title: Molecular excitons in a copper azadipyrrin complex.
abstract: Exciton coupling is investigated in a copper azadipyrrin complex, Cu(L-aza)2. Exciton coupling in Cu(L-aza)2 assuming a single π-π* state on the L-aza ligand fails to account for the electronic structure of Cu(L-aza)2, which displays two almost equal intensity transitions at 15 600 cm(-1) and 17 690 cm(-1). TD-UB3LYP/6-31G(d) calculations suggest multiple π-π* transitions for the L-aza ligands and simple vector addition of the transition dipoles predicts two nearly orthogonal co-planar excitonic transitions that correctly reproduce the absorption band profile. Empirical modelling of absolute resonance Raman intensities using wavepacket dynamics confirms Cu(L-aza)2 has two equal intensity orthogonal exciton transitions. The phenyl substituents at the α- and γ-positions of the pyrrole rings play a central role in determining the orientation of the transition dipoles. Consequently the π-π* transitions for the L-aza ligands are oriented towards the substituent groups and are not in the plane of the pyrrole rings. Mode displacements in the Franck-Condon (FC) region obtained from the wavepacket model suggest that pyrrole ring and phenyl modes control the exciton FC dynamics. Our results suggest that Cu(L-aza)2 is an ideal model for theoretical, computational and experimental investigations of molecular excitons in molecular systems.
authors.affiliation: Institute of Fundamental Sciences, Massey University, Palmerston North, New Zealand. M.Waterland@massey.ac.nz.
authors.forename: T M
authors.initials: TM
authors.lastname: McLean
authors.forename: S G
authors.initials: SG
authors.lastname: Telfer
authors.forename: A B S
authors.initials: AB
authors.lastname: Elliott
authors.forename: K C
authors.initials: KC
authors.lastname: Gordon
authors.forename: M
authors.initials: M
authors.lastname: Lein
authors.forename: M R
authors.initials: MR
authors.lastname: Waterland