Synthesis and photophysical properties of quaternaphthalenes: topology-dependent radiative dynamics in deep-blue...

A series of quaternaphthalenes (QNps) containing four naphthalene chromophores were synthesized and investigated as all-hydrocarbon multi-chromophore emitters. The QNps exhibited near-UV to deep-blue fluorescence in solution and retained emission in the solid state. The negligible solvent dependence of the fluorescence spectra supports locally excited π-π* emissive states rather than intramolecular charge-transfer states. Photophysical analysis revealed that the fluorescence behavior is governed primarily by naphthalene connectivity. Several QNps, especially 126151 and 126261, showed high fluorescence quantum yields and large radiative rate constants, whereas 126272 exhibited a smaller radiative rate and a longer fluorescence lifetime, indicating suppression of the radiative transition probability by a specific linkage topology. TD-DFT calculations showed that highly emissive QNps possess large oscillator strengths and mainly HOMO-LUMO-based S₁ ← S₀ transitions. A positive Strickler-Berg-type analysis including related ternaphthalene data further supports the relationship between topology-dependent oscillator strength and radiative dynamics, while indicating a lower k_f/f trend for 2,7-linked polynaphthalene derivatives. These results demonstrate that efficient deep-blue emission in polynaphthalene systems is achieved not by simply increasing the number of chromophores, but by optimizing the connectivity that preserves a large radiative transition probability.