Macrocyclic Bridgehead Fluorophores, Pyrrolyl-diazabicyclo[8.3.1]tetradecadienones, with Giant Stokes Shifts

A previously unknown class of fluorophores was discovered, which represents 14-membered bridgehead heterocycles, pyrrolyl-diazabicyclo[8.3.1]tetradecadienones, herein referred to as PY-14-ONEs. The new fluorophores are characterized by giant Stokes shifts of ∼8000-10,250 cm^-1 and virtually zero overlap of the absorption and emission bands. They exhibit fluorescence maxima in the blue-green region (454 ≤ λ_em ≤ 513 nm, MeCN), which shift to the red side when converted to their water-soluble salts by alkylation with MeI (478 ≤ λ_em ≤ 516 nm, water). PY-14-ONEs were obtained by an original synthesis from DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene, which reacts with acylethynylpyrroles without catalysts under mild conditions to afford PY-14-ONEs in a 34-58% yield. The reaction represents a ring expansion of DBU. Since acylethynylpyrroles are readily available, the discovered reaction opens promising possibilities for the development of new fluorophores. The results of our time-dependent DFT calculations indicate that the pyrrole ring in PY-14-ONEs plays an important role in the formation of the Stokes shifts, which can be further enhanced by attaching appropriate substituents to it, capable of creating in S₁ an extended conjugated system and causing a substantial alternation of the molecular structure via its planarization.