Electronic transitions phosphorescence

Fig. 5 Schematic representation of the electronic transitions during luminescence phenomena [5]. — A absorbed energy, F fluorescence emission, P phosphorescence, S ground state. S excited singlet state, T forbidden triplet transition.

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

Fig. 14 Schematic representation of the electronic transitions of photochemically excited substances Sq = ground state, Sj = first excited singlet state, T = forbidden triplet transition, N = ground state of a newly formed compound, A = absorption, F = fluorescence, P = phosphorescence.

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.

Fig. 21. Top The general Jablonski diagram for the flavin chromophore. The given wavelengths for absorption and luminescence represent crude average values derived from the actual spectra shown below. Due to the Franck-Condon principle the maxima of the peak positions generally do not represent so-called 0 — 0 transitions, but transitions between vibrational sublevels of the different electronically excited states (drawn schematically). Bottom Synopsis of spectra representing the different electronic transitions of the flavin nucleus. Differently substituted flavins show slightly modified spectra. Absorption (So- - S2, 345 nm S0 -> Si,450nm 1561) fluorescence (Sj — S0) 530 nm 156)) phosphorescence (Ty Sq, 605 nm 1051) triplet absorption (Tj ->Tn,...

The phthalocyanines, naphthalocyanines, and certain of their metal derivatives (Figure 6.17) are infrared fluorophores. 61"64 As a class, they are exceptionally stable compounds, with copper (Cu) phthalocyanine (not a fluorophore) remaining intact above 300 °C in air. First used for textile dyeing in the last century and still widely used, there is a rich chemistry of phthalocyanines. Most derivatives can be made by prolonged heating of a phthalimide or phthalic acid derivative with a metal in powder or salt form at elevated temperature. Several derivatives absorb in the near-IR, and either fluoresce or phosphoresce. The electronic transitions of phthalocyanines are complex and have been extensively studied, at least in part because the symmetry of the molecule makes theoretical calculations of its spectroscopic behavior more tractable. Unsubstituted phthalocyanines and naphthalocyanines are, as a class, very insoluble in solvents other than, for instance, nitrobenzene. Sulfonated phthalocyanines are water soluble and exhibit spectra comparable to the parent derivative. Photolumines-cent phthalocyanines (Pcs) include SiPc, ZnPc, and PC itself. These compounds have been little used for practical infrared fluorometry to date however, Diatron Corpora-... 

Some of the theoretical aspects of fluorescence have already been discussed alongside phosphorescence in section 3.2 and shown schematically in Figure 3.1. In organic fluorophores, especially dyes, excitation from the ground level to the hrst excited state Sj is generally a n-n electronic transition. During the extremely short time (1-10 x 10 s) that the excited molecule spends in the various higher... 

Molecular energy levels in aromatic heterocycles, ir - ir, n - ir electronic transitions in pyridine. Phosphorescence and fluorescence... 

Of all these, the only permitted electronic transition is1 Do < 1S0, while the rest are forbidden according to the spin rule, leading to phosphorescent emissions. However, these are precisely the ones responsible for the emissions found in many luminescent gold complexes, appearing in a range between 500 and 700 nm (665 nm in the gaseous ion). 

Fig. 9.4 Electronic transitions leading to (a) fluorescence emission at Ap and (b) phosphorescence at Ap...

Phosphorescence is emission of light from triplet excited states, in which the electron in the excited orbital has the same spin orientation as the ground state electron. Transitions to the GS are forbidden and the emission rates are slow, so that phophoresence life times are typically milliseconds to seconds. 

Figure 4.5 (a) Spectra of the Cr111 complex with urea [Cr(urea)d3+ absorption (A), phosphorescence (Ph) and fluorescence (F) (b) the relevant electronic transitions (ISC, intersystem... 

The Franck-Condon factors determination is of special interest when the two electronic states, involved in the transition exhibit very different geometries. This is especially the case of electronic transition in the valence shell such as n — tt, which induces conjugation change, as well as geometrical change, in the molecular system. This phenomenon was studied in the fluorescence spectra of acetaldehyde and acetone [62,63], and in the phosphorescence spectra of thioacrolein and thioacetaldehyde [64,65] and thioacetone [66]. 

The electronic excited state is inherently unstable and can decay back to the ground state in various ways, some of which involve (re-)emission of a photon, which leads to luminescence phenomena (fluorescence, phosphorescence, and chemiluminescence) (22). Some biologic molecules are naturally fluorescent, and phosphorescence is a common property of many marine and other organisms. (Fluorescence is photon emission caused by an electronic transition to ground state from an excited singlet state and is usually quite rapid. Phosphorescence is a much longer-lived process that involves formally forbidden transitions from electronic triplet states of a molecule.) Fluorescence measurement techniques can be extremely sensitive, and the use of fluorescent probes or dyes is now widespread in biomolecular analysis. For example, the large increase in fluorescence... 

The phosphorescence spectra of benzophenone dissolved in CCU and adsorbed on MgO, 7-AI2O3, and Si02 (all of which have a similar phosphorescence character) include four separable maxima (Table V) (216). The presence of these maxima, which are attributed to the electronic transitions from the excited state to lower vibrational levels of the ground electronic... 

It has been shown recently by Kapturkiewicz and co-workers [14] that the analysis of the CT absorption CT <— So and the radiative and radiationless charge recombination processes CT So (Figure 4) in selected D-A n-n interacting systems sterically hindered to coplanarity (such as 9-anthryl and 9-acridyl derivatives of aromatic amines [14a,b], carbazol-9-yl derivatives of aromatic nitriles [14c] and ketones [14d] and D-A derivatives of indoles [14e] or phenoxazines and phe-nothiazines [14f]) in terms of the theory of photoinduced ET processes in absorption [52, 53] and emission [53-55] and Mulliken and Murrell models of molecular CT complexes [56, 57] leads to the determination of the quantities relevant for the rate of the radiative ET processes (exemplified by the CT absorption and emission) and to the estimation of the electronic structure and molecular conformation of the states involved in the photoinduced ET. A similar approach can be applied to describe the properties of the fluorescent singlet CT states and phosphorescent triplet CT states [58]. It should be pointed out that the relatively large values of the electronic transition dipole moments of the CT fluorescence indicate a non-... 

In Figure 5.18 the absorption and emission spectra of azulene are shown. The anomalous fluorescence of azulene from the S, state is easy to recognize. The AP(F) spectrum exhibits a deep minimum at 33,900 cm. The small peak in the absorption spectrum at the same wave number is therefore not due to vibrational structure but rather to another electronic transition, the polarization of which had been predicted by PPP calculations. Figure 5.19 shows all four types of polarization spectra of phenanthrene. FP becomes negative at the vibrational maxima of the fluorescence the most intense vibration is not totally symmetric, in contrast to the one which shows up weakly. For all absorption bands, AP(P) = -0.3. The polarization direction of phosphorescence is perpendicular to the transition moments of all transitions lying in the mo-... 

---