Photophysical relaxation dynamics

Although the studies with SPD techniques have provided significant results on the intermittency in quantum dots, the systems of observation were limited to immobile quantum dots in solids, such as polymer films and glass matrices. The immobilization results in intrinsic heterogeneity of the local environment around each quantum dot the SPD cannot cover the photophysical kinetics in quantum dots in solution of a more homogeneous environment. In addition, the SPD approaches needed conventional bin-time longer than 10 ms for reliable determination of on and off states. This also limits the elucidation of relaxation dynamics for shorter time scales. 

In order to better understand the early photophysics of PYP, we have carried out a comparative study of three model chromophores, the deprotonated frans-p-coumaric acid (pCA2 ) and its amide (pCM ) and phenyl thioester (pCT) analogues, in aqueous solution (see structures in Fig. 1). The excited-state relaxation dynamics was followed by subpicosecond transient absorption and gain spectroscopy. 

Like azulenes, thioketones are known to display anomalous S2 —> S0 fluorescence. The S2 — S0 transition is electric dipole allowed and this, combined with large S2 Sl energy gaps in many thioketones, results in relatively large fluorescence quantum yields and nanosecond to picosecond lifetimes [5]. Table 2 gives photophysical information for the S2 state of a variety of thioketones [15-24]. Most of this data was acquired in perfluoroalkane solvents. Maciejewski and Steer note that the S2 relaxation dynamics of these compounds are solvent dependent... 

In the spirit of providing an updated account of the various photophysical and photochemical process to help drive paradigm shifts in solar energy conversion, we will emphasise the specific details of the carrier-relaxation dynamics with respect to... 

The thermal detection of the TG has been used for revealing many unresolved photophysical processes. Dynamics of the excited state should be extracted from the acoustic curve by a theoretical fitting procedure. A vibrational energy relaxation in heme proteins has been studied in relation to the protein conformational change [12b]. A very short lifetime of vibrational modes and the rapid vibrational energy distribution were found. 

It should be stressed that the wave-packet picture of photophysical relaxation and photochemical reaction dynamics described in this chapter is substantially different from the traditional concepts in this area. In contrast to the established picture of radiationless transitions in terms of interacting tiers of zero-order molecular eigenstates, the dynamics is rationalized in terms of local properties of PE surfaces such as slopes, barriers and surface intersections, a view which now becomes widely accepted in photochemistry. This picture is firmly based on ah initio electronic-structure theory, and the molecular relaxation d3mamics is described on the basis of quantum mechanics, replacing previously prevaUing kinetic models of electronic decay processes. Such a more detailed and rigorous description of elementary photochemical processes appears timely in view of the rich and specific information on ultrafast chemical processes which is provided by modern time-resolved spectroscopy. " ... 

For T2a-c containing alkyl substituents in the polymethine chain, the equilibrium between two isomers is shifted from the aU-lrans-isomer in nonpolar media towards the mono-ds-isomer in polar media, thereby reducing Of and O,, . (Table 36.5). The photophysics and subnanosecond relaxation dynamics of T3 and its 9-Me derivative were studied. Implications for the design of near-IR fluorochromes with high fluorescence efficiencies were found from steady-state and picosecond laser studies of a 14 (IR 125) dye. ... 

Sahyim, M.R.V. and Serpone, N., Photophysics of thiacarbocyanine dyes relaxation dynamics in a homologous series of thiacarbocyanines, /. Phys. Chem. A, 101, 9877,1997. 

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... 

Bavykin, Dmitry V. is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998) Luminescent and photocatalytic properties of CdS nanocolloids . Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. 

A time-resolved ion yield study of the adenine excited-state dynamics yielded an excited-state lifetime of 1 ps and seemed to support the model of internal conversion via the nn state along a coordinate involving six-membered ring puckering [187]. In order to determine the global importance of the tict channel, a comparison of the primary photophysics of adenine with 9-methyl adenine will be useful, as the latter lacks a tict channel at the excitation energies of concern here. The first study of this type revealed no apparent changes in excited-state lifetime upon methylation at the N9 position [188] a lifetime of 1 ps was observed for both adenine and 9-methyl adenine. This was interpreted as evidence that the tict is not involved in adenine electronic relaxation. 

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