Phosphorescence study of chlorophyll d photophysics. Determination of the energy and lifetime of the photo-excited triplet state. Evidence of singlet oxygen photosensitization

Abstract

Chlorophyll d (Chl d) is the major pigment in both photosystems (PSI and II) of the cyanobacterium Acaryochloris marina, whose pigment composition represents an interesting alternative in oxygenic photosynthesis. While abundant information is available relative to photophysical properties of Chl a , the understanding of Chl d photophysics is still incomplete. In this paper, we present for the first time a characterization of Chl d phosphorescence, which accompanies radiative deactivation of the photoexcited triplet state of this pigment. Reliable information was obtained on the energy and lifetime of the Chl d triplet state in frozen solutions at 77 K using diethyl ether and aqueous dispersions of Triton X100 as solvents. It is shown that triplet Chl d is effectively populated upon photoexcitation of pigment molecules and efficiently sensitizes singlet oxygen phosphorescence in aerobic solutions under ambient conditions. The data obtained are compared with the previous results of the phosphorescence studies of Chl a and Pheo a, and their possible biological implications are discussed.

Triplet Excited State Energies and Phosphorescence Spectra of (Bacterio)Chlorophylls

“…However, the fluorescence lifetime of the (B)Chls studied lie between 2 and 7 ns, 10 whereas triplet state lifetimes are in the range of 100−900 μs at room temperature 10 and become even longer at low temperatures. 28,41,42 This allows effective gating of the phosphorescence in the time domain using a fast optical shutter. 43−45 A time-gated phosphorescence spectrometer (Figure 3) was built using a sensitive, liquid nitrogen-cooled germanium photodetector (North Coast Scientific Corp., EO-817L) with a measured noise equivalent power (NEP) of 7 f W/(Hz) 1/2 .…”

Electron–Vibrational Coupling and Fluorescence Spectra of Tetra-, Penta-, and Hexacoordinated Chlorophylls c1 and c2

“…Fluorescence lifetimes amount to several nanoseconds . The room-temperature spectrum has a strong 0–0 transition followed by a vibrational shoulder at ∼1200 cm –1 . − This indicates that the emission takes place from the vibrationally equilibrated electronic state. The shapes of the room-temperature fluorescence spectra do not show any obvious dependence on the Chl’s coordination number, in contrast to the absorption spectra .…”

Measuring Ultrafast Spectral Diffusion and Correlation Dynamics by Two‐Dimensional Electronic Spectroscopy

“…The primary function of Chls in plants is to absorb energy from the sun and direct it to the reaction center where the charge separation occurs . Due to its importance in understanding the energy capture and transfer processes in photosynthetic systems, there has been much experimental effort to understand the photophysics of Chls . The first 2DES experiment to characterize the spectral diffusion dynamics of Chls in methanol was performed by Tan and co‐workers .…”

“…[50] Due to its importance in understanding the energy capturea nd transfer processes in photosynthetic systems, there has been much experimental effort to understand the photophysics of Chls. [16,17,45,51,52] The first 2DES experiment to characterize the spectrald iffusion dynamics of Chls in methanol was performed by Tana nd co-workers. [17] The top part of spectrum.T he results reveal af ast sub-100-fs component, attributed to the inertialc omponent of solvation,a% 0.5-ps timescale anda7-ps timescale that are attributed to solvent rearrangement.…”