The Basic Photophysical Processes

Excited states are ordinarily - but not invariably - produced by photoexcitation. Before the photochemical reaction proper can be taken up, the photophysical processes preceding it [1], each with its own symmetry requirements, have to be listed. These are summarized in Fig. 10.1. 

as is nearly always the case, the initial state is Sq, photoexcitation to a triplet is assumed to be so highly forbidden as to be negligible. The selection rules for excitation to the various higher singlets will not be restated [3] let us merely note three qualitative points  

If an excited state higher than Si is produced, it will ordinarily obey KashaRule [7] and relax rapidly to Si. From there it can either fluoresce to Sq or relax non-radiatively by internal conversion (1C) to vibrationally excited Sq the excess vibrational energy is lost rapidly in solution - less so in the dilute gas phase. Kasha s Rule finds expression in Fig. 10.1 by the absence of all processes originating in S2 except internal conversion to Si. 

The selection rules for intersystem crossing (ISC) were given in Section 9.2. If a higher triplet is produced, it too will obey Kasha s Rule and relax to Ti. Since its two modes of relaxation to So, phosporescence and ISC, both involve spin-inversion, Ti will be comparatively long-lived. Except when trapped at very low temperatures, there is ample time for the three components of the triplet T , and to reach equilibrium before it phosphoresces, relaxes by ISC or reacts chemically. 

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Jablonski s diagram, lifetimes of the basic photophysical processes and deexcitation pathways from the lowest-lying excited states of a molecule... 

In 1991, Foote et al. [20,21] carried out the first investigations of basic photophysical properties of pure fullerenes. Since then research has lead to extensive knowledge about the photophysical behavior of the fullerenes in general [22-48], Scheme 1 shows the main photophysical processes. The most relevant photo physical properties are summarized in Table 1. 

The basic photophysical properties of C o and C70 were first described by Arbogast et al. [9,10], These studies have been greatly expanded and extended [11-30], Scheme 1 summarizes the photophysical processes reported. 

The photophysics of type I polymers is very similar to that of the corresponding small-molecule substituent chromophores that they contain. The polymer can be described as an inert scaffold that restricts the movement and relative distance of the chromophores. As a result, the photophysics of these polymers can be strongly affected by the conformation of the polymer backbone and, to a lesser extent, by the aggregation of the polymers. The basic photophysics and energy migration processes of this type of fluorescent polymers have been studied extensively by Guillet and Webber over the past two decades and have been summarized elsewhere [1,11]. 

The present chapter thus has two major goals. First, we will introduce the conceptual and mechanistic foundations of photochemistry. We will see that before we can understand photochemistry, we must understand photophysics. Indeed, photochemistry is controlled by a competition among a variety of rate constants associated with fundamental physical processes, and our goal is to help you develop a sense of the relative values of these rate constants. Second, we will survey the basic photochemical processes of organic chemistry. In reality, a relatively small number of basic reaction types dominate organic photochemistry. 

The basic photophysical properties of a dye molecule are maintained upon immobilisation on a semiconductor surface, but the interaction with the semiconductor may open new reactive routes and/or change the rate of particular photochemical processes. An example of the importance of these routes is the fact that some polypyridyl complexes, intrinsically photolabile in solution, become photostable when bound to the semiconductor titanium dioxide, and actually constitute the class of dyes that has enabled some of the most efficient cells constructed to date [4, 5]. 

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. 

It should be emphasized that best design (each application corresponding to a particular design), proper choice, and correct use of fluorescent probes require a thorough knowledge of the basic phenomena involved in ion recognition medium effect on complexation equilibrium, fundamental photophysical processes, and possible changes from other causes than complexation. 

In Refs. [50-53], two levels of analysis were successively addressed (i) a two-state XT-CT model which is able to capture the basic features of the phonon-mediated exciton dissociation process (ii) a three-state XT-IS-CT model which also comprises an intermediate state (IS), i.e., an additional charge transfer state whose presence can have a significant influence on the dynamics, see Fig. 6. In the latter case, comparative calculations for several interface configurations were carried out, leading to a realistic, molecular-level picture of the photophysical events at the heterojunction. In the following, we start with a summary of the findings reported in Refs. [50,51], where the two-state model was explored (Sec. 5.1). Following this, we address in more detail the analysis of Refs. [52,53] for the three-state model (Sec. 5.2). 

This chapter reviews the methodologies developed over the years to tackle various aspects of surface photoelectrochemistry. Section 2.2 gives an overview of all the photophysical and photochemical processes operative in semiconductor systems, combining findings from solid-state physics and chemistry. For completeness, the effect of quantisation of the band structure is included. The basic principles are presented to enable a smooth transition from purely molecular to purely sohd-state... 

A knowledge of the basic characteristics of luminscent substances permits the determination of the rate constants of photophysical processes discussed in Sect. B. 

In the discussion of the experimental approaches to study molecular resonances, the quantum theory of photon wave packet scattering forms the natural framework. It is thus necessary to recall some of the main features of wave packet scattering (Messiah, 1965 Newton, 1966 Goldberger and Watson, 1965a) with special reference to photophysical phenomena (Shore, 1967). We begin with a brief review of the basic concepts in the formal description of time evolution. Then we consider more in detail the process of scattering of a coherent photon wave packet by a molecule. The expressions for the basic experimental observables are derived, with special emphasis on time-resolved studies. Detection is assumed to take place under short time conditions, in a lateral, nonforward direction so that no coherence of the photon states scattered by different molecules must be considered. 

Figure 4 Upper panel basic photophysical scenario for an isolated molecule. Dotted (dashed) arrows show vibronic absorption (fluorescence) shaded large arrows internal conversion (1C), vibrational relaxation (VR), and intersystem crossing (ISC) processes. Solid black arrow shows photoinduced absorption transitions of triplet excitons taking place in the microsecond time domain. Dot-dashed arrow indicates phosphorescence. Lower panel schematic representation of negative polaron levels and their spin population within the monoelectronic scheme. Black arrows show photoinduced absorption transitions (Pi and P2).

The results of extensive ab initio calculations of the PE surfaces reported above provide the quantitative basis for a qualitative interpretation of the complex photophysics of ESIPT systems. On the basis of the results one can distinguish two basic mechanisms responsible for the ESIPT process and its dynamics, depending on the geometry of the proton transferring center. 

Cp Ir(phen)(H)]+ in CH3CN solution at 293 K). The corresponding chloro complexes showed no interesting photophysical properties. Based on all of these observations the photoassisted transformation of the hydride to H2 is much faster than the other photochemical processes of the present system (eg. photoinduced extrusion of Cf, photoassisted nucleophilic attack of H2 on Ir I-CO or photoinduced reduction of intermediate B to C). The possibility that intermediate B could be formed by insertion of CO into an IrBi-OH (formed in basic media by displacement of the Cl anion) was ruled out by the absence of any thermal or photochemical reactivity of [Cp Iriil(bpy)(OH)] complex Id with CO (eq. 23). 

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