4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) Dyes Modified for Extended Conjugation and Restricted Bond Rotations

Five new, constrained, aryl-substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes (3f,g and 4h−j) were prepared and investigated to see if they have more favorable fluorescence characteristics than the unconstrained systems 2 that were prepared in previous studies. Dye types 3 and 4 have relatively rigid conformations caused by the heteroatom (3f and 3g) or ethylene bridge (4h−j) linkers that preclude free rotation of the substituted-benzene molecular fragments. In the event, the new dye types 3 and 4 have longer λ_{max abs} (620−660 nm) and λ_max fluor (630−680 nm) values than compounds 2. They also exhibit higher extinction coefficients (>100 000 M^-1 cm^-1, except for 3g). Their fluorescent quantum yields are high (up to 0.72 for 4j), with the exception of compound 3g, which has a quantum yield of only 0.05. The redox properties of dyes 3 and 4 have also been examined.