Excitonic Interactions in Bacteriochlorin Homo-Dyads Enable Charge Transfer: A New Approach to the Artificial Photosynthetic Special Pair

Abstract

Excitonically coupled bacteriochlorin (BC) dimers constitute a primary electron donor (special pair) in bacterial photosynthesis and absorbing units in light-harvesting antenna. However, the exact nature of the excited state of these dyads is still not fully understood. Here, we report a detailed spectroscopic and computational investigation of a series of symmetrical bacteriochlorin dimers, where the bacteriochlorins are connected either directly or by a phenylene bridge of variable length. The excited state of these dyads is quenched in high-dielectric solvents, which we attribute to photoinduced charge transfer. The mixing of charge transfer with the excitonic state causes accelerated (within 41 ps) decay of the excited state for the directly linked dyad, which is reduced by orders of magnitude with each additional phenyl ring separating the bacteriochlorins. These results highlight the origins of the excited-state dynamics in symmetric BC dyads and provide a new model for studying the primary processes in photosynthesis and for the development of artificial, biomimetic systems for solar energy conversion.

Figure 1.

(a) Structure of porphyrin, chlorin, and bacteriochlorin. (b) Dyads and monomer investigated in this work.

Figure 2.

UV-vis absorption of BCs in (A) toluene and (B) DMF. The directly linked BC1 (black) shows a red-shifted Q_y band relative to the other dyads. As the number of phenylene bridge units are increased, the Q_y band of the dyads blue-shifts toward that of the monomer BC4 (blue).

Figure 3.

(A) Comparison of the UV-vis absorption spectrum of BC1 dyad (black) and the most distant dyad BC3 (green). The dashed lines indicate the maxima of the absorption peaks. (B) Illustration of the excited-state energies of two BC monomers (Ψ_BC,mono) interacting to form the split BC1 Ψ_bci^- (green) and Ψ_bci⁺ (red) excitonic states.

Figure 4.

(A) fs-TA spectra of BC1 in DMF at indicated delay times (noted in legend) after 500 nm laser pulse excitation. (B) Kinetic traces of the transient bleaching of the dyads BC1-BC4 in toluene and in (C) DMF, monitored at the Q_y band transition.

Figure 5.

Steady-state fluorescence spectra of the dyads BC1-BC3 and the monomer BC4 in (A) toluene and (B) DMF excited at 500 nm. All BCs have similar fluorescence in either solvent, except for BC1, which has a split peak in DMF, whose higher energy is similar to the monomer’s (BC4) fluorescence.

Figure 6.

Time-resolved fluorescence decay kinetics of BCs in (A) toluene and (B) DMF. Data were fit with single exponential decay equations and the obtained lifetimes are summarized in Table 1.

Figure 7.

Time-resolved fluorescence contour plots of BCs in (A-D) toluene and in (E-H) in DMF measured on a high-speed streak camera. From these plots, it is easy to notice trends in fluorescence decay. In DMF, fluorescence is quenched compared to that in toluene, with BC1 showing the largest decrease in fluorescence lifetime. In DMF, the shorter wavelength peak is hypothesized to be due a local excited-state emission and the longer wavelength emission due to the lower-energy excitonic state Ψ_bc+, which is strongly coupled to the charge-transfer (CT) state and as such quenched, owing to the additional relaxation pathway through the CT state.

Figure 8.

Illustration of the relaxation pathways for BC1 in DMF. Following excitation (blue) from the ground state to the excited state, BC1 relaxes to Ψ_BC1^- (green) and then Ψ_BC1⁺ (red) states. From these states, radiative (solid arrow) and nonradiative (not shown) relaxations occur. The CT state (blue) primarily mixes with the Ψ_BC1⁺ state, which leads to accelerated nonradiative relaxation in high-dielectric solvents and possibly to the red-shifted, broad emission centered at 770 nm (see Figure S4).