Electronic States of a Novel Smaragdyrin Isomer: Polarized Spectroscopy and Theoretical Studies

Abstract

Spectral and theoretical techniques were applied to investigate the electronic structure and spectra of two recently synthesized pentapyrrolic macrocycles, isomers of smaragdyrin: 16,20-dibutyl-2,3,6,7,10,11,15,21-octamethyl-[22]pentaphyrin-(1.1.1.0.0) (1) and 16,20-dibutyl-2,3,6,7,10,11,15,21-octamethyl-5-oxa-[22]pentaphyrin-(1.1.1.0.0) (2). Combined use of linear dichroism, magnetic circular dichroism, fluorescence anisotropy and INDO/S calculations resulted in the location and assignments of a number of electronic transitions (eleven for 1 and seven for 2). Even though the spectral pattern differs somewhat from that characteristic of most porphyrins, the results show that the four lowest excited electronic singlet states of both compounds are very well described by a four-orbital model, widely used for the interpretation of spectra in this type of compounds. Fluorescence and transient absorption/bleaching measurements enabled the determination of the rate constants of the radiative and nonradiative S1 depopulation processes. These photophysical properties are consistent with a rigid, nearly planar excited state geometry that is essentially unchanged with respect to the ground-state structure.