Resonance Raman Characterization of Excitonically Coupled meso,meso-Linked Porphyrin Arrays

Abstract

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 B X (0,0), is the supermolecule absorption that is strongly red-shifted, due to exciton coupling. The other feature, designated B y (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. † Part of the special issue "Thomas Spiro Festschrift". Dedicated to Thomas G. Spiro on the occasion of his 65th birthday.

Raman excitation wavelength investigation of single red blood cells in vivo

“…Bhuiyan et al 20 examined the RR scattering characteristics of dimeric, trimeric and tetrameric meso,meso-linked porphyrin arrays and reported novel RR scattering characteristics not previously observed for metalloporphyrins. In particular they noted the intensity enhancement of anomalously polarized bands with B-state excitation and attributed the unusual enhancement to uniaxial excitonic interactions, which contribute to symmetry lowering in the excited states.…”

“…The symmetry lowering promotes novel Frank-Condon and vibronic scattering mechanism in the B state(s) of the arrays. 20 In the present study the unusual enhancement is observed at 632.8 nm approximately 200 nm red shifted from the B state, so the mechanism for enhancement would be different to that reported by Bhuiyan et al 20 Nevertheless, the unusual RR enhancement observed by Bhuiyan et al and the SCD results reported by Goldbeck et al 22 on dimeric and tetrameric haems do set a precedence to explain the unusual enhancement observed in Hb encapsulated within single red blood cells with 632.8 nm excitation. Further experiments are required to validate the excitation coupling hypothesis as a mechanism to explain the unusual enhancement observed at 632.8 nm, which is the subject of continuing studies by our group.…”

“…19 Most studies have focused on covalently linked porphyrin arrays in which the extent of electronic interaction is dependent on both the length and type of linker. 20 The largest electronic interactions are observed in such systems that contain direct meso,mesolinked porphyrins resulting in excitonic interactions that are several tenths of an electronvolt. 21 Although most excitonic coupling usually involves a direct covalent linkage from one porphyrin to the next, it is conceivable that such coupling can also occur in systems such as haems contained within the erythrocyte protein matrix, given the extremely high concentration of the Hb moiety.…”

Resonance Raman spectroscopy provides evidence of heme ordering within the functional erythrocyte

“…Aggregated enhanced Raman scattering (AERS) has been proposed to explain the anomalous enhancement observed in N-protonated porphyrins [4] and cyanine dyes absorbed onto surfaces [5]. Covalently linked porphyrin architectures including meso-meso-linked arrays of copper(II) [6,7] and zinc(II) [8] exhibit strong excitonic interactions showing a distinct splitting of the Soret band and unusual Raman scattering patterns. The propionate linkage between porphyrin units in malaria pigment (hemozoin) may also enable excitonic coupling, although to a lesser extent than in meso-meso-linked arrays.…”

Controlling Chirality and Optical Properties of Artificial Antenna Systems with Self‐Assembling Porphyrins

“…[13] Semisynthetic products, which were derived from chlorophyll a, and which are also capable of selfassembly, were recently described. [15] If practical uses are envisaged, our synthetic strategies, which are high yielding and use cheap starting materials, are superior as the starting material, copper 10,20-bis-(3,5-di-tert-butylphenyl) porphyrin [16] is easily available in multigram quantities.…”