Nonradiative photophysical processes

Fig. 10 Energy level diagram showing the excited states involved in the main photophysical processes (excitation solid lines radiative deactivation dashed lines, nonradiative deactivation processes wavy lines) of the 2 Nd3+ [Ru(bpy)2(CN)2] three-component system. For the sake of clarity, naphthyl excimer energy level has been omitted...

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

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. 

Figure 8.IS Idealized potential energy surfaces illustrating photophysical processes in octahedral complexes of Cr(III) (I) absorption, (2) intersystem crossing, (3) vibrational relaxation, (4) photoreaction from Eg or nonradiative return to ground state, (S) photoreaction from TiB or nonradiative return to ground state, (6) emission from T2b and (7) emission from aE . 

Photophysical processes, that is, ones not involving any change in composition of an A, have become of much interest to the inorganic photochemist, particularly in terms of excited state kinetic schemes. A brief discussion of the phenomenology and theory of radiative and nonradiative deactivations follows. 

The emission properties of lumophores change when included within the microenvironment of a supramolecule bucket. Nonradiative decay processes are generally curtailed within the confines of the bucket interior and luminescence intensity is therefore increased [138,208,209], Because CDs present a more protected microenvironment than calixarenes, the binary complexes of the former supramolecule have been examined most extensively. Spurred by Cramer s pioneering observation that the spectral properties of a lumophore are perturbed by complexation within a CD [210], a large body of work has sought to define the influence of CDs on the photophysics of bound lumophores. Different factors contribute to the enhanced luminescence of 1 1 CDilumophore complexes. These include the following. 

The minimum prerequisite for generation of upconversion luminescence by any material is the presence of at least two metastable excited states. In order for upconversion to be efficient, these states must have lifetimes sufficiently long for ions to participate in either luminescence or other photophysical processes with reasonably high probabilities, as opposed to relaxing through nonradiative multiphonon pathways. The observed decay of an excited state in the simplest case scenario, as probed for example by monitoring its luminescence intensity I, behaves as an exponential ... 

Radiative and Nonradiative Decay Processes - Due to the potential application of these compounds as photosensitizers for photodynamic therapy" the photophysical properties of porphyrins and phthalocyanines, and their corresponding metal complexes, have been investigated extensively over the past decade. The photophysical properties of water-soluble metalloporphyrins, and especially the tetraphenylsulfonates," have been re-examined but nothing new has been found. The disulfonated metallophthalocyanines (MPcS2, where M = Al ", Ga" , or Zn") form complexes with fluoride ions for which the fluorescence yields and lifetimes are decreased with respect to the parent dyes while there are... 

Figure 2.18 Schematic representation of photophysical processes in lanthanide(III) complexes (antenna effect). A = absorption, F = fluorescence, P = phosphorescence, L = lanthanide-centred luminescence, ISC = intersystem crossing, ET = energy transfer S = singlet, T = triplet. Full vertical lines radiative transitions dotted vertical lines nonradiative transitions...

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

The primary photochemical processes performed by electronically-excited molecules. They can be divided further into photophysical processes and photochemical reaction processes. The former include luminescent processes and nonradiative deactivation. 

There are many photophysical processes that are responsible for the de-excitation of molecules. The few examples of intermolecular photophysical processes that induce fluorescence quenching are electron transfer, proton transfer and energy transfer. The following section will focus on energy transfer and most specifically on nonradiative energy transfer. 

The magnetic-field effects on other photophysical processes like absorption and emission, have not been investigated, but in these cases the theoretical approach is straightforward it can proceed in the same manner as in the magnetic-field effect on intramolecular nonradiative processes. 

Excited energy transfer among chromophores is one of the most ftmdamental photophysical processes. According to the mechanism the excited energy transfer is classified into 1.) radiative trivial type, 2.) nonradiative Forster type [286], and 3.) nonradiative Dexter type [285]. 

In this chapter we present an introductory overview of the basic theoretical concepts of computational molecular photoph rsics. First, the nature and properties of electronic excitations are considered, with special attention to transition moments and vibrational contributions. Then, the main photophysical processes involving the electronic excited states are examined, focusing in particular on nonradiative deactivation phenomena. Finally, we present a brief review of computational methods commonly applied for the description of molecular excitations. Special emphasis is given to the configuration-interaction (Cl) method and the time-dependent density functional theory (TD-DFT), discussing some technical details and outlining advantages and limitations. 

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