Electronic states of a novel smaragdyrin isomer: Polarized spectroscopy and theoretical studies

Spectral and theoretical techniques were applied to investigate the electro nic structure and spectra of two recently synthesized pentapyrrolic macrocy cles, 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-octam ethy-5-oxa-[22]pentaphyrin-(1.1.1.0.0) (2). Combined use of linear dichrois m, magnetic circular dichroism, fluorescence anisotropy and INDO/S calculat ions resulted in the location and assignments of a number of electronic tra nsitions (eleven for 1 and seven for 2). Even though the spectral pattern d iffers somewhat from that characteristic of most porphyrins, the results sh ow that the four lowest excited electronic singlet States of both compounds are very well described by a four-orbital model, widely used for the inter pretation of spectra in this type of compounds. Fluorescence and transient absorption/bleaching measurements enabled the determination of the rate con stants of the radiative and nonradiative S-1 depopulation processes. These photophysical properties are consistent with a rigid, nearly planar excited state geometry that is essentially unchanged with respect to the groundsta te structure.