Simultaneous enhancement of transition dipole strength and vibrational lifetime of an alkyne IR probe π-d backbonding and vibrational decoupling

• Published in: Physical chemistry chemical physics : PCCP Vol. 21; no. 45; pp. 24919 - 24925
• Main Authors: Kossowska, Dorota, Lee, Giseong, Han, Hogyu, Kwak, Kyungwon, Cho, Minhaeng
• Format: Journal Article
• Language: English
• Published: 20.11.2019
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c9cp04356j

Alkyne infrared (IR) probes 1-6 with Si and S (or Se) atoms incorporated into the C&z.tbd;C bond were synthesized, and the vibrational properties of their C&z.tbd;C stretch mode were studied using Fourier transform infrared (FTIR) and femtosecond IR pump-probe (IR PP) spectroscopies in combination with quantum chemical calculations. From FTIR studies, the transition dipole strengths (in units of 10 −2 D 2 ) of 1-3 having the Si atom were measured to be 1.85, 3.32, and 2.52, whereas those of 4-6 having no Si atom were measured to be 0.13, 0.20, and 0.17, respectively, in CHCl 3 . Thus, the increase in the transition dipole strength of the C&z.tbd;C stretch mode upon incorporation of the Si atom into the C&z.tbd;C bond is by a factor of about 14 or higher. The large increase in the transition dipole strength of the C&z.tbd;C stretch mode upon such Si incorporation is attributed to π-d backbonding between the C&z.tbd;C group's π and Si atom's d orbitals. From IR PP experiments, the vibrational lifetimes of the C&z.tbd;C stretch mode in 1-3 having none, S, and Se atoms were determined to be 5.7 ± 0.7, 13.0 ± 1.1, and 94.2 ± 5.8 ps, respectively, in CHCl 3 . Thus, the increase in the vibrational lifetime of the C&z.tbd;C stretch mode upon incorporation of the S (or Se) atom between the phenyl ring and the C&z.tbd;C bond is by a factor of about 2 (or 16) or higher. The large increase in the vibrational lifetime of the C&z.tbd;C stretch mode upon such S (or Se) incorporation is attributed to its heavy atom effect impeding vibrational couplings between the C&z.tbd;C stretch and phenyl ring vibrations. From two-dimensional infrared (2DIR) experiments, the large transition dipole strength and long vibrational lifetime of 3 containing the Si and S (or Se) atoms were shown to enable the measurement of its 2DIR spectra up to 500 ps. The strongly absorbing alkynes with long vibrational lifetimes will be a promising probe of molecular dynamics in nonlinear vibrational spectroscopy and imaging on an extended time scale. Alkyne IR probes 1-6 with Si and S (or Se) atoms incorporated into the C&z.tbd;C bond were synthesized, and the vibrational properties of their C&z.tbd;C stretch mode were studied using FTIR and femtosecond IR PP spectroscopies and quantum chemical calculations.