Molecular orbital node engineering in pyrene: linking chemical reactivity with room-temperature phosphorescence.

Substitutions at the nodal and anti-nodal positions of conjugated aromatics exert profound effects on their electronic properties, yet a systematic investigation is lacking on how these underlying quantum mechanical rules are manifested experimentally. Here, using polycyclic aromatic hydrocarbon derivatives as a model system, we systematically elucidate how nodal and anti-nodal substitutions dictate their chemical reactivity and physical properties. It is found that Sonogashira C-C coupling at the nodal position has a noticeably lower product yield than the anti-nodal position under identical reflux conditions due to inhibited molecular orbital amplitude. Comprehensive spectroscopic characterization of the resulting substituted products reveals two striking photophysical differences: (1) Frontier orbital symmetry is largely conserved in node-substituted molecules, whereas the anti-nodal substitution induces strong symmetry-breaking, leading to accelerated fluorescence emission in anti-node-substituted pyrenes; (2) Nodal substitution induces the formation of charge-transfer states due to a twisted geometry, which activates room-temperature phosphorescence via improved singlet-triplet intersystem crossing. This study establishes an effective structure-property relationship linking molecular orbital symmetry to macroscopic cross-coupling reactivity and molecular photophysics within these conjugated aromatics.