The new anionic carbon sulfides C6S10(2-) and C12S16(2-) are described and crystallographically characterized. The C12S16(2-) anion consists of two C6S8 units connected by an exceptionally long (2.157(12) A) S-S bond. In solution, C12S16(2-) exists in equilibrium with the radical C6S8(-*). The equilibrium constant for radical formation (293 K, THF) is 1.2 x 10(-4) M, as determined by optical spectroscopy at varying concentrations. Radical formation occurs through scission of the S-S bond. On the basis of variable temperature EPR spectra, the thermodynamic parameters of this process are DeltaH = +51.5 +/- 0.5 kJ x mol(-1) and DeltaS = +110 +/- 3 J x mol(-1) x K(-1). C6S10(2-) is an oxidation product of C3S5(2-) and consists of two C3S5 units connected by an S-S bond. The S-S bond length (2.135(4) A) is long, and the CS-SC torsion angle is unusually acute (52.1 degrees ), which is attributed to an attractive interaction between C3S2 rings. The oxidation of (Me4N)2C3S5 occurs at -0.90 V vs Fc+/Fc in MeCN, being further oxidized at -0.22 V. The similarity of the cyclic voltammogram of (Me4N)2C6S10 to that of (Me4N)2C3S5 indicates that C6S10(2-) is the initial oxidation product of C3S5(2-).

The molecular third-order optical nonlinearity γR (second hyperpolarizability or nonlinear refractive index) was measured for a series of sulfur-rich molecules:  sulfur (S8), carbon−sulfur compounds, and metal thiolate clusters. Z-scan techniques (pulse width 27 ps, wavelength 527 nm) were used to measure these properties in solution by comparing the solution to the pure solvent. Our approach is an efficient way to evaluate a number of different compounds and to quickly direct synthetic strategies for improved nonlinear and linear optical properties. The optical nonlinearities were evaluated in terms of figures of merit, |W|/I 0 and [T]-1, where |W|/I 0 is the ratio of nonlinear refraction to linear absorption and [T]-1 is the ratio of nonlinear refraction to nonlinear absorption. Among the carbon−sulfur compounds, C6S8O2 had the largest figures of merit:  |W|/I 0 = 4.3 × 10-13 m2 W-1 and [T]-1 > 5. The metal thiolate cluster with the largest second hyperpolarizability was [Zn10S4(SPh)16]4- (γR = −7.8 × 10-56 C m4 V-3, −6.3 × 10-31 esu). This cluster exhibited no measurable linear or nonlinear absorption, so the figures of merit were effectively infinite. Previous work on the second hyperpolarizability of sulfur-rich compounds examined species that were hampered by substantial linear absorption coefficients. The present work shows that high figures of merit can be achieved without significant linear or nonlinear absorption.

New chemical relationships have been established for C2-based carbon sulfides, as shown in the illustration below.

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