Excitonic Interactions in Covalently-Linked Porphyrin Dimers
Selensky, R., Holten, D., Windsor, M.W., Paine III, J.B., David Dolphin, Gouterman, M., and Thomas, J.C.
The absorption spectra, emission spectra, and fluorescence excitation polarization spectra of a series of free base and diprotonated etioporphyrin-I dimers covalently linked through (CH2)n bridges, n = 0-8, were studied. The absorption spectra of the n = 0 and n = 1 dimers show red shifts, which are largest (approximately 15 mm) for the Soret band of the n = 0 dimer. The Soret bands of the diprotonated dimers n = 0-3 show splitting (apprx.500-1000 cm-1) which can be interpreted by an exciton model assuming a reasonable geometry. The fluorescence spectra and quantum yields are similar to that of the monomer, except for the same red shift seen in absorption; however, the n = 0 diprotonated dimer shows an anomalous vibronic structure. The fluorescence excitation polarization spectra for the n = 0 and the n = 1 dimers differ substantially from the monomer; the dimers n ≥ 3 have fluorescence excitation polarization spectra that suggest that some of the excitation stays localized in one moiety while the rest hops to the dimer partner.