ULTRAFAST EXCITED-STATE DYNAMICS IN MOLECULAR AGGREGATES OF AN INDOTRICARBOCYANINE DYE

Abstract

The dynamics of transient absorption spectra of H*- and J-aggregates of an intoricarbocyanine dye was studied using femtosecond pump-probe spectroscopy. Transient bleaching decay of the H*-aggregates proceeds with two time constants of ~3 ps and ~30 ps upon excitation at 400 nm and with a single time constant of ~30 ps upon excitation at 800 nm. Transient bleaching decay of the J-aggregates contains two components with time constants of ~1 ps and ~20 ps, respectively, upon excitation at both 400 nm and 800 nm. The fast component in the transient bleaching decay of the H*-aggregates upon excitation at 400 nm correlates with a direct transition from the top of the exciton band to the ground state. The slow component is due to a competing relaxation from the top to the bottom of the exciton band followed by a transition to the ground state. The fast component in the transient bleaching decay of the J-aggregates corresponds to a direct transition from the nonthermalized excitonic state to the ground state. The slow component is related to the transition to the ground state after intraband exciton relaxation is complete. The longer delocalization length of excitons in the J-aggregates (9 molecules) compared to the H*-aggregates (4 molecules) is manifested in the narrowing of the ground-state absorption band and in the faster decay of the excited states. The H*-aggregates possess a narrow ground-state absorption band at 514 nm, and a weak absorption peaked at 756 nm. Upon excitation at 800 nm the transient bleaching peaks undergo a bathochromic shift as a result of inhomogeneous broadening.