Triplet absorption

The physical properties of the xanthene type dye stmcture in general have been considered. For example, the aggregation phenomena of xanthene dyes has been reviewed (3), as has then photochemistry (4), electron transfer (5), triplet absorption spectra (6), and photodegradation (7). For the fluoresceins in particular, spectral properties and photochemistry have been reviewed (8), and the photochemistry of rhodamines has been investigated (9). 

Slower rates (k = 10 -10 s ) are measured with the method of time-resolved triplet-triplet absorption of the product after flash-photolysis (see, e.g., Grellmann et al. [1983]). 

Figure 9-15. Triplet absorption, singlet emission, and absorption (from left to right) of (from top to bottom) ni-LPPP [I44 and hexaphcnyl films 1145] at 90 K, and /n/ra-quater- and iciplienyl at 77 K in tetrahydro-2-methylfijran [146].

Nanosecond flash photolysis studies of Ct-nitronaphthalene in polar and nonpolar solvents led to the detection of the trip let-triplet absorption spectrum of this nitrocompd (Ref 26). 

By the total internal reflection condition at the liquid-liquid interface, one can observe interfacial reaction in the evanescent layer, a very thin layer of a ca. 100 nm thickness. Fluorometry is an effective method for a sensitive detection of interfacial species and their dynamics [10]. Time-resolved laser spectrofluorometry is a powerful tool for the elucidation of rapid dynamic phenomena at the interface [11]. Time-resolved total reflection fluorometry can be used for the evaluation of rotational relaxation time and the viscosity of the interface [12]. Laser excitation can produce excited states of adsorbed compound. Thus, the triplet-triplet absorption of interfacial species was observed at the interface [13]. 

In Section 3.1 it was shown that the photoreduction of benzophenone can be quenched by addition of small amounts of triplet quenchers such as oxygen or ferric dipivaloylmethide.<60) In fact this was presented as evidence that the benzophenone triplet was involved in the photoreduction. This reaction can also be quenched by naphthalene. In the presence of naphthalene, light is still absorbed by benzophenone and thus benzophenone triplets are produced. However, photoreduction products are decreased. On examining this reaction with flash photolysis, triplet-triplet absorptions were observed but these absorptions corresponded to those of the naphthalene triplet. Thus the triplet excitation energy originally present in the benzophenone triplet must have been transferred to naphthalene and since little of the photoreduction product was observed, this transfer must have been fast in relation... 

As is the case with singlet states, absorption of light by the triplet can occur to produce higher triplet levels. We have previously seen how triplet-triplet absorption by flash photolysis is used as a tool in photochemistry. 

When D and A are similar molecules emission-reabsorption cannot be very important due to the usually small overlap of the emission and absorption spectra. Also, this mechanism should not be important for triplet-triplet energy transfer because of (a) low phosphorescence quantum yields in fluid solutions and (b) the low oscillator strengths for singlet-triplet absorption. 

Porter and Wilkinson(56) measured the rates of quenching for a variety of triplet donors with triplet acceptors at room temperature in fluid solution by flash photolysis. The appearance of the triplet-triplet absorption spectrum of the acceptor and the simultaneous disappearance of the donor triplet-triplet absorption spectrum provided unequivocal evidence for the triplet-triplet energy transfer process. Table 6.5 provides some of the quenching rate constants reported in this classic paper. 

The absorption bands measured by the flash spectrographic method are often assigned by (a) comparison with known singlet-singlet absorption spectra, (b) comparison of the lifetime of the species responsible for the absorption with the phosphorescence lifetime, (c) comparison with calculated energies and intensities of the various possible absorptions by semi-empirical molecular orbital methods, and (d) comparison with published triplet absorption spectra and decay kinetics of model compounds. 

In contrast to the technique of oxygen perturbation to obtain enhanced singlet-triplet absorption spectra, the use of heavy-atom perturbation results in no significant changes in the position or energy of the singlet - singlet... 

Flash photolysis has been used to study the triplet-triplet absorption spectra (T1 -> Tq) of a number of aromatic molecules both in solution and in the gas phase.<34) A disadvantage associated with the flash technique for obtaining triplet-triplet absorption spectra is that the transient absorption can occur for only a short time after the initial flash (determined by the lifetime of triplet... 

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,...

Fig. 22. (A) Comparison of flavin triplet -> triplet absorption spectra (downwards drawn) with bluelight-induced (440 nm) phototropic curvature of aVena coleoptiles as inhibited by strong monochromatic light in the long wave visible region 154). (B) Comparison of the growth response of Phycomyces induced by strong laser light of wavelength longer than 590nm46, with the flavin phosphorescence spectrum los)...

Itoh, M. and Mulliken, R.S. (1969). Singlet-triplet absorption bands of methyl-substituted ethylenes. J. Phys. Chem. 73, 4332-A334... 

Population of the triplet manifold by direct singlet-triplet absorption is a very inefficient process, being spin-forbidden. Instead, the triplet manifold is populated indirectly by excitation into the singlet manifold followed by intersystem crossing. 

Since the photon emitted by D is absorbed by A, the same rules will apply to radiative energy transfer as to the intensity of absorption. Because singlet-triplet transitions are spin-forbidden and singlet-triplet absorption coefficients are usually extremely small, it is not possible to build up a triplet state population by radiative energy transfer. For this... 

B. Hicks, M. White, C. A. Ghiron, R. R. Kuntz, and W. A. Volker, Flash photolysis ofhuman serum albumin Characterization of the indole triplet absorption spectrum and decay at ambient temperature, Proc. Natl. Acad. Sci. U.S.A. 75, 1172-1175 (1978). 

---