Ultrafast Excited-State Dynamics in Molecular Aggregates of an Indotricarbocyanine Dye

Abstract

The dynamics of the transient absorption spectra of H^*- and J-aggregates of an indotricarbocyanine dye was studied using femtosecond pump-probe spectroscopy. Relaxation of the induced transmission of the H^*-aggregates occurs with time constants of ~3 and ~30 ps with radiation pumping at λ_max = 400 nm and ~30 ps with pumping at λ_max = 800 nm. For J aggregates biexponential relaxation with time constants of ~1 and ~20 ps is observed in both cases. The fast component of the kinetics of transmission of the band of the H^* aggregates with pumping at λ_max =400 nm is due to the possibility of direct transition from the upper edge of the exciton band of the aggregate to the ground state, while the slow component is present as a result of concurrent fast transition from the upper edge of the exciton band to the lower edge and subsequent relaxation from this to the ground state. The fast component of the kinetics of transmission of the band of the J aggregates is determined by the possibility of transition from the unthermalized exciton state to the ground state, while the slow component is due to transition to the ground state after relaxation within the limits of the exciton band. The greater length of delocalization of the excitons in the J aggregates (nine molecules) compared with the H^* aggregates (four molecules) appears not only as decrease of the half-width of the stationary absorption band but also as more rapid quenching of the excited states. It was shown that the H^* aggregates have a narrow stationary absorption band at 514 nm and weak absorption with a maximum at 756 nm. With pumping at λ_max = 800 nm there is a bathochromic shift of the maxima of the transmission band of the system as a result of the appearance of nonuniform broadening.