Excited state absorption and relaxation dynamics in a series of heptamethine dyes under femtosecond and nanosecond excitations

Abstract

We show that the near-infrared-absorbing polymethine cyanine dyes exhibit nonlinear absorption (NLA) in the visible region, where they have negligible or very low linear absorption. In chloroform solvent, the studied cyanines were found to be photo-unstable and exhibited photo-degradation under prolonged exposures as compared to methanol and dimethyl sulphoxide solvents, where they were stable. Excitation with femtosecond (fs) pulses in the visible region (400–750 nm) exhibits excited state absorption (ESA) as characterized by positive differential absorption (ΔOD) in transient absorption signal. Non-resonant excitations in the visible region are governed by reverse saturable absorption (RSA) mechanism leading to ESA. Excitation with nanosecond pulses also shows NLA owing to ESA from triplet states as compared to that from singlet states under fs excitations. Single beam Z-scan studies were performed with nanosecond (ns) pulses to evaluate ground (σgr) and excited state (σex) absorption cross-sections, which confirms the mechanism of RSA as σex were found to be greater than σgr. The population relaxation from higher excited singlet states shows ultrafast behavior with multi-exponential decay components. The fast time component decay varies from a few hundreds of fs (τ1) to some hundreds of picoseconds (τ2), and the long-time decay is in the ns domain (τ3).