Unveiling the effect of thiol functionalization on the photophysical and nonlinear optical properties of higher diamondoids: insights from the DFT approach

The rational control of the optoelectronic and nonlinear optical (NLO) properties of higher diamondoids (HDs) is the focus of the present study, which remains challenging. Functionalizing the HDs with a single thiol (-SH) group plays a vital role in tuning the energy gap, enhancing charge transfer and improving NLO behaviour. Density functional theory (DFT) calculations were performed for the pristine HDs (from tetramantane to heptamantane, including their isomeric structures) and thiolated HDs. Within the time-dependent framework of DFT using the PBE0 level of theory, the absorption and emission profiles were calculated and their structure-property relationship was presented. Larger Stokes shifts obtained for the functionalized HDs reveal enhanced fluorescence detection sensitivity. While the pristine units exhibit phosphorescence in the UV region, thiolated units show red emission by stabilizing the triplet excited states. Furthermore, the thiolated HD units exhibit higher dipole moments and polarizabilities, confirming their enhanced NLO properties. All these findings highlight the transformative potential of functionalized diamondoids in the design of advanced materials for optoelectronic and photonic technologies.