ENVIRONMENT-DEPENDENT PHOTOPHYSICS OF POLYMER-BOUND J-AGGREGATES DETERMINED BY TIME-RESOLVED FLUORESCENCE SPECTROSCOPY AND TIME-RESOLVED NEAR-FIELD SCANNING OPTICAL MICROSCOPY

Exciton migration and annihilation dynamics in J aggregates comprised of a pseudoisocyanine (PIC) dye and poly(vinyl sulfate) (PVS) are stud ied by time-correlated, single-photon-counting (TCSPC) methods coupled with near-field scanning optical microscopy (TCSPC-NSOM) and also by conventional, far-field TCSPC. Far-field TCSPC studies of the aggregat es in aqueous solution demonstrate that the exciton lifetime in the ab sence of annihilation (at low excitation intensities) is 789 +/- 36 ps with biexponential exciton decay occurring at high excitation intensi ties due to exciton annihilation. Analysis of the intensity-dependent decay with conventional exciton decay theories demonstrates that the o bserved decay dynamics are consistent with exciton migration limited t o finite molecular domains. In contrast to the solution studies, depos ition of a thin PIC/PVS aggregate film onto fused quartz results in a dramatic reduction in the exciton lifetime to similar to 10 ps. This l ifetime demonstrates only modest dependence on excitation intensity. P ossible mechanisms for the reduction in exciton lifetime for the adsor bed aggregates are discussed. Finally, the dependence of the exciton l ifetime on the nanostructure of the aggregates is explored with TCSPC- NSOM. A new TCSPC-NSOM instrument with an instrument response of 30 ps and spatial resolution of similar to 100 nm is presented and used to perform the first direct measurement of exciton lifetimes for a single aggregate. TCSPC-NSOM studies demonstrate that the exciton lifetime i s not sensitive to the structural details of the aggregates on the sim ilar to 100 nm length scale. The combination of measurements presented here demonstrate that the exciton diffusion length is limited to shor t distances (similar to 100 nm) relative to the aggregate size. The ho mogeneous spectral properties observed for these aggregates are theref ore not due to site-averaging resulting from long-range exciton migrat ion but instead are due to the structural homogeneity of the aggregate s on the length scales investigated here.