Picosecond transient absorption spectroscopy on 13 2-hydroxy-bacteriopheophorbide-methylester (0H-BPME) molecules in solution and embedded in mixed Langmuir-Blodgett films

Abstract

Transient absorption spectroscopy was carried out on OH-BPME in methanol and on multilayer mixed OH-BPME/behenic acid Langmuir-Blodgett (LB) films after resonant excitation of the S 2 state by 17 ps single pulses. The time dependences of saturable absorption of the S 0–S 2 and S 0–S 1 transitions and of the Stokes shifted transient amplification of the probe photons are investigated. From the latter a lifetime of the S 1 state of 1.7 ns and from the first and ISC quantum efficiency of 0.5 are obtained on OH-BPME in methanol, which is supported by the strong induced absorption band observed around 600 nm caused by triplet-triplet absorption processes. In comparison to the saturable absorption a delayed starting of the transient amplification is observed speaking for a relatively long lifetime of the molecules in the S 2 state. The spectra of the LB films are dominated by aggregates of OH-BPME caused by the high package density. An excitation induced decrease of the absorption coefficient at the S 0–S 1 transition of the aggregate at 815 nm is observed with a very short decay time of 85 ps.