ARTICLES The Nature of the Intramolecular Charge Transfer Excited State in p-Pyrrolocyanobenzene (PBN) and Other Derivatives of Benzene Substituted by Electron Donor and Acceptor Groups Shmuel Zilberg and Yehuda Haas Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem, Israel 91904 ReceiVed: July 9, 2001 The title compound is a representative of a family of molecules known to exhibit dual fluorescence in polar solvents. A theoretical analysis of these compounds, in which benzene is substituted by an electron withdrawing group and an electron donating group para to it is offered. The first excited state is derived from the 1 L b state of benzene and is of a covalent nature. Light emission from this state is due to local excitation of the benzene moiety (LE fluorescence). The second excited state of benzene ( 1 L a ) evolves in the presence of these substituents in two highly polar structures. Depending on the substituents, one or two energy minima may form on this surface, both having a charge transfer character. Of these structures, one has a quinoid nature, whose minimum is in the planar form. The other may be termed an anti-quinoid (AQ) structure: the distance between the two bonded central carbon atoms in the benzene ring is longer than in benzene. This structure has a larger dipole moment than the quinoid one, and a minimum at the perpendicular form. The AQ structure minimum is found also on the excited state potential of benzene substituted by an electron donor only, such as pyrrolobenzene, but not for an acceptor only substituted molecule such as benzonitrile. This minimum on the excited state surface is reported here for the first time; it appears to conform with all the experimentally observed characteristics of TICT molecules. The quinoid form is the one predicted by the PICT model. The dual fluorescence of these compounds is due to LE and the CT emissions; the latter arising from either the Q or the AQ structures. I. Introduction The dual fluorescence of some substituted benzene derivatives (the typical molecule being p-(N,N′-dimethylamino)benzonitrile (DMABN), Scheme 1), was discovered over 40 years ago. 1 These molecules fluoresce in two different bands when solvated in a polar solvent. The “normal” fluorescence exhibits the familiar mirror image to the absorption spectrum and is only slightly red-shifted upon increasing the polarity of the solvent. The second, so-called “anomalous” emission, is strongly red shifted and appears often as a separate band, whose intensity increases at the cost of the “normal” emission as the solvent’s polarity is increased. Intramolecular charge transfer was sug- gested as the mechanism leading to this phenomenon, 2 as shown in Figure 1. Initial excitation is typically into the locally excited (LE) state, that correlates with the 1 1 B 2u (L b , Platt’s nomen- clature 3 ) state of benzene. A radiationless intramolecular process leads to the population of the CT state, which correlates with the 1 1 B 1u (L a ) state of benzene. The LE and CT states are often referred to (by their symmetry in C 2 point group) as the B and A states, respectively. Ever since its discovery, the nature of the charge transfer state has been the target of many theoretical studies. Several models were suggested; Bimolecular models, © Copyright 2002 by the American Chemical Society VOLUME 106, NUMBER 1, JANUARY 10, 2002 10.1021/jp012573j CCC: $22.00 © 2002 American Chemical Society Published on Web 12/12/2001