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.

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 pi-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.

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Aromatic and antiaromatic compounds are resonance hybrids of two cyclic covalent Kekulé structures. In both, two combinations can be formed, an in-phase and an out-of-phase one. In aromatic compounds having an odd number of conjugated double bonds, the in-phase combination is the ground state and the out-ofphase one is an excited state. In antiaromatic compounds, having an even number of conjugated electron pairs, the situation is reversed; the ground state is formed by the out-of-phase combination. This causes the ground state of these molecules to be a non-totally symmetric one, which in turn means that it has a biradical character. Moreover, the out-of-phase combination is necessarily unstable, being a transition state between the two bond-alternating Kekulé structures. By comparison to noncyclic biradicals such as perpendicular olefins, the antiaromatic cyclic structures are strongly stabilized, reducing the activation barrier from around 50-60 kcal/mol to around 3-5 kcal/mol. Therefore, the bond-alternating structures are easily interconverted at ambient temperatures and in the process acquire biradical character, making them highly reactive and difficult to synthesize. The in-phase combination of the two Kekulé structures is a strongly stabilized totally symmetric excited state which has a similar geometry to that of the ground transition state.

ABSTRACT:It is shown that the antiaromatic character of certain conjugated cyclic hydrocarbons is due to the presence of an even number of distinct electron pairs in the Ž . system such as, but not necessarily electrons . In these systems, the ground state is Ž . constructed from an out-of-phase combination of two valence bond VB structures, and its equilibrium geometry is necessarily distorted along the coordinate that interchanges these structures. If a new symmetry element appears during the transition between the two structures, the ground electronic state at the symmetric point transforms as one of the nontotally symmetric irreducible representations of the point group. The conjugate excited state, formed from the in-phase combination of the same two structures, transforms as the totally symmetric representation of the group and is strongly bound. Its structure is similar to that of the ground state at the symmetric point, and the energy separation between the two states is small compared to that of conjugated cyclic hydrocarbons having an odd number of distinct electron pairs. Motion along the ''Kekule-type'' vibrational mode on the excited-state potential surface is very similar tó motion along the reaction coordinate connecting the two distorted structures on the ground-state surface. It is characterized by a significantly higher vibrational frequency compared to frequencies of similar modes in ground-state molecules. These qualitative predictions are supported by quantum chemical calculations on cyclobutadiene, cyclooctatetraene, and pentalene.

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A method to locate conical intersections between the ground-state potential surface and the first electronically excited states of polyatomic molecules is described. It is an extension of the Longuet-Higgins sign-change theorem and uses reaction coordinates of elementary reactions as the starting point of the analysis. It is shown that the complete molecular landscape [1] can be partitioned into 2-D domains, each bordered by a Longuet-Higgins loop formed from reaction coordinates of elementary reactions. A domain may contain a conical intersection and if it does, it contains only one (the uniqueness theorem), whose energy is higher than the neighboring minima or transition states. The method can be helped by symmetry, but applies also to systems having no symmetry elements. It is demonstrated for some simple cases. The presence of a conical intersection is manifested by the nature of ground-state thermal reactions, as shown for instance by the fact that the transition state in the ring opening of the cyclopropyl radical is nonsymmetric.

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