* Corresponding author. Fax: 40-32-213330. E-mail address: vlah@uaic.ro (A. Vlahovici) Journal of Luminescence 82 (1999) 155}162 Photophysics of some indolizines, derivatives from bipyridyl, in various media Alexandru Vlahovici*, Ioan Drut 7 a y , Mioara Andrei, Mircea Cotlet, Rodica Dinica y  Faculty of Physics, **Al. I. Cuza++ University, Bd. Copou No. 11, 6600, Iasi, Romania Faculty of Chemistry, **Al. I. Cuza++ University, Bd. Copou No. 11, 6600, Iasi, Romania Received 12 October 1998; received in revised form 10 March 1999; accepted 10 March 1999 Abstract A series of photoluminescent properties of some new indolizinic compounds, recently synthesized, derivatives from bipyridyl, are analyzed. Signi"cant di!erences are observed between monoindolizinic compounds and bisindolizinic ones regarding the absorption and #uorescence in the UV-VIS spectral range. Extended conjugation in bisindolizinic compounds causes a strong increase in the molar absorptivity and #uorescence quantum yield and a shift of the #uorescence spectrum towards violet. Unlike monoindolizinic compounds, the bisindolizinic ones realize both an almost planar conformation and a rigidization of the molecule, in the sense that it changes less its dimension after excitation. It is estimated that the transitions of the analyzed molecules are of the type PH and that the mixing of the S  and S  states is achieved to a small extent.  1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Molecular #uorescence; Absorption spectra; Indolizines; Bipyridyl; Fluorescence quantum yield; Oscillator strength 1. Introduction A series of organic compounds, #uorescent deriv- atives of pyridine, already have practical applica- tions as markers, scintillators, activators and active media for lasers [1]. However, in some of these compounds a more intense #uorescence appears in water or in acid media upon proton transfer only [2}5]. The bipyridyl and the majority of its organic derivatives either do not exhibit or have a weak #uorescence; the origin of this #uorescence is not clear yet. The #uorescence of some complex combinations of bipyridyls with atoms of rare earth has intensive- ly been studied [6}11], the #uorescence of some `purelya organic compounds being studied to a les- ser extent. Referring to the #uorescence properties of indolizines and their substituted derivatives, there is less information available in literature [12]. The aim of this paper is to study the photo- luminescent properties of some `purelya organic compounds, recently synthesized from bipyridyl, some of them exhibiting a strong #uorescence in sol- vents which do not favor proton transfer. The data reported in this work bring certain contributions to 0022-2313/99/$ } see front matter  1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 0 2 7 - 7