Volume 175,number 5 CHEMICAL PHYSICSLETTERS 21 December 1990 Two-photon spectroscopy of antiaromatic molecules: the case of biphenylene * V.T. Bich ‘, R. Bini, P.R. Salvi Laboratoria di Spettroscopia Mo[ecolare, Dipartimento di Chimica, Universitri di Firenze, via G. Capponi 9, 50121 Florence, Italy and Laboratorio Europe0 di Spettroscopie Non Lineari, Dipartimento di Fisica, L’niversitddi Firenze, Largo E. Fermi 2, 50135 Florence, Ital> and G. Marconi lstifuto FRAE de1 CNR, via de’ Castagnoli I,40126 Boiogna, Italy Received 17 September 1990 The two-photon spectrum and polarization ratio of biphenylene have been measured at room temperature in the spectral range 225-370 nm, combining fluorescence-excitation and thermal-lensing techniques. The first excited 4 state, 24, has been ob- served around 265 nm and the second excited Br,, 2B,,, has its onset at z 225 nm. Similarities and differences with altemant aromatics are pointed out. MO calculationsof two-photon intensities support the interpretation of the experimental data. 1. Introduction induced by bZU vibrational modes [ 7,8]. Biphenylene belongs to the class of cyclic planar conjugated hydrocarbons with 4n R electrons in their perimeter (antiaromatic systems) [ 11. In contrast with aromatics whose 4n+2 K electrons are delo- calized over the molecular ring, biphenylene has a more localized n arrangement [ 2,3 1. Unlike most ar- omatics, its lowest excited state is one-photon in- active, being of B,, symmetry [ 41 (assumed struc- ture DZh with x, y long and medium in-plane molecular axes). Also, the photophysical properties are different from aromatic behaviour. Biphenylene is a weakly fluorescent molecule with strongly Stokes- shifted S, +S, emission [ 5-71. The fluorescence spectrum has been related to structural changes in S, following electronic excitation and to Sr-Sz coupling On the other hand, conjugated cyclic hydrocar- bons may be classified as alternant or non-alternant according to the topology of the skeletal carbon at- oms [9-l I 1. From this point of view, both biphen- ylene and polyacenes are altemant. Therefore, it may be envisaged that the same pseudoparity ( + ; - ) se- lection rules valid for aromatic hydrocarbons [ 121 are satisfied in the biphenylene case. In recent years, much work has been done on this point for aromatic systems [ 13 1. In particular, the pseudoparity selec- tion rules were shown to be a reliable criterion for discussing two-photon absorption spectra [ 14,15 1. Similar studies have not been performed, to our knowledge, on antiaromatics. Here, we wish to report on the two-photon (TP) excitation spectrum of biphenylene in solution at room temperature in the 225-370 nm spectral range. * This work was supported by the Italian Minister0 Universiti e Ricerca Scientifica e Tecnologica (MURST) and Consiglio Thermal lensing and fluorescence-excitation were the Nazionaledelle Ricerche. probing techniques. Due to the presence of the in- ’ On leave from the Institute of Physics of Vietnam, Hanoi, version center, states of g total symmetry are ob- Vietnam. served. The first excited A, state, 2A,, occurs around 0009-2614/90/$ 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland ) 413