Photonic materials derived from the [closo-B10H10]2− anion: tuning photophysical properties in [closo-B10H8-1-X-10-(4-Y-NC5H5)]−

Abstract

The parent pyridine [closo-B₁₀H₉-1-NC₅H₅]⁻ was substituted either at the antipodal B(10) position with CN, OAc, N₃, I, Br, SCN, pyridine, OEt, and morpholine, or at the C(4) position of the pyridine ring with CN, COOEt, Me, and OMe groups. The substituent effects on electronic absorption and emission properties, and also on the boron cage geometry were investigated experimentally and with DFT (B3LYP/Def2TZVP) computational methods. Experimental and theoretical results were correlated with Hammett σ_p parameters. Fluorescence was also investigated in the solid state and from aggregates (AIE). Solvent effects on photophysical properties of [closo-B₁₀H₉-1-NC₅H₅]⁻ were correlated with E_T30 parameters, giving a slope of 0.71 for absorption and 0.17 for emission. Results demonstrated the substantial impact of the B(10) substituent on the HOMO and the C(4) substituent on the LUMO of the derivatives, which allows variation of the energy of the (π,π*) intramolecular charge transfer band in the range of 330–450 nm, and the emission energy in the range of 530–580 nm in MeCN solutions. The substituent effect on excitation energy is 2.4 times greater for substitution at the pyridine ring (LUMO control) than for the B(10) position (HOMO control). Additivity of the substituent effect was tested on [closo-B₁₀H₈-1-(NC₅H₄CN)-10-OEt]⁻ with λ_max = 501.5 nm in MeCN and 560 nm in THF. These studies indicate that a substantial degree of control over photophysical properties is possible in derivatives of [closo-B₁₀H₁₀]²⁻ through a combination of substituent and solvent (medium) effects.