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J. Phys. Chem. A 2002, 106, 2318-2321
Dipolar Excitonic States of Oligo-Anthrylenes† Jacob J. Piet and John M. Warman* Radiation Chemistry Department, IRI, Delft UniVersity of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands
Martin Baumgarten and Klaus Mu1 llen Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany ReceiVed: July 25, 2001; In Final Form: October 31, 2001
Photoexcitation of dimeric, trimeric, and tetrameric oligomers of anthracene in benzene results in the formation of relaxed S1 excitonic states with dipole moments of 6.9, 8.0, and 9.1 D respectively. Dipole reversal occurs with a relaxation time in the range of 8 ( 2 ps and is driven by a combination of fluctuations in the solvent environment and torsional motion about the central sigma bonds. The excitonic interaction at any given moment in time involves mainly only two neighboring anthracene moieties rather than delocalization over all chromophoric units.
1. Introduction The relaxed S1 state of 9,9′-bianthryl has been the subject of a large body of experimental and theoretical research since the initial finding by Schneider and Lippert that the fluorescence of bianthryl was strongly solvent-dependent.1,2 Despite the underlying D2 symmetry, the fluorescence data clearly indicate an excited-state dipole moment close to 20 D in medium to high polarity solvents which corresponds to close to complete, center-to-center charge separation. The symmetry breaking required has been rationalized for polar solvents in terms of vectorial pinning and stabilization of a charge-separated state by the preferential orientation of the dipolar solvent molecules around the charged centers. The resulting charge-separated state is considered to have a minimum-energy configuration with an orthogonal (90°) relative orientation of the anthryl moieties.3,4 To explain the fluorescence results, the persistence time of the charge separated state must be considerably longer than the dielectric relaxation times of the solvents used, i.e., tens of picoseconds. Subsequent time-resolved microwave conductivity (TRMC)5-7 and electrooptical emission (EOEM)8 measurements have shown that the relaxed S1 state of bianthryl has a net dipolar character even in weakly (quadru)polar solvents such as benzene and paradioxane and in nonpolar saturated hydrocarbons. In this case, it has been suggested that torsional relaxation after initial local excitation results in the formation of a delocalized excitonic state which may have a large polarizability because of the admixture of charge resonance states. Lifting of the degeneracy of the charge resonance states resulting in a net (transient) dipole moment has been attributed to local density and/or structural fluctuations in the solute environment.7 In contrast to the charge separated state formed in polar solvents, the excitonic state of bianthryl is found to have a much lower dipole moment of ca. 8 D and undergoes rapid flip-flop dipole reversal on a time scale of a few picoseconds in saturated hydrocarbon solvents and close to 10 ps in benzene and dioxane.6,7 A study of the fluorescence of the longer oligomers of anthracene in Figure 1 has shown them to display properties †
Part of the special issue “Noboru Mataga Festschrift”. * To whom correspondence should be addressed.
Figure 1. Molecular structures of the compounds studied together with the pseudonyms used in the text.
qualitatively similar to those of bianthryl,9 i.e., only a slight, if any, solvent dependence in weakly polar media but large bathochromic shifts in medium to high polarity solvents. On the basis of the bathochromic shifts, dipole moments of 20, 27, and 42 D have been estimated for the relaxed S1 states of the dimer, trimer, and tetramer, respectively. The tetramer was found to display in general a much greater propensity toward charge separation than the trimer even in low-temperature polar glasses. In the present work, we have carried out TRMC measurements on flash-photolyzed benzene solutions of the anthracene oligomers with the aim of seeing if the trend which is apparent from the optical properties mentioned above is paralleled in the properties of the excitonic states formed in a weakly polar solvent. 2. Experimental Section The molecular structures of the solute molecules investigated are shown in Figure 1. Dilute solutions (ca. 10-4 M) in benzene
10.1021/jp012870o CCC: $22.00 © 2002 American Chemical Society Published on Web 01/19/2002
Dipolar Excitonic States of Oligo-Anthrylenes
J. Phys. Chem. A, Vol. 106, No. 10, 2002 2319
TABLE 1: Lifetimes, τ, of the S1 States of the Compound and the Parameters ∆Im and ∆Re Obtained from Fits to the TRMC Transients Shown in Figure 2 together with the Values of the Dipole Relaxation Times, Θ*, and Dipole Moments, µ*, Derived using Equations 3 and 4 compounda
τ (ns)
∆Im (10-60 C2 m2)
∆Re (10-60 C2 m2)
Θ*b (ps)
µ*c (D)
A AA AAA AAAA
4.3 11.7 9.2 9.8
