Stimulated emission depletion dynamics in push-push polyenes

Abstract

Two-photon fluorescence polarisation and stimulated emission depletion dynamics are investigated in three high two -photon cross-section push-push polyenes: OM62, LP79 and OM77 and compared to the behaviour of a standard fluorophore (rhodamine 6G). Two-photon fluorescence anisotropy measurements (R(0) and Ω) were undertaken using picosecond time-correlated single photon counting (TCSPC). For OM62 and LP79 these are consistent with a diagonal two-dimensional transition tensor with S XX ≤S YY . For OM77 the contribution of off-diagonal elements (S XY & S YX ) appears significant. Two-photon fluorescence anisotropy decay data is combined with streak camera measurements of excited state population depletion to determine stimulated emission cross sections and ground state vibrational relaxation times. Cross-sections for STED in all.three polyenes were found to be significantly higher than those for rhodamine 6G. The efficiency of STED is however dependent on the degree to which the S 1 →S 0 transition is saturated by the DUMP pulse; this is mediated by fast ground state vibrational relaxation. Of the three polyenes, LP79 is seen to combine a large stimulated emission cross-section (c.a. 13σ(r6G)) with rapid ground state relaxation (τ R =240fs).