Upper singlet state

The existence of this TTA bimolecular process makes it possible to carry out cooperative excitation of energy rich molecular states by low energy quanta. In particular, formation of upper excited singlet states as a result of TTA has been observed for several aromatic hydrocarbons (1) and metalloporphyrins ( ) by the demonstration of annihiTation-induced delayed fluorescence (AOF) from upper singlet states. 

This may undergo decomposition (D) or may be collisionally deactivated to the ground vibrational level of the upper singlet state (Ao1)-... 

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

The details of the photochemical primary process in cyclopentanone are fairly well understood (33). Since the spectrum in the first absorption region appears to be similar to that of the simple aliphatic ketones, it has been assumed that initial excitation is to the upper singlet state and that the zero-zero band is near 3600 A. 

Absorption of radiation by polyatomic molecules composed of light atoms follows, as regards change in multiplicity, the same rules as those for atoms and diatomic molecules. Since most polyatomic molecules have ground singlet states the vast majority of absorption processes form upper singlet states. In the presence... 

Carbon-halogen bond cleavage in aryl halides is believed to involve initial excitation into an upper singlet state that is stable with respect to cleavage, followed by intersystem crossing to an upper triplet state that is either dissociative itself or can cross to an upper dissociative (a, a ) triplet [65-67]. The two-color approach has been used to demonstrate directly that excitation to an upper triplet state results in carbon-halogen cleavage. For example, in 2-bromonaphthalene (81) and 9-bromo-phenanthrene (82) [68], promotion of Tj to an upper triplet by dye laser excitation... 

Intramolecular cycloaddition for 5-phenyltricyclo[5.2.1.02i6]deca-4,8-dien-3-one, 102, producing the cage product, 103 has been reported to proceed through an upper singlet state [82] (Scheme 8). The participation of an upper singlet was determined by a comparison of yields obtained using different excitation wave-... 

Rate coefficients and activation energies for proton-transfer reactions of species in the upper singlet state at 25 °C... 

The rates of intramolecular proton-transfer reactions, which are not limited by the need for reagents to diffuse together have also been studied by the fluorescence-quenching technique [85], For example, salicylic esters undergo protomeric isomerisation in the upper singlet state... 

It is realized that the mechanism just postulated is a greatly simplified version of a very complex situation unless either the fluorescence yield is unity and independent of pressure or there is no crossover to the triplet state. It must be mentioned that l/fc = to is the radiative lifetime of the upper singlet state, i.e., the quantity which in principle may be calculated from integrated absorption coefiScients. The observed lifetime will be l/13fci[M], where is the sum of all of the quantities... 

The data in Tables II-VI may not be used, therefore, to indicate precisely which absorption bands will be important with various mercury lines. It is evident on inspecting these tables that not only the vibrational energy but the modes of vibration in the upper singlet state will depend on the spectral characteristics of the absorbed light. Not enough data are at present available to relate mode of vibration to behavior but studies of this type should be made. 

The first electronic transition in butadiene has been the subject of many experimental and theoretical studies.The absorption, which has a maximum at 2100 A., is strong and represents a tt tt transition from a ground singlet to an upper singlet state. Analysis of the spectrum, which shows very little structure, has not been carried out. Since no fluorescent radiation has ever been detected on excitation of any of the simple dienes even at low temperature, a definite assignment of the 0 — 0 band has not been made. The 0 — 0 band had been placed at 2300 A. (124 kcal./mole), at which point the absorption is only /so as intense as at its maximum. The oscillator strength is 0.53, which leads to a radiative lifetime of 10 sec. Since emission of radiation has not... 

Excited States. The first absorption region in the ultraviolet spectrum of cycloheptatriene extends from 3300 to 2250 A. The absorption is intense and the transition is presumably a ir — ir excitation from the ground singlet to an upper singlet state. The radiative lifetime has been calculated to be about 10 sec. No fluorescence has actually been observed. There are no data available on the triplet states, but by comparison with the singlet — triplet absorption spectrum of 1,3,5-hexa-triene, which has been recorded by Evans, it is reasonable to assume that l,3,o-cycloheptatriene has its lowest triplet state at 47 kcal./mole or less. A second triplet state has also been reported for 1,3,5-hexatrienc by Evans. ... 

The second primary process, which leads to toluene, cannot occur from the same excited state as the first one. This excludes the upper singlet state. The quenching of the formation of toluene at high pressure indicates that the energy content in the reactive state is a critical factor... 

As a consequence of light absorption, excitation to the vibrationally excited upper singlet state occurs, viz. 

More detailed information concerning the mode of decomposition of the triplet and the upper singlet states was expected from the results of flash photolytic investigations However, the difficulties encountered in determining quantum yields and in obtaining monochromatic light under such conditions limit the usefulness of these results. Here, the discussion will be restricted to the pressure effect observed at high intensities. 

The above studies do not completely exclude the possibility of the upper singlet state playing some role at 3130 A at shorter wavelengths, the singlet decomposition is undoubtedly of significance. 

It has been established that the reciprocal of the quantum yield of the decomposition increased nearly linearly with the concentration of the quencher at low concentrations, but approached a limiting value at high quencher concentrations. Assuming that the limiting quantum yield is related to the decomposition occurring from the upper singlet state, the contribution of the excited singlet and triplet... 

At 3650 A, the efficiency of luminescence increases with increasing biacetyl pressure . This is just the opposite of what would be expected on the basis of the Stern-Volmer relation. Moreover, it was established that the primary decomposition quantum yield decreases with increasing pressure (the plot of Ijcf) versus biacetyl concentration gives a straight line). The results were explained by the assumption that the molecules absorbing radiation of 3650 A are excited to high vibrational levels (of the upper singlet state) from which dissociation can occur, but luminescence cannot. Luminescence can only occur if vibrational excitation is removed by collision. 

Very little is known about the photochemical behaviour of biacetyl at short wavelengths. At 3130 A and at shorter wavelengths, which are beyond the absorption minimum of biacetyl, light absorption excited the molecules to the second upper singlet state Heicklen claimed that at these wavelengths the... 

Th is the vibrationally excited, upper singlet state, the energized intermediate I might have a valene structure as suggested in the studies on arylthiophenes vide infra) but it is not a triplet state B is some other intermediate, probably a triplet since it is scavenged by O2. The Hg( Pi)-sensitized decomposition of thiophene probably proceeds by the same basic mechanism although the overall product quantum yields are much reduced. 

Other Processes - The involvement of the upper singlet state in the photochemistry of some 1,1-diarylethenes such as (29) has been assessed. Irradiation of (30) at 365 nm in chloroform solution brings about a 1,3-hydrogen... 

If photochemical reactions and intersystem crossings do not take place in the upper singlet states, then the general kinetic pattern of the photoprocesses occurring during absorption of light quanta can be expressed as follows ... 

The fluorescence emission discussed so far is produced by direct excitation of a molecule M to one of its excited singlet states that, after IC to Sx when an upper singlet state was initially populated, emits prompt fluorescence from Si with a lifetime on the order of nanoseconds. In addition, several processes can be envisaged that permit repopulation of Sx following ISC to Tx, which can give rise to emission that has the spectral characteristics of fluorescence, but a lifetime much longer than that of prompt fluorescence, that is, to delayed fluorescence. These processes can be sub-classified as P-type and 4E-type delayed fluorescence,32 We do not consider artificial delayed fluorescence due to impurities, or due to ionization followed by recombination with the formation of 1M. ... 

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