The Triplet State Emission

The key issue can be best appreciated with reference to a series of contribution by Lewis and colleagues culminating in a seminal 1944 paper [2], where Lewis and Kasha proposed to explain how a spectroscopic study of phosphorescence (long-lived emission, afterglow) provided quantitative data concerning an extremely important 

State of every type of molecule, the triplet state. They began by excluding from the discussion one of the two classes of light-emitting materials, that is, phosphors such as mineral phosphors. These contained some inhomogeneity in the crystals, and in this case an electron was ejected. When this returned, light was emitted as an afterglow. Thus the phenomenon involved chemiluminescence. 

The stumbling block for the rationalization of such molecular afterglow had been the existence in some dyes of two long-lived emissions, indicated by Lewis and Kasha as a and p (see Fig. 3.1) [3]. The first one gave an emission spectrum identical with that of the short-lived emission (fluorescence) and had a shape 

Even when impurities were excluded, multiple emission could be observed. Formation of complexes, solvation, conformational equilibria, the intervention of different excited states, and energy transfer were invoked as the possible cause of such multiform emission, and actually aU of these mechanisms have been found to participate to various degrees in different cases. 

Jablonski added to Perrin s hypothesis the further assumption that the metastable state could emit, although the probability of such transition M-N was very small. Therefore when the temperature was sufficiently high, a large majority of molecules were raised thermally from the level M to F and were then able to emit the band F-N (long-lived emission at room temperature). At low temperatures, direct transitions M-N took place (see Fig. 3.2). 

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The ratio of the emission quantum yield and lifetime of the triplet state emission (phosphorescence) yields the product of the intersystem crossing efficiency and radiative decay rate constant. Determination of intersystem crossing efficiencies is generally not straightforward and often techniques other than emission spectroscopy, such as time-resolved photoacoustic calorimetry, are used. 

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

Fessenden R W and Verma N C 1976 Time resolved electron spin resonance spectroscopy. III. Electron spin resonance emission from the hydrated electron. Possible evidence for reaction to the triplet state J. Am. Chem. Soc. 98 243-4... 

Since the phosphorescence emission from (6) (68.8 kcal/mole) is very similar in energy and vibrational structure to benzophenone, and has a short lifetime (0.5 msec), it was proposed that the photorearrangement takes place via the triplet state. A Zimmerman-like mechanism is as follows for the formation of the cyclopropyl ketone (7) from dienone (6) ... 

Phillips and Schug (24) have suggested that the 390 nm emission, observed when PET is excited with high energy electrons, might be from a triplet state or an excimer. Since the triplet states of both PET and DMT are lower in energy (MSO nm), it is unlikely that the emission is from a triplet state. In addition, excimer formation and emission should not effect the absorption-excitation processes therefore, it is unlikely that the 390 nm emission is from an excimer. 

Seminal studies on the dynamics of proton transfer in the triplet manifold have been performed on HBO [109]. It was found that in the triplet states of HBO, the proton transfer between the enol and keto tautomers is reversible because the two (enol and keto) triplet states are accidentally isoenergetic. In addition, the rate constant is as slow as milliseconds at 100 K. The results of much slower proton transfer dynamics in the triplet manifold are consistent with the earlier summarization of ESIPT molecules. Based on the steady-state absorption and emission spectroscopy, the changes of pKa between the ground and excited states, and hence the thermodynamics of ESIPT, can be deduced by a Forster cycle [65]. Accordingly, compared to the pKa in the ground state, the decrease of pKa in the... 

From Si, internal conversion to So is possible but is less efficient than conversion from S2 to Si, because of the much larger energy gap between Si and So1 . Therefore, internal conversion from Si to S0 can compete with emission of photons (fluorescence) and intersystem crossing to the triplet state from which emission of photons (phosphorescence) can possibly be observed. 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

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