Thiophosgene monomer–dimer equilibria in perfluoroalkane solutions: spectroscopic and photophysical implications of self-association in the ground state

Abstract

The S2–S0 absorption, emission, and emission excitation spectra of thiophosgene in a perfluoroalkane solvent have been measured as a function of the analytical concentration, C, of the solute. Strong evidence of the formation of van der Waals dimers at 10−4 M < C < 10−1 M and of higher oligomers at C > 10−1 M has been found. Variations in the absorption spectra at C < 10−1 M have been analyzed successfully in terms of a single [Formula: see text] equilibrium, for which the value of K is (5.0 ± 0.4) × 102 M−1 at 295 K. The mole fractions of monomer and dimer have been calculated at each C, and have been used to construct the absorption spectrum of the dimer. Both the monomer and dimer have similar S0 → S1, and S0 → S2 absorption spectra, but the dimer has an oscillator strength that is about twice that of the monomer. The effects of self-aggregation on the S2–S0 emission and excitation spectra have been analyzed. Absorption by the dimer in the ultraviolet also leads to emission, but with a different quantum efficency compared with direct excitation of the monomer. The implications of these observations on the interpretation of the spectroscopy, photophysics, and photochemistry of thiophosgene in solution are assessed.

Fluorescence and Phosphorescence from Higher Excited States of Organic Molecules

“…214 Most of the research on the emission properties of thiophosgene was carried out in the vapor phase, but the emission from the three excited states, S 2 , S 1 , and T 1 , has been observed also in solution. 215,216 The S 1 absorption of thiophosgene is located at ∼550 nm and the much stronger S 2 absorption is located at about 260 nm, with a relative S 2 /S 1 absorption intensity ratio of ∼10 3 (ΔE(S 1 − S 2 ) = ∼20 000 cm −1 ). The quantum yield of the S 2 fluorescence of thiophosgene vapor is reported to be as high as 0.5−1.0, indicating the lack of efficient nonradiative processes from S 2 .…”

Spectral and Photophysical Properties of Aromatic Thiones in Micellar Systems: Xanthione as Convenient Absorption and Emission Probes

“…PT solubility determined in the presence of 0.15 M of SDS is about 100 times higher tive description of PT properties in SDS micelles alone. Changes in the absorption spectrum of PT in water within hand, this band for PT in water has a very similar shape to that in micelle and shows a strong hypsochromic shift when the concentration range (0.5-6) 1 10 03 M are attributed to the process of dimerization of thioketone molecules, in anal-compared to the spectrum in ACN, resulting in the shift of this band to the long wavelength part of the much more ogy to what has been observed for thiophosgene in perfluoroalkanes (69,70) and as it has been shown for a number intense S 2 (p, p*) band. Such a significant shift of the S 1 band to the blue probably results from the formation of of various compounds in water (71).…”

Evidence of a solvent-mediated barrier to radiationless decay in the state of thiophosgene in solution

“…In almost all experiments C12CS concentrations were kept below ca. 4 x lo4 M to minimize the effects of ground state van der Waals dimer formation known to be a problem at higher concentrations (12).…”

“…Perfluoroalkanes are very "poor" solvents (24), so it is reasonable to assume that the energy gained through dispersive interactions among clustered CC14 or nhexane molecules could offset the entropic deficit associated with their formation. Thiophosgene itself forms van der Waals dimers and oligomers at relatively low concentrations in perfluoroalkanes (12). It is also interesting to note that for systems such as mixed noble gas liquids in which dispersive interactions are responsible for solvation enthalpies, the coordination number of the "solute" by the "solvent" is found to be either somewhat greater or somewhat smaller than those of self-solvation (1 1-13), depending upon the relative sizes and polarizabilities of the species involved (22 The effects of such clustering would be difficult to observe by standard spectroscopic means.…”

“…For example, Okabe (9) has reported the quantum yield of C1 atom production is 1 at I,, = 253.7 nm (39 417 cm-I), whereas Ondrey and Bersohn (10) In solution the formation of ground state van der Waals dimers of C12CS (12) and the existence of a nearby very weak absorption, tentatively assigned to a "dark", doubly excited state (5), must be taken into account in order to model quantitatively the photophysical processes of the B" state (8,13). When the relaxation of only monomeric C12CS (B") is untangled from these complications, one finds that, in contrast to the isolated molecule, radiationless processes account for a very large fraction of decay events in solution at room temperature even when bound portions of the B state are initially populated.…”