Vibronic Origin of the Q_y Absorption Tail of Bacteriochlorophyll a Verified by Fluorescence Excitation Spectroscopy and Quantum Chemical Simulations

Abstract

The long-wavelength tail of the lowest-energy Q singlet absorption band of bacteriochlorophyll a in triethylamine peaking at 768.6 nm was examined by means of fluorescence excitation spectroscopy at ambient temperature of 22 ± 1 °C. The tail, usually considered a Gaussian, does in fact weaken quasi-exponentially, being clearly evident as far as 940 nm, nearly 2400 cm (∼12 kT) away from the absorption peak. Quantum chemical simulations identified vibronic transitions from the thermally populated normal modes and their overtones in the ground electronic state as the origin of this tail. Because energy transfer and relaxation processes generally depend on vibronic overlap integrals, these findings may have important implications on the interpretation of numerous photoinduced phenomena that involve bacteriochlorophyll and similar molecules, including photosynthesis.

Establishment of the Qy Absorption Spectrum of Chlorophyll a Extending to Near-Infrared

“…From Figure 1 B, one can also perceive that the absorption/absorptance band shape of Chl a can be broadly described by a combination of central Gaussian and red-side exponentially decreasing parts (drawn by dashed line). Similar general dependence was formerly observed for BChl a [ 30 ]. However, as it will be elaborated below in more detail, compared to BChl a , the Gaussian part of Chl a is significantly narrower and, most remarkably, there is a prominent “hunch” apparent between 700 and 750 nm.…”

“…One may suspect photoselection by partially polarized laser excitation in fluorescence excitation measurements, conceivable due to near mixing between the Q y and Q x states [ 14 , 15 ] of Chl a . Yet, a similar effect was observed in BChl a [ 30 ], where the Q y and Q x states are energetically well separated from each other.…”

“…In order to learn about the physical origin of the quasi-exponentially decreasing red tail, its temperature dependence was studied by recording the spectra at several temperatures between the ambient temperature of 295 and 220 K. The latter temperature, which is reasonably higher than the freezing temperature of TEA (158 K [ 31 ]) was chosen to avoid any sample solidification effects on the data. A reasoning behind the temperature-dependence measurements is that if the long tail is related to the absorption from the hot (thermally populated) vibrational levels of the ground electronic state, as suggested in the case of BChl a [ 30 ]; the tail absorbance is, according to the Boltzmann distribution, bound to diminish exponentially with lowering the temperature.…”

“…Besides, the TEA molecule is fairly compact, hence, computationally well manageable. As was convincingly demonstrated in [ 30 ], application of excitation spectroscopy in the measurements of weak absorption tails of fluorescing samples is superior compared with absorption spectroscopy due to the much higher sensitivity of this method. Recording absorbance via fluorescence also adds specific selectivity to the method.…”

“…In a previous work [ 30 ], we measured the absorption red tail of BChl a molecules dissolved in triethylamine (TEA). The tail turned out to decrease roughly exponentially toward near-infrared, being at ambient temperature well measurable up to ~2400 cm −1 from the Q y band maximum.…”