Photophysical Impact of Diacetylenic Conjugation on Classical Donor-Acceptor Electronic Energy Pair

Organic fluorophores with extended π-conjugation are important for their widespread applications. The present work provides photophysical insights into a diacetylene bridged classical donor-acceptor electronic energy pair, naphthalene-pyrene, in comparison with its constituents' molecular structures, naphthyl and pyrenyl acetylenes, as well as parent naphthalene and pyrene chromophores. The diacetylenic dye loses the individual spectral identities of the donor and acceptor fluorophores exhibiting a locally excited (LE) emission (∼411 nm) from the overall molecular entity with high fluorescence quantum yields (0.55-0.84) in nonaqueous media. In contrast to the parent pyrene, the hybrid derivative shows a strongly allowed S₀ → S₁ transition. In mixed-aqueous media, the dye forms aggregates displaying a new red-shifted absorption (∼425 nm) as well as emission (∼510 nm) band. Unlike the hybrid dye, the naphthyl and pyrenyl acetylenes do not form aggregates. In the aggregate state of the hybrid fluorophore, electronic energy transfer takes place from the naphthyl moiety to pyrenyl ring. The excited-state photophysical properties of the dye are exploited in vapor sensing in the solid state.