Photoexcited singlet excited state

We can illustrate the application of PAC to a simple photochemical reaction. Acetone is readily excited to its singlet excited state which rapidly undergoes efficient intersystem crossing to its triplet state. The triplet state decays in solution primarily by radiationless decay. The PAC experimental waveforms obtained from the photoexcitation of acetone in air and argon-saturated cyclohexane are shown in Fig. 1. In addition, the waveform obtained from the calibration compound 2-hydroxybenzophenone is also shown. 

While phenols and amines are usually more acidic in the singlet excited state than in the ground state, some substances, e.g., aromatic ketones, may become more basic in the photoexcited state. 

Photoluminescence intensity of the amorphous polymers was generally much larger than that of the more crystalline polymers. The energy level of the lowest singlet excited state Es was evaluated to be 2.5-2.7 eV for the amorphous polymer pristine films, and 2.0 eV for the more crystalline polymers. The Stokes shifts were also observed to be much larger for the amorphous polymer films compared with those of the more crystalline polymer films. This indicates a larger structural relaxation of the amorphous polymers following photoexcitation. 

Indisputably, photoexcitation is followed by a rapid deactivation of the singlet-excited state of the oPPE moiety resulting in the generation of a charge-separated species, i.e. the radical-ion-pair state exTTF +-oPPE -C o, which is apparently lower in energy than the corresponding triplet state of C o- The radical ion pairs decay on the ps time-scale with charge-recombination rates that prove wire-like... 

Photoexcitation of a deaerated PhCN solution of Acr+-Mes by a nanosecond laser light flash at 430 nm results in the formation of Acr -Mes+ with a quantum yield close to unity (98 %) via photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the acridinium ion moiety ( Acr -Mes) [54]. The decay of Acr -Mes+ obeyed second- rather than first-order kinetics at ambient temperature as observed in the case of Fc+-ZnP-H2P-C60 , when the bimo-lecular back electron transfer predominates owing to the slow intramolecular back electron transfer (see above) [50]. In contrast, the decay of Acr -Mes+ obeys first-order kinetics in PhCN at high temperatures (e.g. 373 K). This indicates that the rate of the intramolecular back electron transfer of Acr -Mes4 becomes much faster than the rate of the intermolecular back electron transfer at higher tempera-... 

Photoexcitation of OPVn in Solution. The photoexcitations of oligo(p-phenylene vinylene)s (OPVns, n = 2-7) [106] can be summarised as follows. The singlet excited state OPVn(5i) decays radiatively or non-radiatively to the ground state and via intersystem crossing to the OPVn(Ti) triplet state. The singlet excited state lifetimes t have been determined for OPV3 (r =... 

Photoexcitation of MP-Ceo in Solution. The photoexcitation of MP Cgo in toluene or ODCB results in weak fluorescence at 1.74 eV and a long-lived triplet excited state. The fluorescence quantum yield in toluene is known to be 6 x 10-4 [107]. Singlet excited state lifetimes of 1.45 ns [103] and 1.28 ns [107] have been reported for toluene solutions. The quantum yield for intersystem crossing from MP G eofA i) to MP-Ceo(Ti) is near unity [107] and the lifetime of this triplet state is about 200 (is [103]. The triplet state PIA spectrum of MP Ceo exhibits a Tn <— Ti absorption at 1.78 eV with a characteristic shoulder at 1.54 eV [103]. The energy level of the MP -Coo(7j) triplet state has been determined from phosphorescence to be at 1.50 eV above the ground state level [107]. 

In the present chapter, we will focus on the simulation of the dynamics of photoexcited nucleobases, in particular on the investigation of radiationless decay dynamics and the determination of associated characteristic time constants. We use a nonadiabatic extension of ab initio molecular dynamics (AIMD) [15, 18, 21, 22] which is formulated entirely within the framework of density functional theory. This approach couples the restricted open-shell Kohn-Sham (ROKS) [26-28] first singlet excited state, Su to the Kohn-Sham ground state, S0, by means of the surface hopping method [15, 18, 94-97], The current implementation employs a plane-wave basis set in combination with periodic boundary conditions and is therefore ideally suited to condensed phase applications. Hence, in addition to gas phase reference simulations, we will also present nonadiabatic AIMD (na-AIMD) simulations of nucleobases and base pairs in aqueous solution. 

The isolated N-terminal BLUF domain exhibits a photocycle identical to that observed with full-length AppA [48]. Photoexcitation of AppA involving a singlet excited state in the flavin chromophore leads to the formation of... 

We wish to discuss briefly the photoexcited state of zinc(II) porphyrin. Zinc(II) porphyrin has several singlet excited states that are formed by visible light absorption [62]. Of those singlet excited states, the first and the second, Si and S2, have been investigated well [63]. Energy levels of these excited states are schematically shown in Scheme 27. 

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