Fate Of Excited State

This section discusses the processes that can take place following an excitation created by light absorption, which takes a molecule to an excited state. These processes can be radiative, where a photon is emitted (emission) to bring the molecule back to the ground state. They can be nonradiative, where the excited-state energy is dissipated as a heat or in producing a chemical reaction (photochemistry). The return to the ground state may also involve a combination of both. The nonradiative processes producing heat involve crossing from one electronic level to another of lower energy, with the excess energy converting to vibrational energy by an interaction called electronic-vibrational state coupling. Subsequently, the excess vibrational energy is converted to heat by coupling to translation (this process is called vibrational relaxation). These processes are schematically represented in Table 4.1. In this table the star sign, such as in A*, signifies that A is in the excited state. The processes of

TABLE 4.1. Schematic Representation of Processes Involved in Electronic Excitation

Electronic excitation in a molecule/molecular aggregate

Photophysical processes

Photoinduced electron transfer

Photochemistry

Radiative Nonradiative process process

Excited-state complex formation (i) Excimer

Fluorescence (i) State-to-state crossing Phosphorescence (ii) Vibrational relaxation

Photoisomerization such as cis-trans isomerization

Photodecomposition

energy transfer and excited-state complex formation occur only when more than one molecule are interacting. Therefore, for such processes a minimum size molecular aggregate is a dimer (A2) or a bimolecular (AB) unit. An excimer [an excited-state dimer, (A-A)*] or an exciplex [an excited-state complex, (A-B)*] may return to the ground state radiatively (by emitting light) or nonradiatively. An example of excimer formation is provided by an aromatic dye, pyrene, which shows a broad structureless fluorescence peaked at ~500 nm, well shifted to the red from the emission (at ~390 nm) of the single pyrene molecule. In biological fluid media, the excimer formation is diffusion-controlled. Therefore, excimer emission (such as from the pyrene dye) has been used to study diffusion coefficient (a quantity defining the diffusion rate) in membranes.

Exciplexes are excited-state complexes formed between two different molecules (or molecular units), A and B, when one of them (e.g., A) is excited (designated as A* in Table 4.1). Exciplexes can form between aromatic molecules, such as naphthalene and dimethylaniline. Just like in the case of an excimer, the resulting emission from the exciplex (A-B*) is red-shifted, compared to that from A*. Of biological interest has been the exciplex formation between a metalloporphyrin and a nucleic acid or an oligonucliotide, which can provide structural information on the microenvironment of the metallo-porphyrin (Mojzes et al., 1993; Kruglik et al., 2001). Exciplex formation has been studied for double-stranded polynucleotides and natural DNA having regular double-helix structures.

The photochemical processes, listed under photochemistry in Table 4.1, are of considerable significance to biology, because they occur in biological materials with important consequences. These processes in biological materials are discussed in detail in Chapter 6, "Photobiology," with specific examples provided there.

The state-to-state crossing and the various possible radiative and nonra-diative processes in an organic structure are often represented by the so-called Jablonski diagram shown in Figure 4.3. In this diagram, the radiative processes are represented by a straight arrow, whereas nonradiative processes (also sometimes referred to as radiationless transition) are represented by a wiggly arrow.

The ground state of most molecules (organics in particular) involves paired electrons; therefore, their total spin S = 0 and the spin multiplicity 2S + 1 = 1. These are singlet states and, in the order of increasing energy from the ground-state, singlets are labeled S0, S1, S2, and so on. An exception is the common form of O2, where the ground-state is a triplet with the spin S = 1 and the spin multiplicity 2S + 1 = 3. Therefore, the ground state of oxygen is T0. This case is not represented in Figure 4.3, which only depicts the case of molecules with a singlet ground state, S0. For molecules whose ground states are S0, the excitation of an electron from a paired electron pair to an excited state can produce either a state where the two electrons are still paired (like S1) or where the two electrons are unpaired (a triplet, T state). The excited

triplet configurations are labeled as T1, T2, and so on, in order of increasing energies.

Quantum mechanical considerations show that for the excitation to the same orbital state, the energy of the excited triplet state (say T1 state) is lower than that of its corresponding singlet state (51 in this case). In Figure 4.3 the possibilities for the fate of an excitation to a higher singlet S2 manifold are described. The horizontal closely spaced lines represent the vibrational levels. Suppose the excitation is to an electronic level, S2. A nonradiative crossing from the S2 state to S1 is generally the dominant mechanism. Only very few molecules (e.g., azulene), show emission (radiative decay) from S2. This crossing between two electronic states of the same spin multiplicity (such as from S2 to S1) is called internal conversion (IC). This IC process is then followed by a rapid vibrational relaxation where the excess vibrational energy is dissipated into heat, the molecule now ending up at the lowest, zero-point vibration level (v = 0, see Chapter 2 on vibration) of the S1 electronic state. From here, it can return to the ground electronic state S0 by emitting a photon (radiatively). This emission from a state (S1) to another state (S0) of same spin multiplicity is called fluorescence and is spin-allowed (observes the rule of no change of spin value). It, therefore, has a short lifetime of emission, generally in the nanoseconds (10-9-sec) range. Alternatively, the excitation may cross from S1 to T1 by another nonradiative process called intersystem crossing (ISC) between two states of different spin. This crossing (change) of spin violates the rule of no change of electron spin during a change of electronic state and is thus called a spin-forbidden transition. This spin violation (or occurrence of a spin-forbidden transition) is promoted by spin-orbit coupling, described in

Chapter 2, which relaxes the spin property by mixing with an orbital character. Followed by a rapid vibrational relaxation, the excitation ends in the zero-point vibrational level of the T1 state. A radiative process of emission from here leading to S0 is spin-forbidden and is called phosphorescence. Again, the spin-violation occurs because of spin-orbit coupling (Chapter 2). This is a weaker emission process and, therefore, has a long lifetime. Some of the phosphorescence lifetimes are in seconds. Many photochemical processes originate from this type of long-lived triplet state. Heavy metals, molecular oxygen (having a triplet ground state), paramagnetic molecules, and heavy atoms such as iodine increase the intersystem crossing rate, thus reducing the fluorescence and enhancing the process taking place from the excited triplet state.

Finally, there can also be a nonradiative intersystem crossing from T1 to S0.

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