Ultrafast Vibrational Dynamics at Cyano Defect Sites in Carbon Nitride Revealed by Two-Dimensional Infrared Spectroscopy

We investigate the ultrafast structural and vibrational energy-transfer dynamics of a cyano-modified graphitic carbon nitride hydrocolloid (CMCNC) using linear and two-dimensional infrared (2D IR) spectroscopies. The dominant species is identified as a cyanodefect tris-s-triazine dimer. Three distinct cyano stretching bands exhibit noticeable inhomogeneous broadening, reflecting enhanced local flexibility and solvent interactions due to partial disruption of the conjugated framework of g-CN. In contrast to the nearly static line shapes of pristine g-CN, 2D IR reveals pronounced spectral diffusion on sub-ps to ps time scales, indicating increased molecular mobility and dynamic solvent coupling─features that correlate with improved charge-carrier stability in the corresponding solid photocatalyst. Intramolecular vibrational energy redistribution (IVR) among the cyano modes occurs with time constants of ∼2 ps, while vibrational relaxation proceeds with an average lifetime of ∼1 ps. These IVR rates are slower than those among delocalized C═N modes in unmodified g-CN, consistent with the more localized character of the cyano oscillators. This work provides the first direct observation of ultrafast vibrational dynamics at cyano defect sites in carbon nitride, offering a molecular-level description of tailored structural flexibility and picosecond energy flow in a modified carbon nitride system, with implications for the design of efficient photocatalytic materials.