Macrocyclic Bridgehead\nFluorophores, Pyrrolyl-diazabicyclo[8.3.1]tetradecadienones,\nwith Giant Stokes Shifts

A previously unknown class of fluorophores was discovered,\nwhich\nrepresents 14-membered bridgehead heterocycles, pyrrolyl-diazabicyclo[8.3.1]tetradecadienones,\nherein referred to as PY-14-ONEs. The new fluorophores are characterized\nby giant Stokes shifts of ∼8000–10,250 cm<sup>–1</sup> and virtually zero overlap of the absorption and emission bands.\nThey exhibit fluorescence maxima in the blue-green region (454 ≤\nλ<sub>em</sub> ≤ 513 nm, MeCN), which shift to the red\nside when converted to their water-soluble salts by alkylation with\nMeI (478 ≤ λ<sub>em</sub> ≤ 516 nm, water). PY-14-ONEs\nwere obtained by an original synthesis from DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene,\nwhich reacts with acylethynylpyrroles without catalysts under mild\nconditions to afford PY-14-ONEs in a 34–58% yield. The reaction\nrepresents a ring expansion of DBU. Since acylethynylpyrroles are\nreadily available, the discovered reaction opens promising possibilities\nfor the development of new fluorophores. The results of our time-dependent\nDFT calculations indicate that the pyrrole ring in PY-14-ONEs plays\nan important role in the formation of the Stokes shifts, which can\nbe further enhanced by attaching appropriate substituents to it, capable\nof creating in S<sub>1</sub> an extended conjugated system and causing\na substantial alternation of the molecular structure via its planarization.