Rational Design of an Acetylenic Infrared Probe with Enhanced Dipole Strength and Increased Vibrational Lifetime.

Developing infrared (IR) probes is of great interest in biomolecular imaging and spectroscopy. We report our attempt to improve the IR properties of alkyne-derivatized compounds. The vibrational properties of the alkyne (C≡C) stretch mode of aromatic silylacetylene 1 and aliphatic silylacetylene 2 were studied using Fourier transform infrared and femtosecond IR pump-probe spectroscopies. We find that the insertion of silicon at the position adjacent to the alkyne group, separating it from the compound's main body, causes an approximately 10-fold increase in the dipole strength of the C≡C stretch mode and a lengthening of its vibrational lifetime from 5.6 ps for the acetylenic compound without a silicon atom acting like a thermal insulator to 50.6 and 50.4 ps for 1 and 2, respectively. The enhanced dipole strength and the increased lifetime of 1 allowed us to measure the 2D IR spectra for long waiting times up to 450 ps, which suggests that the dynamic observation range of 2D IR spectroscopy with these IR probes can be extended into the subnanosecond range where protein skeletal movements occur.