290309-94-3Relevant academic research and scientific papers
Resonance Raman characterization of excitonically coupled meso,meso-linked porphyrin arrays
Bhuiyan, Anwar A.,Seth, Jyoti,Yoshida, Naoya,Osuka, Atsuhiro,Bocian, David F.
, p. 10757 - 10764 (2007/10/03)
Resonance Raman (RR) spectra are reported for dimeric, trimeric, and tetrameric porphyrin arrays in which the porphyrins are directly linked at the meso position. The RR spectra of two monomeric building blocks of the arrays are also examined. The close proximity and orthogonal orientation of adjacent porphyrins in the arrays result in exceptionally strong excitonic interactions along the axis defined by the meso,meso-linkage-(s) and negligible interactions along the orthogonal axis. The coupling scheme breaks the degeneracy of the B excited state and leads to two B(0,0) absorption features. One feature, designated Bx(0,0), is the supermolecule absorption that is strongly red-shifted, due to exciton coupling. The other feature, designated By(0,0), is the superposition of absorptions of the orthogonal, individual porphyrins. This latter absorption occurs at approximately the same energy as the B(0,0) band of a monomeric porphyrin, due to the absence of excitonic interactions. The exciton coupling in the Q state is much weaker than that in the B state, due to the smaller oscillator strength of the former transition. The B-state excitation RR spectra of the meso,meso-linked arrays exhibit a complex and unusual scattering pattern. The most striking features are (1) the appearance of only polarized and anomalously polarized modes in the RR spectrum, (2) the intensity enhancement of anomalously polarized vibrations with B-state excitation, and (3) the large differential enhancement of symmetric versus nontotally symmetric vibrations with excitation. across the B-state absorptions. All of these scattering characteristics are due to the effects of symmetry lowering. The asymmetric meso substitution pattern inherent to the meso,meso-linked arrays contributes to symmetry lowering in both the ground and excited electronic states. The strong uniaxial excitonic interactions make an additional contribution to symmetry lowering in the excited state(s). This latter characteristic promotes novel Franck-Condon and vibronic scattering mechanisms in the B state(s) of the arrays. Collectively, the studies of the meso,meso-linked arrays provide insight into the type of RR scattering that might be anticipated for other types of systems that exhibit strong excitonic interactions among the constituents.
