Ultrafast internal conversion

Pecourt J-ML, Peon J, Kohler B (2000) Ultrafast internal conversion of electronically excited RNA and DNA nucleosides in water. J Am Chem Soc 122 9348... 

Kang H, Jung B, Kim SK (2003) Mechanism for ultrafast internal conversion of adenine, J Chem Phys 118 6717... 

Zgierski MZ, Patchkovskii S, Fujiwara T, Lim EC (2005) On the origin of the ultrafast internal conversion of electronically excited pyrimidine bases. J Phys Chem A 109 9384-9387... 

Merchan M, Serrano-Andres L (2003) Ultrafast internal conversion of excited cytosine via the lowest it it electronic singlet state. J Am Chem Soc 125 8108... 

Chen H, Li SH (2005) Theoretical study toward understanding ultrafast internal conversion of excited 9H-adenine. J Phys Chem A 109 8443-8446... 

Seel, M., and Domcke, W. (1991), Femtosecond Time-resolved Ionization Spectroscopy of Ultrafast Internal-Conversion Dynamics in Polyatomic Molecules Theory and Computational Studies, J. Chem. Phys. 95,7806. 

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]. 

Our objective is to understand how the noncovalent interactions responsible for nucleic acid secondary structure (i.e. base stacking and base pairing) affect the photophysics of these multichromophoric systems. Here we describe initial experimental results that demonstrate dramatic differences in excited-state dynamics of nucleic acid polymers compared to their constituent monomers. Although ultrafast internal conversion is the dominant relaxation pathway for single bases, electronic energy relaxation in single-stranded polynucleotides... 

D. R. Cyr and C. C. Hayden, /. Chem. Phys., 104,771 (1996). Femtosecond Time-Resolved Photoionization and Photoelectron-Spectroscopy Studies of Ultrafast Internal Conversion in 1,3,5-Hexatriene. 

Stock, G. and Domcke, W. (1990). Theory of femtosecond pump-probe spectroscopy of ultrafast internal conversion processes in polyatomic molecules, J. Opt. Soc. Am. B 7, 1971. 

Figure 8. Time-resolved photoelectron spectra revealing vibrational and electronic dynamics during internal conversion in DT. (a) Level scheme in DT for one-photon probe ionization. The pump laser prepares the optically bright state S2. Due to ultrafast internal conversion, this state converts to the lower lying state Si with 0.7 eV of vibrational energy. The expected ionization propensity rules are shown S2 —> Do + e (ei) and Si —> D + (b) Femtosecond time-...

Figure 10. Energy level scheme for TRPES of PH, an example of a Type (II) ionization correlation, (a) The pump laser prepares the optically bright st al e. Due to ultrafast internal conversion,...

Surface Hopping, Excited States, Density Functional Theory, Ultrafast Internal Conversion, Conical Intersections, Nucleobases, Base Pairs, Photostability, UV Genetic Damage... 

Pecourt JML, Peon J, Kohler B (2001) DNA excited-state dynamics Ultrafast internal conversion and vibrational cooling in a series of nucleosides. Journal of the American Chemical Society 123 10370-10378. 

In this chapter, we review the progress we have made to date, and describe the issues that need to be addressed by further studies. The chapter is divided into three parts. The first deals with the ultrafast internal conversion of photoexcited nucleobases, and the role the out-of-plane deformation (twisting of a double bond, leading to biradicaloid geometry) plays in the photoprocess. The second is concerned with the highly efficient ICT process in dialkylaminobenzonitriles and related EDA molecules, where in-plane bending of the triple bond (which yields ira state of... 

The above results on the covalently modified nucleobases support the theoretical conclusion that ultrafast internal conversion occurs via the conical intersection of the... 

Benchmark ab initio quantum dynamical studies are carried out for the prototypical naphthalene and anthracene radical cations of the PAH family aiming to understand the vibronic interactions and ultrafast decay of their low-lying electronic states. The broadening of vibronic bands and ultrafast internal conversion through conical intersections in the Da — — D2 electronic states of these species is... 

Theoretical papers on effects directly observable in the very short time regime are notable in this years collection. The theory of femtosecond pump-probe spectroscopy of ultrafast Internal conversion processes in polyatomic molecules has been developed using the behaviour of the excited pyrazine molecule as an example . The solvation dynamics for an ion pair in a polar solvent can now be examined by the time dependence of fluorescence and by direct observation of photoinduced charge... 

The radiationless decay has been investigated by ultrafast polarization spectros-copy [44] and time-resolved fluorescence [45,46]. The results confirm that the radiationless decay occurs by an ultrafast internal conversion, due to intramolecular motion about the bridging bond of the chromophore in the excited state, that the isomerization is nearly barrierless, and that there is only a very weak dependence on medium viscosity, thereby implying that the isomerization occurs by a volume-conserving motion such as a hula twist [47]. 

The excited state lifetime of the GFP chromophore is very long in the protein (cfl. 3 ns) but much shorter (less than 0.3 ps) in solution. The mechanistic hypothesis is that the decay is due to a Z/E isomerization. Thus, while in solution the fluorophore may undergo an ultrafast internal conversion, the protein should act by restraining the isomerization. In contrast in Rh the excited state lifetime is ca. 150 fs. However, if we look at the solution lifetime this is increased of one order of magnitude. Furthermore, one has 24% quanmm yield in solution and 65% quantum yield in the protein. Thus, in this case the protein is catalyzing the reaction. The absorption maxima (A]iiax) of... 

Farmanara P, Stert V, Radloff W (1998) Ultrafast internal conversion and fragmentation in electronically excited C2H4 and C2H3CI molecules. Chem Phys Lett 288 518... 

Tao H, Allison TK, Wright TW, Stooke AM, Khutmi C, Tilborg JV, Liu Y, Falcone RW, Belkacem A, Martinez TJ (2011) Ultrafast internal conversion in ethylene. I. The excited state lifetime. J Chem Phys 134 244306... 

Ultrafast internal conversion in the triplet manifold then leads to emitting Ti (phosphorescence lifetime under nitrogen 9 and 16ps respectively for the two complexes. A similar ultrafast ISC had previously been determined for... 

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