Singlet state dynamics

AIMD simulations have also been carried out on the chromophore present in the rhodopsin photoreceptor (retinal). In the primary event of vision, retinal passes from the ground state (GS) to an excited state (ES) and isomerizes from 11-cis to all-trans within 200 fs. A series of papers [46-50] have analyzed the GS isomerization process. More recently, calculations were extended to the first singlet ES [51] within a recently developed scheme for singlet state dynamics [52]. This work characterizes structural and energetic changes during the photoisomerization process and points to the crucial role of environment effects. 

Cohen B, Crespo-Hernandez CE, Kohler B Strickler-Berg analysis of excited singlet state dynamics in DNA and RNA nucleosides. Faraday Discussions 2004, 127 137-147. 

Gustavsson T, Sarkar N, Lazzarotto E, Markovitsi D, Barone V, Improta R (2006) Solvent effect on the singlet excited-state dynamics of 5-fluorouracil in acetonitrile as compared with water. J Phys ChemB 110 12843-12847... 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

Pillai ZS, Sudeep PK, George Thomas K (2003) Effect of viscosity on the singlet-excited state dynamics of some hemicyanine dyes. Res Chem Intermed 29(3) 293-305... 

Theoretical and time-resolved spectroscopic studies of triene photochemistry The dynamics of relaxation of the excited singlet states of E- and Z-l,3,5-hexatriene (HT) have recently been studied in the gas phase and in solution. In the gas phase, population of the 21 / state of the Z-isomer by internal conversion from the spectroscopic 11B state has been estimated to occur with a lifetime Tig of about 20 fs, while the lifetime of the 21 / state has been determined to be T2A =730 fs47. The lifetime of the latter in ethanol solution has been determined by Fuss and coworkers to be T2a = 470 fs52. A similar 21A lifetime has been reported for -l,3,5-hexatriene in cyclohexane and acetonitrile solution by Ohta and coworkers48. 

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

Fig. 1. Rise of the perylene cation absorption, which reflects the electron injection dynamics, after excitation of the first singlet state with a 15 fs pump pulse. Variation of the electronic coupling via a change in the anchor group as well as the insertion of one or two -CH2- groups leads to a systematic change in the time scale of both electron injection and charge recombination.

Efforts continue to elucidate the excited-state dynamics and ligand substitutional reactivities of d6 transition metal complexes.201-203 Fluorescence from incompletely relaxed ligand field singlet states has been detected for some haloamminerhodium(III) complexes.204... 

Photodissociation dynamics of this molecule were studied using the TOF technique to determine the velocity distribution of the fragments when the molecule is photolyzed with a KrF laser at 248 nm (182). Absorption at this wavelength leads to the excitation of the second singlet state, the B- -Ai state, from the X A ground state. From the TOF spectra of the CS fragment they were able to show that both of the following reactions occur ... 

Addition of a ca. 10% concentration of CA to a mesophase solution (mixture E) of BN quenches completely its photoresolution. CA quenches singlet states of BN in n-hexane at a nearly diffusion-controlled rate (kq = 101- -tf -s- assuming tbn = 3 ns (34-36)). Thus, even In a very viscous medium like cholesteric mixture E, static and dynamic quenching should preclude formation of BN triplets. 

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