Phosphorescence of Hydrogen-Capped Linear Polyyne Molecules C₈H₂, C₁₀H₂ and C₁₂H₂ in Solid Hexane Matrices at 20 K

Round 1

Reviewer 1 Report

The authors performed a comprehensive experimental study on the phosphorescence of three unstable polyyne molecules C8H2, C10H2, and C12H2 in solid hexane at 20 K. For each species, the phosphorescence 0-0 band and the associated vibrational progressions were observed and compared with the observations in the gas phase and in the hexane solution at room temperature. The lifetimes of the excited triplet state at 20 K were also determined. Furthermore, the detailed annealing experiments for the phosphorescence bands of C8H2 in solid hexane were performed and discussed. All the experiments were well designed, the spectroscopic data were carefully analyzed, and the paper is well prepared. The analysis is convincing and supported by the rigorous comparison with the gas-phase studies. Given the interest of polyynes in the interstellear carbon-chemistry and the fundamental photophysical information they provide based on the spectral analysis, I believe this manuscript meets the scope of Photochem. and can be published after considering a minor comment on the experimental setup.

In the section of Method, it was mentioned that the hexane solutions of the polyynes were sprayed onto a nearby cold cupper slab through a tubing of 0.25 mm. I supposed polyynes have quite different volatility from the solvent, then how to make a homogeneous solution (or solid matrix) during the sample deposition in the vacuum? Since polyynes polymerize easily, did the title compounds polymerize during the the course of deposition due to increased concentration by solvent evaporation? Perhaps more details can be described in the Method section.

Author Response

Polyyne C8H2 is volatile as the solvent hexane is. C10H2 and C12H2 are less volatile, but the deposition of the solution is so quick as to condense it even as droplets. Thus, possibility of dimerization or polymerization of polyyne molecules upon are minimized. Some detailed are described in the revised manuscript.

Reviewer 2 Report

The authors aim to study the phosphorescence of hydrogen-capped linear polyyne molecules C8H2, C10H2, and C12H2 isolated in solid hexane matrices at low temperatures (20 K). UV laser excitation was used to excite the molecules and the 0-0 to 0-4 phosphorescence bands could be observed. Phosphorescence excitation spectra were also obtained and is similar to the one taken in hexane solution and in gas phase at room temperature, although some minor redshift could be noticed. The lifetime of the triplet state was shown to be inversely proportional to the length of the carbon chain. The symmetry-forbidden vibronic transitions for C10H2 were also detected.

The manuscript is well-written and it was a really interesting read. I wholeheartedly recommend it for publication, I have found a couple of minor typos that should be corrected though.

Line 155: gauss functions -> Gaussian functions

Lines 156–158: "Among the two sets of the narrow-band features, the one is the major component of high intensity and the other is the minor component at the shoulder closely locating to the position of peaks in the solution at ambient temperature." remove the "the" after comma: "Among the two sets of the narrow-band features, one is the major component of high intensity and the other is the minor component at the shoulder closely locating to the position of peaks in the solution at ambient temperature."

Line 338: Define HOMO and LUMO here. (The definitions are given in Lines 348 and 350, delete them.)

Lines 430–432: "Under the room temperature condition of the solution, fluorescence was detectable but phosphorescence, reasonably because of the fast relaxation via internal conversion at ambient temperature." something is missing in the second clause of the sentence.

Author Response

Descriptions in the lines 155, 156-158, 338, 348-350, and 430-432 are modified according to the suggestions by Reviewer 2. Other changes for clarification are listed in the attached cover letter.

Author Response File: Author Response.pdf