Electronic excitation energy phosphorescence

Fig. 1.1. Electronic and vibrational energy levels (schematic, rotational are omitted for simplicity), So singlet ground state, Sj excited singlet state (higher electronically excited states are omitted in this figure), T triplet electronically excited energy state. Straight lines symbolise radiative processes (absorption (Ab) as well as emission F, fluorescence P, phosphorescence). Wavy lines give the radiationless transitions, ic, internal conversion isc, intersystem crossing sd, radiationless deactivation te, thermal equilibration.

The low fluorescence quantum yield and the fact that no phosphorescence could be observed in oligothio-phenes leads to the conclusion that most of the electronic excitation energy decays radiationless. There are different views of the participation of triplet states in this decay process in thin films. In solution, however, the relaxation via triplet states is well agreed. Tliis is due to the fact that singlet oxygen is produced and the absorption attributed to triplet-triplet excitation (i.e. at 1.80 eV [139] respectively 1.92 eV [38], theoretically predicted value around 2.7 eV [172]) is completely quenched by exposure to air. 

Figure 10. Electron excitations in radicals (a) Collective representation of one-electron transitions of the A, B, and C types if denotes MO (b) LCI energy-level scheme (Jablonski diagram) for doublet and quartet states indicating why with radicals fluorescence (- - -) but not phosphorescence is observed. Spin-forbidden transitions are represented by dashed lines.

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

The possible fate of excitation energy residing in molecules is also shown in Figure 2. The relaxation of the electron to the initial ground state and accompanying emission of radiation results in the fluorescence spectrum - S0) or phosphorescence spectrum (Tx - S0). In addition to the radiative processes, non-radiative photophysical and photochemical processes can also occur. Internal conversion and intersystem crossing are the non-radiative photophysical processes between electronic states of the same spin multiplicity and different spin multiplicities respectively. 

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. 

The fluorescence and phosphorescence spectra of a complex molecule are generally discussed by reference to an energy level diagram such as that shown in Figure 1. Absorption of light raises the molecule from the ground state to one of the upper electronically excited singlet states. At... 

The fluorescence quantum yield of 448 is 0.14, a sixfold increase relative to that of the parent. In comparison, the fluorescence quantum yield of 449 (0.01) is comparable to that of the parent compound. The phosphorescence emission quantum yield of 449 is 0.56 in a frozen matrix as expected as a result of the intramolecular heavy atom effect. In contrast, the phosphorescence is effectively shut off in the anti-isomer where the quantum yield is only 0.04. This observation suggests that the electronic excited state structures and nonradiative decay channels very considerably with constitution of the isomers. The optical gap energy was 3.1 (3.3) eV for 448 (449). 

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