The excited states and vibronic spectroscopy of diphenyldiacetylene and diphenylvinylacetylene

Abstract

Laser induced fluorescence (LIF) excitation scans and dispersed fluorescence (DFL) spectra have been recorded for two four-carbon α,ω-diphenyl systems, diphenyldiacetylene (DPDA, ϕ-CC–CC-ϕ) and trans-diphenylvinylacetylene (DPVA, ϕ-CHCH–CC-ϕ) as isolated molecules cooled in a supersonic expansion. While these molecules have similar conjugation length, they exhibit strikingly different vibronic spectroscopy and photophysics. The near-UV LIF excitation spectrum of diphenyldiacetylene has its electronic origin at 32 158 cm⁻¹, and a strong progression in the CC stretch (2156 cm⁻¹). All transitions are inherently broad, with widths of ∼30 cm⁻¹ fwhm or greater. The S₁ origin DFL spectrum is composed of sharp transitions with Franck–Condon activity mirroring that in the excitation spectrum, and broad emission shifted well to the red ascribable to phosphorescence on the μs timescale. Using ab initio calculations, it is possible to show that DPDA exists as a single, planar conformer with D_2h symmetry. In contrast, trans-diphenylvinylacetylene shows intense sharp transitions in both LIF and DFL spectra with an S₀–S₁ origin of 31 183.2 cm⁻¹ and long progressions involving the in-plane fundamentals ν₅₃ (bridge-phenyl bending) and ν₅₁ (bridge-phenyl stretch). A sharp reduction in fluorescence yield in DPVA occurs within 300 cm⁻¹ of the S₁ origin. Possible causes for the photophysical processes occurring in the two molecules are discussed.