How Progressive Hydrogenation Shapes the Optical Response of Carbon Clusters

Hydrogenated carbon clusters represent structural intermediates conveniently placed between isolated molecular hydrocarbons and extended carbon materials in which hydrogen content and chemical functionalization critically determine both structure and optical response. In this work, we present a computational investigation of medium-sized carbon clusters (ranging from 20 to 35 carbon atoms) with different degrees of hydrogenation, nitrogen doping, and amino-functionalization. A fairly large ensemble of structures was generated using ab initio methods showing that hydrogenation induces a significant structural reorganization, progressively disrupting π-conjugation and increasing sp3 character. Time-dependent density functional theory calculations were carried out for the C30 family of clusters, showing that increasing hydrogenation induces a systematic blue shift of the absorption spectra and a depletion of low-energy transitions, consistent with the reduction of π-conjugated domains. In contrast, nitrogen doping and amino-functionalization partially counteract this effect by inducing the presence of localized electronic states that produce absorption in the visible range. Partially hydrogenated C30 clusters exhibit large Stokes shifts, further enhanced by amino functionalization, while N-doping promotes visible emission and excited-state localization. We conclude that hydrogenation and nitrogen doping generally act as two complementary driving forces on the optical properties of carbon clusters and disordered carbon-based systems.