonker, S. A.; Warman, J. M.; Leinhos, U.; Ku ¨hnle, W.; Zachariasse, K. A. J. Phys. Chem. 1992, 96, 10809. Scheme 1. Structures and Atom Numbering Convention for Dimethylaniline (DMA), Dimethylaminobenzonitrile (DMABN), N-Phenylpyrrole (PP), and para-Phenylbenzonitrile (PBN) Published on Web 02/18/2006 10.1021/ja0548945 CCC: $33.50 © 2006 American Chemical Society J. AM. CHEM. SOC. 2006, 128, 3335-3345 9 3335