Extremely sensitive detection of transient species in laser flash photolysis of ultrathin organized molecular films by optical waveguide

Transient species upon pulsed laser excitation of ultrathin polymer and Langmuir-Blodgett (LB) films was detected sensitively by the optical waveguide (OWG) method. Tetratolylporphyrin in poly(methylmethacrylate) films and amphiphilic tetraphenylporphyrin in mixed LB films with dioctadecyldimethylammonium were prepared on the surface of OWGs. These films were excited with the second harmonics of a ns Nd:YAG laser in controlled environment. The transient absorption of excited triplet porphyrins was detected sensitively by Ar+ laser, He-Ne laser, or diode lasers as a probe. Transient absorption in LB films with only two monolayers was observed by the present method. It was about hundred times increase of sensitivity as compared with a conventional normal incidence measurement. Different decay dynamics were observed for excited triplet state porphyrins in polymer films and in LB films, which was attributed to the different spatial distribution of chromophores.