Chcmid Physics 22 (1977) 167-173 0 North-Holland Publishing Company THEEXCITEDSINGLETSTATE ABSORFTIObiSPECTRUMOF 1,4-DIPHENYLNAPHTHALENE Eric L. RUSSELL, Allen J. TWAROWSKI, David S. KLIGER and Eugene SWITKES Department of Chemistry, Diuisiorr of Natural Sciences. Universify ofCa[iforrria, Santa CTILZ, California 95064. ilSA Reccivcd 12 January 1977 The escitcd sindet sk~tespectrum of 1,4diphenylnaphthalene (DPN) has been mcasurcd and analyzed in term theoretical calculations. The results show that there is si@icant geometrical zyxwvutsrqponmlkjihgfedcbaZYXWVUT rearmn~cment in DPN upon excitation and that consid~r&n of this clrangc in gcomctry is important in cillcubting the cscited stale spectrum. A new method is also suggested for selecting the configurations to be used in the excited st;Ltc spectral calculation. Caku~ations using an expanded set of configurations (S+S*) provide a more consistent analysis of the esperimentsl spectrum than those using only singly excited configurations (s). 1. Introduction Since 1967, when Novak and Windsor reported the absorption spectrum of the first excited singlet state, S,, in coronene [I], many excited singlet state spectra have been measured. However, in only a few cases have the excited state lransitions been careful- iy characterized and compared to thcoreticai cslcula- tions [2-51.h each of these studies the nlolecules under investigation were relatively rigid and the ground state geometries were used in calculating the excited state spectra. This straigbtfonvard procedure is, unfortunately, not applicable to the characteriza- tion of excited state spectra in a number of systems of chemical interest. Once excited from the ground state to an initial excited state many molecules will rapidly relax to a new equilibrium geometry, or even to a neighboring excited state, prior to absorbing a second photon. Thus, experimental ground state ge- ometries may be inappropriate for describing excited state spectra. Such a change in geometry has been shown to be very important in the interpretation of the excited state spectrum of l,l’-binaphthyyl [6]. We report here another example in which a change in excited state geometry must be considered in as- signing the ~3,~ f S1 absorption spectrum, 1,4di- phenylnaphtbalene (DPN). We have been investigating the Process of exciplex formation and decay in a series of phenylated poly- scenes. In addition to monitoring these events through fluorescence decay, we have followed the exciplex dynamics by monitoring the excited singlet and trip- let absorption spectra of the esciplex and the parent molecule. In another paper we will report the details ofexciplex dynamics in the DPN-triethylamine sys- tern [7]. In this paper we concentratt 011 techniques for analyzing the S,, + SI absorption in DPN itself. As is the case in many bicyclic molecules [6], the large Stokes shift observed between the ground state absorption and fluorescence spectra in DPN suggests a significant geometrical rearrangement in its excited state. in our calculations on DPN we allow for both a bond length variation and a change, upon excita- tion, in the tilt of the phenyl rings relative to the naphthalene pIane. Semi-empirical pi-electron meth- ods are usually calibrated to reproduce ground state spectra. These techniques may require modification in order to describe transitions to highly excited states not directly accessible from the ground sMe. In the excited state calculations reported here, we propose a way of augmenting the set of singly excited con- figurations usually used for ground state spectral caC culations. This augmented set of configurations should provide a mote reaIistic description of excited state transitions when the initial state invalved in the transition is dominated by a single configuration.