Spectroscopic Evaluation of Novel Adenine/Thymine-Conjugated Naphthalenediimides: Preference of Adenine-Adenine over Thymine-Thymine Intermolecular Hydrogen Bonding in Adenine- and Thymine-Functionalized Naphthalenediimides

The synthesis and spectroscopic characterization of novel nucleobase (adenine/thymine)-conjugated naphthalenediimides (NDIs), namely, NDI-AA, NDI-TT, and NDI-AT have been successfully achieved. NDI-AA, NDI-TT and NDI-AT have similar absorption in the 300–400 nm region. The effect of solvent on the absorption spectrum indicates aggregation, either through intermolecular π-σ interaction among the main chromophore or through intermolecular hydrogen bonding between adenine and adenine group. Addition of water does not assist hydrogen bond formation between thymine-thymine, rather increasing the polarity of the solvent encourages π-σ interaction among NDI-TTs. No spectral change for NDI-TT with increasing temperature confirms hydrogen bonding is not playing a crucial role in NDI-TT. A fluorescence study on NDI-AA also establishes excimer formation along with ground state aggregation. As the water content in the solvent mixture increases, aggregation of NDI-AA is discouraged due to adenine-adenine hydrogen bonding in accordance with earlier results. At the same time, the NDI-TT emission spectrum does not shift to the blue region and the intensity of the peak around 535 nm increases at the expense of fluorescence in 411 nm. Thus, increasing water content in the solvent mixture facilitates aggregation through π-σ interaction in NDI-TT as thymine-thymine hydrogen bonding is less pronounced.