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In recent years visible pump/mid-infrared (IR) probe spectroscopy has established itself as a key technology to unravel structure-function relationships underlying the photo-dynamics of complex molecular systems. In this contribution we review the most important applications of mid-infrared absorption difference spectroscopy with sub-picosecond time-resolution to photosynthetic complexes. Considering several examples, such as energy transfer in photosynthetic antennas and electron transfer in reaction centers and even more intact structures, we show that the acquisition of ultrafast time resolved mid-IR spectra has led to new insights into the photo-dynamics of the considered systems and allows establishing a direct link between dynamics and structure, further strengthened by the possibility of investigating the protein response signal to the energy or electron transfer processes. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Light harvesting complex II (LHCII) is the most abundant protein in the thylakoid membrane of higher plants and green algae. LHCII acts to collect solar radiation, transferring this energy mainly toward photosystem II, with a smaller amount going to photosystem I; it is then converted into a chemical, storable form. We performed time-resolved femtosecond visible pump/mid-infrared probe and visible pump/visible probe absorption difference spectroscopy on purified LHCII to gain insight into the energy transfer in this complex occurring in the femto-picosecond time regime. We find that information derived from mid-infrared spectra, together with structural and modeling information, provides a unique visualization of the flow of energy via the bottleneck pigment chlorophyll a604.

The photosynthetic Reaction Center (RC) from the purple bacterium Rhodobacter sphaeroides has unique photoconversion capabilities, that can be exploited in assembly biohybrid devices for applications in solar energy conversion. Extending the absorption cross section of isolated RC through covalent functionalization with ad-hoc synthesized artificial antennas is a successful strategy to outperform the efficiency of the pristine photoenzyme under visible light excitation. Here we report a new heptamethine cyanine antenna that, upon covalent binding to RC, forms a biohybrid (hCyN7-RC) which, under white light excitation, has doubled photoconversion efficiency versus the bare photoenzyme. The artificial antenna hCyN7 successfully meets appropriate optical properties, i.e. peak position of absorption and emission maximum in the visible and NIR region respectively, large Stokes shift, and high fluorescence quantum yield, required for improving the efficiency of the biohybrid in the production of the charge-separated state in the RC. The kinetics of energy transfer and charge separation of hCyN7-RC studied via ultrafast visible and IR spectroscopies are here presented. The antenna transfers energy to RC chromophores within <10 ps and the rate of QA reduction is doubled compared to the native RC. These experiments further demonstrate hCyN7-RC, besides being an extremely efficient white light photoconverter, fully retains the charge separation mechanism and integrity of the native RC photoenzyme, thus allowing to envisage its suitability as biohybrid material in bioinspired systems for solar energy conversion.