Realization of nitroaromatic chromophores with intense two-photon brightness

Strong fluorescence is a general feature of dipyrrolonaphthyridinediones bearing two nitrophenyl substituents. Methyl groups simultaneously being weakly electron-donating and inducing steric hindrance appear to be a key structural parameter that allows for significant emission enhancement, whereas Et2N groups cause fluorescence quenching. The magnitude of two-photon absorption increases if 4-nitrophenyl substituents are present while the contribution of Et2N groups is detrimental. Effective manipulation of electronic effects within substituted nitroaromatics based on the dipyrrolonaphthyridinedione (DPND) core leads to fluorescent dyes with enhanced emission intensity as well as two-photon response.