Excited-state dynamics of polymer-bound J-aggregates

The excited-state dynamics of polymer-bound J-aggregates formed in aqueous mixtures of pseudoisocyanine (PIC) chloride and poly(vinyl sulfonic acid sodium salt) (PVS) (MW ≈ 11 200) have been studied by picosecond time-correlated single-photon counting and picosecond polarized pump-probe spectroscopy. At a concentration of 40 μM PIC and 5 × 10^-4 g/dL PVS, the absorption spectrum of the J-aggregate is characterized by a J-band at 565 nm with a fwhm of ≈500 cm^-1. For this particular mixture, a dye molecule is bound to each of the SO₃^- groups on the polymer chains. The physical size of the aggregate is therefore determined by the number of polymer residues (≈87) per chain. The fluorescence lifetime and fluorescence quantum yield of these J-aggregates are 17 ± 3 ps and 0.022 ± 0.003, respectively. The lifetime is independent of the excitation intensity. From an analysis of the photophysical parameters, we infer a coherence size of ≈ 5 ± 1, which is smaller than the physical size of the aggregate. The pump-probe signal at 565 nm is entirely due to bleaching and consists of a fast component, with a decay time comparable to the fluorescence lifetime, and a slow component. The decay kinetics of the induced bleaching are independent of the excitation intensity. The signal at 558 nm consists of an absorption component at early times and a slowly decaying bleaching component at long times. The anisotropy at 565 nm was constant over the 160-ps time range of the signal. The kinetics are rationalized in terms of a model involving the singlet exciton states of the J-aggregate and a long-lived bottleneck state. The induced absorption at early times for excitation on the blue edge of the J-band is consistent with a one-exciton to two-exciton transition.