Electronic Structures of 9,10-Anthrylene Dimers and Trimers in Solution:  Formation of Charge Separation States Depending on Alkyl Substituent Groups

Electronic structures of 9,10-anthrylene dimers and trimers analogous to 9,9‘-bianthryl (BA) have been investigated by means of steady-state fluorescence and transient absorption spectroscopy and dipole moment evaluation based on the solvatochromic fluorescence shifts and integrated electrooptical emission measurements (IEOEM) in various solvents. Formation of the charge-separation (CS) state in the excited state was strongly dependent on the substituent group: unsubstituted and n -hexyl-substituted anthrylenes exhibited almost the same behavior compared to BA, whereas the tert -butyl-substituted dimer and trimer relaxed into an excitonic state rather than the CS state even in a polar solvent. The lower possibility of the CS state for the tert -butyl-substituted anthrylenes was mainly ascribed to the smaller solvation energy because of the larger effective Onsager cavity due to bulky tert -butyl groups. The limitation of a simple continuum model using Onsager's reaction field was also discussed in relation to the solvent-induced electronic structure change in the excited state of anthrylenes.