The Q-Band Energetics and Relaxation of Chlorophylls a and b as Revealed by Visible-to-Near Infrared Time-Resolved Absorption Spectroscopy

Chlorophyll (Chl) is the most abundant light-harvesting pigment of oxygenic photosynthetic organisms; however, the Q-band energetics and relaxation dynamics remain unclear. In this work, we have applied femtosecond time-resolved (fs-TA) absorption spectroscopy in 430–1,700 nm to Chls a and b in diluted pyridine solutions under selective optical excitation within their Q-bands. The results revealed distinct near-infrared absorption features of the B_x,y ← Q_y and B_x,y ← Q_x transitions in 930–1,700 nm, which together with the steady-state absorption in 400–700 nm unveiled the Q_x(0,0)-state energy that lies 1,000 ± 400 and 600 ± 400 cm^–1 above the Q_y(0,0)-state for Chls a and b, respectively. In addition, the Q_x-to-Q_y internal conversion time constants are estimated to be less than 80 fs for Chls a and b. These findings may shed light on understanding the roles of the Chls in the primary excitation energy transfer reactions of photosynthesis.