Nitrogen effect in aromatic cores on the photophysical properties of 2,6‐diphenyl‐substituted naphthalene derivatives

Abstract Designing luminophores with tunable emission properties is crucial for the development of responsive optical materials, such as pH ‐responsive compounds. Here, we report a structure–property investigation on a series of diphenyl‐substituted aromatic structures S8 (2,6‐diphenylnaphthalene), S11 (3,7‐diphenylquinoline), and S14 (3,7‐diphenylcinnoline) that vary by the number of nitrogen atoms incorporated into their aromatic cores. Through systematic photophysical characterization across multiple solvents, we demonstrate that nitrogen insertion progressively bathochromic‐shifts absorption and emission bands while concurrently decreasing fluorescence quantum yield and excited‐state lifetimes. To modulate this behavior, we employed both Brønsted acid protonation with trifluoroacetic acid ( TFA ) and Lewis acid coordination with tris(pentafluorophenyl)borane ( BCF ). Protonation induces red‐shifted absorption and emission with solvent‐ and counter‐ion‐dependent fluorescence recovery, whereas BCF adduct formation in chloroform provides clean isosbestic behavior, quantitative suppression of non‐radiative decay, and strong fluorescence turn‐on. Time‐resolved fluorescence studies and time‐dependent density functional theory ( TD ‐ DFT ) calculations support these findings, revealing that protonation effectively suppresses the n–π* and facilitates π–π* transitions. This study establishes a connection between nitrogen incorporation, protonation state, and emission behavior in 2,6‐diphenylnaphthalene derivatives. The acid‐triggered fluorescence activation observed in S14 highlights its potential as a promising platform for stimuli‐responsive fluorescent probes, pH indicators, and molecular optical switches.