Integration of Cyanine, Merocyanine and Styryl Dye Motifs with Synthetic Bacteriochlorins

Understanding the effects of substituents on spectral properties is essential for the rational design of tailored bacteriochlorins for light-harvesting and other applications. Toward this goal, three new bacteriochlorins containing previously unexplored conjugating substituents have been prepared and characterized. The conjugating substituents include two positively charged species, 2-(N-ethyl 2-quinolinium)vinyl- (B-1) and 2-(N-ethyl 4-pyridinium)vinyl- (B-2), and a neutral group, acroleinyl- (B-3); the charged species resemble cyanine (or styryl) dye motifs whereas the neutral unit resembles a merocyanine dye motif. The three bacteriochlorins are examined by static and time-resolved absorption and emission spectroscopy and density functional theoretical calculations. B-1 and B-2 have Q_y absorption bathochromically shifted well into the NIR region (822 and 852 nm), farther than B-3 (793 nm) and other 3,13-disubstituted bacteriochlorins studied previously. B-1 and B-2 have broad Q_y absorption and fluorescence features with large peak separation (Stokes shift), low fluorescence yields, and shortened S₁ (Q_y) excited-state lifetimes (~700 ps and ~100 ps). More typical spectra and S₁ lifetime (~2.3 ns) are found for B-3. The combined photophysical and molecular-orbital characteristics suggest the altered spectra and enhanced nonradiative S₁ decay of B-1 and B-2 derive from excited-state configurations in which electron density is shifted between the macrocycle and the substituents.