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

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 ¹L_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 (¹L_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.