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.

Absorption and emission spectra of europium perchlorate and europium nitrate have been measured in protonated and deuterated solvents (water, dimethyl sulfoxide, methanol, and acetonitrile). The line shape of these spectra is shown to depend strongly on the solvent and anion. Ion pairing takes place, depending on the combination of solvent and anion. Quantum yields and lifetimes of fluorescence from the ;Do level are reported. An isotopic enhancement factor of 40 is found in water, while in other solvents it is much smaller.The rate constants for radiative and radiationless processes are calculated, The radiationless constants range from about lo4 sec-' for light water to about 10 for deuterated dimethyl sulfoxide in which the fluorescent quantum yield on direct excitation to 5 D~ approaches unity. The suitability of this system for use as a liquid laser is indicated. It is shown that the anions also quench fluorescence. Radiationless rate constants for CNS-and NOa-anions are calculated to be about loa sec-l. Fluorescence from the 5D1 level is weak in all solvents. Its quantum yield is about to 10-6. The isotopic effect for this fluorescence is only 3. The probability for decay from the 6D1 level directly to ground state is about the same for all systems, ~0 . 5 , suggesting a common mechanism for this process.

The photochemical [small alpha]-cleavage of acetone is analyzed in view of recent results obtained for the isolated molecule in supersonic jets. The fluorescence decay time of the isolated molecule spans a range of more than six orders of magnitude, from approximately 10(-6) s near the origin of the S(0)-S(1) transition to less than 10(-12) s at about 20 kcal x mol(-1) excess energy. In contrast, the decay time of the excited singlet (S(1), (1)n pi) in the bulk is around 10(-9) s and independent of excitation wavelength. Initial excitation to the (1)npi state is followed by internal conversion (IC) to the ground state and intersystem crossing to the lowest-lying triplet. The rate constants of these processes are comparable to the radiative decay rate constant for excess energy up to 7 kcal x mol(-1) above the origin of the S(0)-S(1) transition. Beyond that energy, the triplet state becomes dissociative and the ISC rate becomes much larger than other processes depleting S(1). The primary reaction on the triplet surface is a barrier-controlled alpha-cleavage to form the triplet radical pair CH(3)(*)+ CH(3)CO(*). Direct reaction from the S(1) is negligible, and the non-quenchable reaction (by triplet quenchers) observed in the bulk gas phase is due to hot triplet molecules that dissociate on the timescale of 10(-12) s or less. The singlet-state decay time measured in the bulk (approximately 1-2 ns) arises from collision-induced processes that populate low-lying levels of S(1). The analysis is aided by detailed state-resolved studies on related molecules (in particular formaldehyde and acetaldehyde) whose photophysics and photochemistry parallel those of acetone.

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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.