Excited state photophysical effects

Khopde, S.M. et ah. Effect of solveut ou the excited-state photophysical properties of curcumin, Photochem. Photobiol., 72, 625, 2000. 

As we have discussed in depth elsewhere, despite the similarities in the structures of hypericin and hypocrellin, which are centered about the perylene quinone nucleus, their excited-state photophysics exhibit rich and varied behavior. The H-atom transfer is characterized by a wide range of time constants, which in certain cases exhibit deuterium isotope effects and solvent dependence. Of particular interest is that the shortest time constant we have observed for the H-atom transfer is 10 ps. This is exceptionally long for such a process, 100 fs being expected when the solute H atom does not hydrogen bond to the solvent [62]. That the transfer time is so long in the perylene quinones has been attributed to the identification of the reaction coordinate with skeletal motions of the molecule [48, 50]. 

In order to study solvent effects on the excited state photophysical properties and nonradiative decay of uracil, additional na-AIMD simulations of uracil in liquid water have been carried out [64], Figure 10-7 shows the periodic simulation cell containing uracil and 39 water molecules. We have verified that at any time in... 

MODELING OF PLASMONIC EFFECTS ON MOLECULAR PHOTOPHYSICS SEPARATION OF ABSORPTION, EMISSIVE RATE ENHANCEMENT AND EXCITED STATE QUENCHING EFFECTS... 

Kincaid and co-workers have exploited the steric effect in altering the excited-state photophysical properties of [Ru(bpy)2(daf)] +, where daf = 4,5 diazafluorene [32]. Aqueous solutions of this complex exhibit very weak luminescence, with lifetimes shorter than 10 ns at room temperature. Upon encapsulating this molecule into zeolite Y supercages, the fluorescence intensity increases by a factor of 100 and the excited-state lifetime is increased to 302 ns. 

The effect of thione structure and rigidity on the excited state photophysical and photochemical decay constants [60,64,67,73]... 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

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... 

Kawski A (1992) Solvent-shift effect of electronic spectra and excitation state dipole moments. In Rabek JR (ed) Progress in photochemistry and photophysics. CRC, New York, pp 1-47... 

It should be noted that no emission from the zwitterionic form of the proton-transferred tautomer was observed from any of the benzotriazoles studied in the present work. This implies that non-radiative relaxation processes from the excited state of this species are very efficient in all of the solvent and polymer environments studied. Thus no information is available on the effect of the medium polarity on the room-temperature photophysics of the zwitterionic form using fluorescence techniques. 

The formation of excimers and exciplexes are diffusion-controlled processes. The photophysical effects are thus detected at relatively high concentrations of the species so that a sufficient number of collisions can occur during the excited-state lifetime. Temperature and viscosity are of course important parameters. 

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

Patterson et al. performed a thorough photophysical study of the solid state [10] and solution ground and excited states properties [11,12] of the potassium salt, suggesting that in the solid state the strength of the metal-metal interactions has a great effect on luminescence. 

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... 

These discussions provide an explanation for the fact that fluorescence emission is normally observed from the zero vibrational level of the first excited state of a molecule (Kasha s rule). The photochemical behaviour of polyatomic molecules is almost always decided by the chemical properties of their first excited state. Azulenes and substituted azulenes are some important exceptions to this rule observed so far. The fluorescence from azulene originates from S2 state and is the mirror image of S2 S0 transition in absorption. It appears that in this molecule, S1 - S0 absorption energy is lost in a time less than the fluorescence lifetime, whereas certain restrictions are imposed for S2 -> S0 nonradiative transitions. In azulene, the energy gap AE, between S2 and St is large compared with that between S2 and S0. The small value of AE facilitates radiationless conversion from 5, but that from S2 cannot compete with fluorescence emission. Recently, more sensitive measurement techniques such as picosecond flash fluorimetry have led to the observation of S - - S0 fluorescence also. The emission is extremely weak. Higher energy states of some other molecules have been observed to emit very weak fluorescence. The effect is controlled by the relative rate constants of the photophysical processes. 

The analysis of the transient fluorescence spectra of polar molecules in polar solvents that was outlined in Section I.A assumes that the specific probe molecule has certain ideal properties. The probe should not be strongly polarizable. Probe/solvent interactions involving specific effects, such as hydrogen-bonding should be avoided because specific solute/solvent effects may lead to photophysically discrete probe/solvent complexes. Discrete probe/solvent interactions are inconsistent with the continuum picture inherent in the theoretical formalism. Probes should not possess low lying, upper excited states which could interact with the first-excited state during the solvation processes. In addition, the probe should not possess more than one thermally accessible isomer of the excited state. 

The present contribution deals mainly with novel 9-substituted anthracenes in which the substituent either incorporates or by itself represents a 7r-system, and whose effect on the overall molecular shape is such as to have major photochemical and photophysical repercussions [33]. Not discussed are anthracenophanes [34] and various types of bichromophoric anthracenes whose excited state properties have been reviewed previously [8,25,35]. Considered beyond the scope of this contribution are the photochemistry and photophysics of anthraceno crown ethers and cryptands [36-38], and of intramolecular exciplexes derived from anthracenes linked to aromatic amines [39-41],... 

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