Revealing vibronic coupling in chlorophyll c1 by polarization-controlled 2D electronic spectroscopy
Graphical abstract
Introduction
Chlorophylls and chlorophyll-like molecules have received a significant amount of attention through the years due to their major role in photosynthesis, as they belong to the main molecular group responsible for light absorption, excitation energy transfer and charge separation during the solar energy utilization process [1]. Naturally, a lot of effort has been dedicated to trying to understand the electronic structure and excitation dynamics of these molecules. The well-known Gouterman model, originally applied to the porphyrin molecules, describes two main transitions in chlorophylls as separate electronic transitions, denoted as Qy and Qx, with almost perpendicular mutual orientation of transition dipole moments [2]. However, a recent comprehensive study of a wide range of chlorophyll-like molecules has shown that vibronic coupling between these two states must be taken into account in order to describe the photophysical properties and excitation dynamics in these molecules correctly [3].
Here we use a definition of the vibronic coupling as follows: it is an interaction between the purely electronic transition to the higher energy state (Qx) and vibronic transition of the lower energy state (Qy + a vibrational quantum ). This vibronic coupling entails sharing of the oscillator strength between the states, as well as mixing of their characters, resulting in reorientation of the transition dipole moments [3], [4]. The mixing of the states depends both on coupling strength and resonance detuning between Qx and Qy + transitions. It is noteworthy, that vibronic coupling in molecular dimers and larger aggregates was suggested to speed up energy transfer in multichromophore complexes [4], [5]. Although vibronic coupling in chlorophylls is anticipated, it is very difficult to observe, which is the aim of this study.
Two-dimensional electronic spectroscopy (2DES) is very well-suited for exploring couplings and superpositions between quantum states [6], [7]. Specifically, it has been employed for studying vibrational coherences in chlorophyll-type molecules [8], [9], [10]. However, to observe the vibronic coupling between two electronic energy states is challenging. There are several technical complications involved. Since the energy splitting between the tentative Qy and Qx transitions is large, it is difficult to excite their superposition with the same laser pulse [3]. Additionally, the Qx transition (higher in energy) is generally very weak and 2D signals scale super-linearly with the transition dipole moment. Therefore, high dynamic range of detection is required to discern signals arising from the coupling between the two states [11]. Lastly, the most common 2DES experiments are done with the polarization of all the laser pulses set parallel to each other. Such experiments have no selectivity for detecting vibronically coupled transitions, instead they are dominated by the Franck-Condon vibrational wavepacket signals on the ground and excited electronic states.
To explore the vibronic coupling between the Qy and Qx states in chlorophyll molecules, we performed polarization-controlled 2DES at 77 K on chlorophyll (Chl) c1 molecules extracted from the diatom Cyclotella meneghiniana, where they are found in light-harvesting complexes. In contrast to most chlorophylls, which are chlorins, Chl c is a porphyrin with an acrylic acid side chain (see [12] and Fig. S1). Our choice of the Chl c1 is based on the fact that it features two lowest absorption bands (tentatively denoted as Qy and Qx) having similar intensity. Close position in energy also makes it feasible to spectrally cover both of them with a single laser pulse (Fig. 1). Due to these favorable properties, we expected vibronic coupling to be detectable. Two sets of 2DES experiments were performed: a standard all-parallel arrangement, with all four laser pulses having the same linear polarization (〈0°,0°,0°,0°〉, AP), and a double-crossed polarization sequence (〈45°,−45°,90°,0°〉, DC). The major advantage of the DC polarization sequence is that it is specifically sensitive to detecting vibronic coupling in molecular systems [13], [14], [15].
Section snippets
Sample preparation
Chl c1 (structure shown in Fig. S1 in Supplementary Information (SI)) was isolated from the diatom Cyclotella meneghiniana (Culture Collection Göttingen, strain 1020-1a) that had been cultured for 7 days at 18 °C in modified ASP medium according to [16] under 40 μmol quanta m−2 s−1 of white light, with 16 h light and 8 h dark constant shaking at 120 rpm.
Cells were harvested by centrifugation at 4304 g for 15 min at 4 °C and homogenized in liquid nitrogen using pistil and mortar. All steps were
Chlorophyll c1 absorption at cryogenic temperature
The low temperature (77 K) absorption spectrum of Chl c1 is presented in Fig. 1 together with the laser spectrum used in the 2DES experiments. Absorbance was obtained by measuring the transmitted laser spectrum (intensity) in the 2DES setup with (I) and without (I0) the sample and taking the common logarithm of the ratio of the two intensities (I0/I). We note here that this type of absorption measurement is susceptible to the effect of the self-interference of the laser beam, which could result
Discussion
The energy relaxation between Q states in chlorophylls is known to take place on the 100 fs timescale [26], [27]. In the presented data we did not directly observe Qx → Qy energy relaxation in the Chl c1 molecule, as there is neither an observable decay in the upper diagonal peak nor a rise of the lower cross-peak in the time traces after 80 fs (see Fig. S8). As mentioned above, the first 80 fs in the measurements are unreliable, because of the unavoidable presence of the multiple artefacts.
Conclusions
While the role of the vibronic coupling in photosynthetic functions remains to be clarified, with the help of polarization-controlled 2DES we unveil vibronic coupling between two electronic states of the photosynthetic pigment Chl c1. This coupling appears to be realized via at least two vibrational modes. It is also clear that inhomogeneous broadening in the linear spectrum hides the features arising from the complexity of the vibronic coupling, which is nevertheless revealed in the study of
CRediT authorship contribution statement
Eglė Bukartė: Investigation, Formal analysis, Writing - original draft. Anja Haufe: Resources, Writing - original draft. David Paleček: Formal analysis, Methodology, Writing - original draft. Claudia Büchel: Resources, Funding acquisition. Donatas Zigmantas: Conceptualization, Investigation, Funding acquisition, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Work in Lund was supported by the Swedish Research Council. CB and AH acknowledge support from the Deutsche Forschungsgemeinschaft [grant number DFG Bu 812/10-1] and from the European Union’s Horizon2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 675006 (SE2B).
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