Photophysical dynamics

Tinnefeld, R, Herten, D.P., and Sauer, M. 2001. Photophysical dynamics of single molecules studied by spectrally-resolved fluorescence hfetime imaging microscopy (SFLIM). J. Phys. Chem. A 105 7989. 

Lim EC (1986) Proximity effect in molecular photophysics Dynamical consequences of pseudo-Jahn-Teller interaction. Journal of Physical Chemistry 90 6770-6777. 

The photophysical dynamics of o-HBP was investigated by the Temp.G method with a picosecond pulsed laser (pulse width 30ps) [164]... 

This compound has been frequently used as a photochemical stabilizer and the photophysical dynamics is interesting in relation to the efficient energy dissipation mechanism [168, 169], The TG signal from o-HBP in water was fitted well by a superposition of the Dens.G and Temp.G signals. At 4°C, the signal consisted of the pure Temp.G component (Fig. 28). 

Yip W-T, Hu D, Yu J, Vanden Bout D A and Barbara P F 1998 Classifying the photophysical dynamics of single- and multiple-chromophoric molecules by single molecule spectroscopy J. Chem. Phys. A 102 7564-75... 

In principle the photophysical dynamics of a molecule under coherent excitation has two components. In the coherent motion driven by the laser field, the pseudospin representing the density matrix of the electronic system performs a Rabi precession between the singlet ground and excited states. Additionally, incoherent transitions through coupling to the vacuum field modes or to intramolecular vibrational modes will cause radiative and non-radiative population transfer between the electronic... 

In general, the correlation technique as well as the quantum jump technique are powerful tools to unravel complicated molecular photophysical dynamics for a single absorber. This statement is exemplified by the investigation of the chromophore terrylene, for which no kinetical parameters of the triplet state were known from ensemble measurements. The ISC rates presented here were determined solely by experiments on single molecules. Actually, it would be quite difficult to measure absolute rates of photophysical ISC parameters by other techniques when the triplet quant yield is smaller than 10 . Recently, fluorescence correlation spectroscopy was also proposed as an appropriate method for the determination of triplet parameters of fluorophores in solution [75]. Additionally, it is a helpful tool to investigate spectral diffusion of single absorbers as discussed in Sections 1.4 and 1.5. 

Excited-states simulations were mainly limited to small and medium-sized molecules before the 90s. However, many important photophysical processes, as for example, the photoisomerization of rhodopsin, take place in a biological environment, seldom not without the presence of an enzyme. To study photochemical processes in the large-size systems, alternative methods are required. One such method, the QM/MM method," was developed by Warshel and Levitt in 1976. This approach combines the accuracy of quantum chemical models with the speed of molecular mechanics. An alternative method to combine different quantum chemical approaches, the ONIOM method, was developed by Morokuma and co-workers." These methods were initially used in the context of ground-state reactions. Early applications of the QM/MM hybrid method to photochemical processes can be found as early as 1982," however, it was not until at the beginning of this century that the method started to be used extensively for photochemical and photophysical dynamics. To find representative investigations of that time consult the reference list." " ... 

Dendrimers with a polyphenyl core around a central biphenyl unit decorated at the rim with peryleneimide chromophores have been investigated both in bulk and at the single-molecule level in order to understand their time and space-resolved behavior [28]. The results obtained have shown that the conformational distribution plays an important role in the dynamics of the photophysical processes. Energy transfer in a series of shape-persistent polyphenylene dendrimers substituted with peryleneimide and terryleneimide chro-mophoric units (4-7) has been investigated in toluene solution [29]. 

The validity of the above conclusions rests on the reliability of theoretical predictions on excited state barriers as low as 1-2 kcal mol . Of course, this required as accurate an experimental check as possible with reference to both the solvent viscosity effects, completely disregarded by theory, and the dielectric solvent effects. As for the photoisomerization dynamics, the needed information was derived from measurements of fluorescence lifetimes (x) and quantum yields (dielectric constant, where extensive formation of ion pairs may occur [60], the observed photophysical properties are confidently referable to the unperturbed BMPC cation. Figure 6 shows the temperature dependence of the... 

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. 

The electronic structure method used to provide the energies and gradients of the states is crucial in photochemistry and photophysics. Ab initio electronic structure methods have been used for many years. Treating closed shell systems in their ground state is a problem that, in many cases, can now be solved routinely by chemists using standardized methods and computer packages. In order to obtain quantitative results, electron correlation (also referred to as dynamical correlation) should be included in the model and there are many methods available for doing this based on either variational or perturbation principles [41],... 

Their distinguishing feature is the high speed of response. This response develops on the scale of fluorescence lifetime of photophysical or photochemical events that provide the response and can be as short as 10 x-10 10 s. Because of that, the fluorescence reporting is never time-limiting, so that this limit comes from other factors, such as the rate of target - sensor mutual diffusion and the establishment of dynamic equilibrium between bound and unbound target. 

The sensitizers display a crucial role in harvesting of sunlight. To trap solar radiation efficiently in the visible and the near IR region of the solar spectrum requires engineering of sensitizers at a molecular level (see Section 9.16.3).26 The electrochemical and photophysical properties of the ground and the excited states of the sensitizer have a significant influence on the charge transfer (CT) dynamics at the semiconductor interface (see Section 9.16.4). The open-circuit potential of the cell depends on the redox couple, which shuttles between the sensitizer and the counter electrode (for details see Section 9.16.5). 

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

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