Coherent internal conversion of pyrene revealed by pump-probe and ultrabroad 2D-UV spectroscopy

Summary form only given. The 2D measurements on a light harvesting complex by Fleming and coworkers showed long lived oscillations [1] suggesting the existance of quantum-coherent effects in large macromolecules. In order to gain a deeper insight into these coherent phenomena, the intramolecular energy dissipation in restricted size molecules has to be investigated. Here we present a combined pump-probe and 2D-UV investigation of pyrene in solution, showing that in this molecule the coherent excitation of vibrational states survives the S₂-S₁ internal conversion (figure 1a). The temporal evolution of the transient absorption of pyrene is shown in figure 1b. The S₂ excited state absorbtion band maximum at 575 nm reveals that the IC occurs with a time constant of 85 fs. The ultrafast decay of this band is accompanied by the rise of S₁ excited state absorpion bands in the range 340 - 380 nm and 450 - 525 nm. Both bands show oscillations that can be ascribed to wavepackets in the S₁ state indicative for coherent internal conversion. 2D-UV measurements on pyrene performed with our new collinear setup are shown in figure 2. As excitation we use UV pulses broadened by self-phase modulation to 25 nm to cover two vibronic bands of the S₂¹B_2u S0 electronic transition of pyrene at 312 nm. An ultrabroad supercontinuum was used as probe. The observed S₁ excited state absorption between 340 - 380 nm (29400 - 26300 cm-1) displays an excitation dependent Franck-Condon envelope with an additional shoulder at 394 nm (25350 cm-1) only present at the high energy excitation side (figure 2b). The intensity of this shoulder does not increase steadily with increasing excitation energy, but displays a rather discrete step between the two excited vibronic bands. The 2D-UV and pump-probe measurements thus provide the complete experimental framework needed to identify the mechanism behind the co- erent internal conversion on the vibrational level.