Excited state relaxation dynamics in condensed phase by femtosecond transient absorption spectroscopic method

Abstract

Photophysical investigation of molecules in condensed phase is sufficiently important works because it is intimately connected from our body to surrounding environment. However, lifetime of molecular excited state is very short and interaction with solvent molecules makes difficult to interpretation of ‘pure’ molecular dynamics. After development of femtosecond laser system and various analytic tools for spectroscopy, it is possible to observe the ultrafast phenomena on molecular excited state. Here, we have investigate the relaxa-tion dynamics of excited state for light-harvesting systems from bulk to nanomaterial with femtosecond broadband transient absorption (TA) spectroscopic method. Femtosecond TA is one of the widely used pump-probe technique and it is useful to investigate the non-emissive or dark states after photoexcitation. First topic is Cu(Ⅱ)porphyrins. Porphyrin molecules have been spotlighted for a long time because they are physiologically important components in heme and chlorophylls. In contrast to free-base porphyrin, the Cu(Ⅱ) porphyrin shows unique relaxation dynamics by forming five-coordinate complex with solvent molecule due to lone pair electron of Cu atom $(3d^94S^0)$. In non-coordinating solvents such as benzene or 2,6-lutidine, triplet state of Cu(Ⅱ)porphyrin decays very slowly extending from tens to hundreds of ns. However, in nitrogen-containing solvents like pyridine and piperidine, the charge transfer state drops lower to triplet state and then it is dominant relaxation channel whereas the metal excited (d,d) state acts as a competitive relaxation pathway in oxygen-containing solvents like 1,4-dioxane and tetrahydrofuran. Meanwhile, benzo-nitrile and furan do not coordinate to Cu ion due to poor coordinating ability although they contain N-or O-atom. Second topic is metal/semiconductor hybrid systems. Metal-tipped CdSe nanorod (NR) composites are good candidate for photocatalysis on visible light excitation due to efficient charge separation from CdSe NR to metal domain. After photoexcitation, negative bleach and stark-shifted positive absorption signals are observed by state-filing mechanism and formation of biexciton state. Subsequently, a number of electron-hole pair (exciton) relax via electron cooling, surface trap or nonradiative/radiative recombination. On the other hand, in presence of metal, electron transfer occurs dominantly with time constant of 0.94 ± 0.02 ps, 0.9 ± 0.01 ps, 0.87 ± 0.01 ps and 0.79 ± 0.02 ps for Pt/CdSe, Pt/CdSe/Pt, Au/CdSe/Pt and Au/CdSe/Au nano-composite, respectively. Interestingly, the bleach kinetics of Au-tipped CdSe NRs shows unusual rising behav-ior and its amplitude is remarkably enhanced accompanying oscillatory feature as pump power increases. These phenomena also observed in Pt/CdSe/Pt @ Ag, where Ag metal and CdSe NR are not contacted direct-ly. Moreover, after 800 nm pulse whose energy is insufficient to excite the CdSe NR, transient signal probed at CdSe NR is observed like 400 nm excitation. Therefore, it means that plasmon energy affects the number of electrons in CdSe NR overcoming Schottky barrier. Electromagnetic field simulation with FDTD method also support the plasmon effect on exciton relaxation dynamics in Au-tipped CdSe NRs.