Radiationless decay

Totally deuterated aromatic hydrocarbons yield measured phosphorescence lifetimes greater than their protonated analogs.182 This behavior is ascribed to the closer spacing of vibrational levels in deuterated compounds with a consequent decrease in probability for nonradiative T - S0 transitions. Quantum mechanical tunnelling may also contribute to the rate of the radiationless process with the normal compounds. 

Another less-common form of unimolecular decay is also temperature dependent and results in the phenomenon known as is-type delayed fluorescence.183 For example, the lowest excited singlet and triplet states of eosin are quite close together in energy, so that excited 

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Edington M D, Riter R E and Beck W F 1997 Femtosecond transient hole-burning detection of interexciton-state radiationless decay in allophycocyanin trimers J. Phys. Chem. B 101 4473-7... 

Fig. 3. A configurational—coordinate diagram showing mechanisms of radiationless decay to the ground state. Nonradiative decay to the ground-state...

Knowledge of photoiaduced electroa-transfer dyaamics is important to technological appUcations. The quantum efficiency, ( ), ie, the number of chemical events per number of photons absorbed of the desired electron-transfer photoreaction, reflects the competition between rate of the electron-transfer process, eg, from Z7, and the radiative and radiationless decay of the excited state, reflected ia the lifetime, T, of ZA ia abseace ofM. Thus,... 

The photochemical behavior of butadienes has been closely studied. When these compounds are exposed to light, they move from the ground state to an excited state. This excited state eventually returns to one of the ground state conformations via a process that includes a radiationless decay (i.e., without emitting a photon) from the excited state potential energy surface back to the ground state potential energy surface. 

The excited-state molecules may either undergo radiationless decay to the ground state leading to the formal generation of heat under conditions of high radiation flux or radiative decay (i.e., phosphorescence), thereby emitting light. 

Since the negligible contribution of intersystem crossing to the S, radiationless decay of cyanine-like systems has been firmly established [26,58,59], henceforth we will consider only the properties of the S, and Sq potential surfaces involved in the... 

Radiationless decay takes place in the coordinates X X2 as one passes through the conical intersection diabatically (the VB structure does not change, see, for example. Fig. 9.4). 

Computational Photochemistry, Conical intersections, Radiationless decay, Ab initio... 

All the nucleic acid bases absorb UV radiation, as seen in Tables 11-1, 11-2, 11-3, 11-4, and 11-5, making them vulnerable to the UV radiation of sunlight, since the energy of the photons absorbed could lead to photochemical reactions. As already mentioned above, the excited state lifetimes of the natural nucleobases, and their nucleotides, and nucleosides are very short, indicating that ultrafast radiationless decay to the ground state takes place [6], The mechanism for nonradiative decay in all the nucleobases has been investigated with quantum mechanical methods. Below we summarize these studies for each base and make an effort to find common mechanisms if they exist. 

The electronically excited singlet states of uracil, and how they can lead to efficient radiationless decay to the ground state, were initially investigated using MRCI methods by Matsika [147] and later with other methods that agree with the MRCI results in the more general features [92, 126, 128, 148-150], The discussion in this section describes MRCI results for free uracil in detail, along with the studies that have been... 

Figure 11-5. Energy level diagram of minima and conical intersections involved in the radiationless decay in uracil. Energies and structures taken from Ref. [147, 224]...

As discussed earlier, thymine is very similar to uracil in its excited states pattern. This is also true for its radiationless decay mechanism except from the fact that the excited state lifetime in thymine is somewhat longer than in uracil. Theoretically the mechanism for radiationless decay has been studied using CASPT2 electronic structure methods [150, 152],... 

Cytosine was the first nucleobase whose radiationless decay was studied with quantum mechanical methods. Nevertheless, its first excited states are not so clearly separated as in uracil and thymine, and this causes complications in the computational studies of the photophysics. So, many computational studies have been reported to elucidate the mechanisms for radiationless decay to the ground state but, not always with the same conclusions. 

An important question in the topic of radiationless decay in nucleobases is whether a common mechanism exists that operates in all of them. The studies above show that the mechanism in each nucleobase depends on the details of the excited states... 

A question that becomes obvious at this point is what happens to the molecules that have similar structures to the natural bases but have different photophysical properties, i.e. they fluoresce. These molecules have similar main structure to the bases, similar ring systems and double bonds, and so, according to the previous discussion, similar conical intersections should be expected. If that is true, and conical intersections facilitate efficient radiationless decay, why do these molecules fluoresce instead of decaying nonadiabatically That is a question that has occupied a number of scientists and some answers and insights are given in the following section. 

Perun S, Sobolewski AL, Domcke W (2005) Ab initio studies on the radiationless decay mechanisms of the lowest excited singlet states of 9H-adenine. J Am Chem Soc 127 6257-6265... 

Matsika S (2004) Radiationless decay of excited states of uracil through conical intersections. J Phys Chem A 108 7584... 

Zgierski MZ, Fujiwara T, Kofron WG, Lim EC (2007) Highly effective quanching of the ultrafast radiationless decay of photoexcited pyrimidine bases by covalent modification photophysics of 5,6-trimethylenecytosine and 5,6-trimethyleneuracil. Phys Chem Chem Phys 9 3206-3209... 

Zgierski MZ, Patchkovskii S, Lim EC (2005) Ab initio study of a biradical radiationless decay channel of die lowest excited electronic state of cytosine and its derivatives. J Chem Phys 123 081101... 

Kistler KA, Matsika S (2007) Radiationless decay mechanism of cytosine an ab initio study with comparisons to die fluorescent analogue 5-methyl-2-pyrimidinone. J Phys Chem A 111 2650—2661... 

Yamazaki S, Kato S (2007) Solvent effect on conical intersections in excited-state 9H-adenine radiationless decay mechanism in polar solvent. J Am Chem Soc 129 2901—2909... 

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