Assignment of carotene S* state features to the vibrationally hot ground electronic state

Abstract

The so-called S* state has been suggested to play an important role in the photophysics of beta-carotene and other carotenoids in solution and photosynthetic light-harvesting complexes, yet its origin has remained elusive. The present experiments employing temperature-dependent steady-state absorption spectroscopy and ultrafast pump-supercontinuum probe (PSCP) transient absorption measurements of beta-carotene in solution demonstrate that the spectral features of S* are due to vibrationally excited molecules in the ground electronic state S(0). Characteristic spectral signatures, such as a highly structured bleach below 500 nm and absorption in the range 500-660 nm result from the superposition of hot S(0) absorption ("S(0)*") on top of the ground-state bleach of room-temperature molecules. Appearance and disappearance of the S(0)* molecules can be completely described by a global kinetic analysis employing time-dependent species-associated spectra without the need to invoke the population of an intermediate electronically excited state.

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile

“…8(B). 43 The development in the remaining two panels, Fig. 8(C) and (D), is dominated by multiexponential decay kinetics.…”

“…5(A), including fit results from a global analysis procedure using the simple kinetic model S 1 -S 0 with a time-dependent spectrum for the S 1 state. 42,43 In the transient at 522 nm (red), S 0 -S 1 GSB and S 1 -S 0 SE appear first. These then quickly change into S 1 -S n ESA due to sub-picosecond solvation dynamics (the SE shifts to longer wavelengths, and then ESA overwhelms the underlying GSB contribution).…”

Ultrafast dynamics of the photo-excited hemes b and cnin the cytochrome b6f complex

“…The broad negative signal in the 140 fs DAS component ( Figure 6) is about twice as large in amplitude as expected if it would solely represent the Car S 2 to Car S 1 internal conversion. 27,35 Part of this broad negative signal in the 140 fs DAS component can be attributed to the recovery of an initially bleached heme c n . Indeed, the broad underlying negative signal present in the 150 fs transient spectra (Figure 4) indicates the presence of an initially excited pool of heme c n .…”

“…It is important to mention that since multiple processes are taking place in the same spectral window with comparable lifetimes, the exponential components can only be taken as approximations. For example, the decay of the Car S 1 , which is about 9 ps when analyzed separately, 35 is now associated with the ground state recovery of the heme b -band bleach in the 7.3 ps DAS. As previously mentioned, residual transient signal from Car in the double difference data is expected to be ~5% of the absolute Car signal, which is about 40 mO.D.…”

“…While the sharp undulations are solely assigned to the heme b activity, the broad feature can either be attributed to the heme c n signal that is expected to peak around 550 nm 26 or to the Car S 1 signal that maximizes at ~580 nm. 35 The broad negative signal in the 140 fs DAS, the bimodal feature with the zero crossing point at 690 nm in the 710 fs DAS as well as the broad positive signal in the 7.3 ps DAS are all typical, in spectral features and dynamics, of the Car excited state relaxation mechanisms: 35 Internal conversion from S 2 to the hot S 1 state in 140 fs, followed by a 710 fs intramolecular vibrational relaxation to the lowest S 1 state and subsequent ground state recovery in 7.3 ps. It is important to mention that since multiple processes are taking place in the same spectral window with comparable lifetimes, the exponential components can only be taken as approximations.…”