Triplet ground state direct observation

Both l3A"-4b and 23A"-4b are predicted to be minima on the potential surface at the CASSCF level.61 This is in contrast with the situation for 4a, where one triplet state is calculated to be a transition state for pseudorotation of the other.55-57 A derivative of triplet 4b has been implicated in solution trapping experiments,49 but there have been no reports of the direct observation of 4b. Facile intersystem crossing from the triplet ground state (13A") to the open-shell singlet state ( A"), followed by vibrational relaxation to one of the enantiomers of 3b (Fig. 12), would be expected to rapidly depopulate the 13A" state of 4b. Thus, direct detection of triplet 4b may prove to be much more difficult than detection of 4a was.lla... 

The transient T-T absorption in the gas phase has been measured recently for aromatic molecules such as naphthalene (119,211) and anthracene (80,81) using flash kinetic spectroscopy and tandem laser pulse absorption techniques. Particularly, the later technique (211) provides time-dependent absorption spectra of the "isolated" unrelaxed triplet molecules because of its capability for rapid monochromatic excitation and detection. It will certainly provide a wealth of Important kinetic and spectroscopic information about the evolution and decay of triplet states. Direct observation of the formation of transient hot ground-state (Sq) molecules through an internal conversion process has also been achieved with laser excitation and laser... 

ESR parameters for triplet carbenes15 and nitrenes16 have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices.17 The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the S0 nitrene (Amax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, Amax 415 nm) with a rate constant of about 4.4 x 107 sec. Tt is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (T2) was about 7 nsec. T dimerizes to azopyrene.18... 

The matrix-isolation technique is the method of choice for the direct spectroscopic observation of carbenes . However, efforts to generate and to observe silylcarbenes in solid matrices at cryogenic temperatures met with limited success. When (trimethylsi-lyl)diazomethane, (dimethylsilyl)diazomethane or bis(trimethylsilyl)diazomethane were irradiated in an argon matrix at <10 K, no IR spectra of the corresponding carbenes 3a-c could be obtained " . However, weak ESR signals were observed which were typical for a linear carbene with a triplet ground state . These results indicate that at least small amounts of carbenes 3a-c were present in the matrix. 

For a triplet to result, two electrons must have different orbitals and the same spin (the Pauli principle forbids having two unpaired electrons in the same orbital since all four quantum numbers would be the same). We saw in Chapter 1 that upon light absorption an electron is promoted from one orbital to a higher orbital. If we were to excite directly from the ground state singlet to the triplet state we would have to simultaneously change orbitals and electronic spins. Since this process is relatively improbable, direct absorption to the triplet state is seldom observed (later in this chapter,... 

In contrast to the behavior of BA, triplet sensitized formation of XA in the presence of alcohol gives exactly the same result as does the direct irradiation ether is formed in very high yield. Thus, even when 3XA is formed first, the chemical behavior observed is that of XA. This finding is consistent with a carbene having a singlet ground state a view at least supported by the absence of an epr spectrum for the triplet carbene as noted above. 

Thus, reaction (8.30) could specify either an excited singlet or triplet S02. The excited state may, of course, degrade by internal transfer to a vibration-ally excited ground state that is later deactivated by collision, or it may be degraded directly by collisions. Fluorescence of S02 has not been observed above 2100 A. The collisional deactivation steps known to exist in laboratory experiments are not listed here in order to minimize the writing of reaction steps. 

In the delayed emission spectrum of eosin in glycerol or ethanol two bands are present, the relative intensities of which are strongly temperature-dependent (see Fig. 12). The visible band at 1.8 has a contour identical with that of the fluorescence band. It no doubt corresponds to the visible phosphorescence observed by Boudin.26 To interpret the results it was assumed that this band of delayed fluorescence was produced by thermal activation of the eosin triplet to the upper singlet level followed by radiative transition from there to the ground state. The far red band was assumed to correspond to the direct transition from the triplet level to the ground state and was therefore called phosphorescence. To determine the relationship between the intensities of the two bands we write the equations for the formation and consumption of triplet molecules as follows ... 

The problem of the decay of exciplexes arises directly from their stability. There is no simple and unambigous relation between the electron donor ability of an excited complex A, electron withdrawing property of its reaction partner Q, and the decay rate constant kq. This experimental observation may be understood on the basis of two deactivation pathways of exciplexes (A — Q) a spin-forbidden conversion to the singlet ground state (GS) molecules A and Q, e.g. for a triplet exciplex 3(A — Q)... 

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