Boron Dipyridylmethene (DIPYR) Dyes: Shedding Light on Pyridine-Based Chromophores

Boron dipyrromethene (BODIPY) derivatives have found widespread utility as chromophores in fluorescent applications, but little is known about the photophysical properties of pyridine-based BODIPY analogues, dipyridylmethene dyes. Indeed, it has been reported that boron difluoride dipyridylmethene (DIPYR) is nonemissive, and that derivatives of DIPYR have modest, if any, luminescence. In this report, we explore this little-touched area of chemical space and investigate the photophysical properties of three simple DIPYR dyes: boron dipyridylmethene, boron diquinolylmethene, and boron diisoquinolylmethene. The three dyes strongly absorb in the blue-green part of the spectrum (λ_em = 450-520 nm, ε = 2.9-11 × 10⁴ M^-1 cm^-1) and display green fluorescence with high quantum yields (Φ_PL = 0.2, 0.8, and 0.8, respectively). Key photophysical properties in these systems were evaluated using a combination of TD-DFT and extended multiconfigurational quasidegenerate second-order perturbation theory (XMCQDPT2) methods and compared to experimental results, revealing that high quantum yields of the quinoline and isoquinoline derivatives are a result of the relative reordering of S₁ and T₂ state energies upon benzannulation of the parent structure. The intense absorption and high emission efficiency of the benzannulated derivatives make these compounds an intriguing class of dyes for further derivatization.