RESONANCE RAMAN-SCATTERING FROM EXCITON-COUPLED STATES IN PORPHYRIN MONOLAYER ASSEMBLIES

The utility of resonance Raman (RR) spectroscopy toward elucidating exciton structure in molecular aggregates is assessed in terms of a detailed RR investigation of a metalloporphyrin monolayer (ML) assembly which exhibits well-resolved exciton effects in its absorption spectrum. RR spectra are reported for (5,10,15,20-tetrakis[4-(1-octyloxy)phenyl]porphinato)copper(II) in solution and in mixed ML assemblies supported at air/glass [1:1 molar ratios with 4-(1-octyloxy)benzaldehyde] and air/water [1:3.6 with 4-(1-hexadecyloxy)analine] interfaces. The RR bands exhibit only subtle frequency shifts between the various sample environments, and analyses of the observed shifts are therefore speculative; yet the shifts of certain structure-sensitive bands are similar to those observed in previous studies of porphyrin aggregation and are therefore consistent with the reported formation of pi-pi aggregates in the ML environments. Also, small frequency shifts observed between glass- and water-supported ML samples for modes localized at the periphery of the porphyrin macrocycle indicate the presence of measurable effects due to the nature of the substrate. The most compelling evidence for exciton interactions is observed in the intensity enhancement patterns of the various modes as observed for the glass-supported ML. The Raman excitation spectra (RES) of totally symmetric porphyrin modes show clear evidence of enhancement via both components of the split B state and can be modeled satisfactorily by considering only the two scattering excited states. The simulations indicate that the enhancements of totally symmetric modes are characterized by A-term scattering associated with small excited-state displacements and that substantial interference occurs between the scattering terms associated with the two scattering states. An anomalous RES is observed for a non-totally symmetric mode at 772 cm-1 which is not enhanced with B-state excitation of the solution sample. The observed intensity dispersion cannot be reproduced satisfactorily by the three-state (one ground, two excited) calculation, even upon inclusion of possible vibronic coupling (B term) effects, and thus may be indicative of more complex scattering mechanisms associated with the exciton structure of the ML. The results of this study suggest that RR spectroscopy can yield important information about exciton structure in molecular aggregates but that detailed work, including RES compilation and simulation, is necessary in order to extract definitive characterizations.