Substituent Effects on Photochemistry of Anthracene–Phenol–Pyridine Triads Revealed by Multireference Calculations

Abstract

Inverted region behavior for concerted proton-coupled electron transfer (PCET) was recently demonstrated for biomimetic anthracene-phenol-pyridine molecular triads. Photoexcitation of the anthracene to a locally excited state (LES) is followed by concerted electron transfer from the phenol to the anthracene and proton transfer from the phenol to the pyridine, forming a relatively long-lived charge separated state (CSS). The long-lived CSS and the inverted region behavior associated with the decay from the CSS to the ground state through charge recombination were experimentally observed only for triads with certain substituents on the anthracene and the pyridine. To explain this distinction, we computed the proton potential energy curves in four substituted triads using the complete active space self-consistent-field method and multireference perturbation theory, including solvent effects with a dielectric continuum model. The calculations revealed a local electron-proton transfer (LEPT) state, in which both the electron and proton transfer from the phenol to the pyridine. When the LEPT state is lower in energy than the CSS, it may provide an alternative pathway for fast decay from the LES to the ground state and thereby preclude detection of the CSS and the inverted region behavior. These calculations predict that substituents stabilizing negative charge on the pyridine and destabilizing negative charge on the anthracene will favor the LEPT pathway, while substituents with the reverse effects will favor the CSS pathway, which could exhibit inverted region behavior. These insights about the stabilization of energy-storing charge-separated states have implications for designing and controlling PCET reactions in artificial photosynthetic systems and other energy conversion processes.

Pathways towards Boosting Solar‐Driven Hydrogen Evolution of Conjugated Polymers

“…Incorporating functional substituents into CPs' building block is a facile and scalable method to modulate the porous structure, physical-chemical property, and electronic configuration, which has been widely employed in the field of gas adsorption and catalysis. [99][100][101] For example, when introducing electron-withdrawing substituent into the skeleton of porphyrin-metal, the 2p core electrons of functional groups can be excited to the empty 3d orbitals of the transition metal, which would rearrange the electronic structures of the active sites for further accelerating catalytic activity. [102] Similarly, different functional substituents can modulate the stacking pattern and π-electron delocalization and then affect the visible light absorption.…”

Reaction Mechanisms of Photoinduced Quinone Methide Intermediates Formed via Excited-State Intramolecular Proton Transfer or Water-Assisted Excited-State Proton Transfer of 4-(2-Hydroxyphenyl)pyridine

“…During investigation of the photochemistry of the anthracene phenol pyridine triad, it is noted that after PT to the pyridine in the excited state, a biradical local electron− proton transfer (LEPT) state was produced, while a zwitterionic state was generated in the ground state. 45 In a higher polarity solvent, the transformation of the excited LEPT to the proton-transferred ground state was more favorable. It is thus reasonable to assume that for 1 the deactivation rate of QM2(S 1 ) to QM2(S 0 ) in ACN−H 2 O (1:1, v:v) could be faster than that in ACN−H 2 O (9:1, v:v).…”

Excited State Molecular Dynamics of Photoinduced Proton-Coupled Electron Transfer in Anthracene–Phenol–Pyridine Triads

“…27 All of these simulations were performed with the Q-Chem software package. 28 Even though solvent effects stabilize the charge transfer states and are important for quantitative accuracy, 21 the inclusion of explicit solvent increases the complexity and computational cost of TDA-TDDFT molecular dynamics simulations. Moreover, nonequilibrium excited state dynamics simulations in conjunction with a polarizable continuum model for solvation is challenging because the solvent should also be out of equilibrium for a meaningful description.…”

“…The five lowest-energy states upon photoexcitation were tracked along the trajectories, and the character of each state was monitored by analysis of changes in the net charge of each fragment compared to the ground state, as depicted in Figure 2 and described in the SI and in Table S5. 21,33 For all of the initial conditions, the system was propagated on the S 1 adiabatic state, which corresponded to the LES until it changed character to the CSS for triads 1 and 3 or to the LEPT state for triads 4 and 6. For all trajectories, the LEPT state remained significantly higher in energy than the S 1 state for triads 1 and 3, and the CSS remained higher in energy for triads 4 and 6.…”

“…These simulations elucidate the dynamical differences between the triads that exhibit a long-lived CSS and inverted region behavior and the triads that do not exhibit these properties. In our previous work, we probed the electronic structure of four different triads (Figure ) using ab initio multireference calculations to explain the different behavior observed experimentally for these two types of triads. Our previous electronic structure study revealed the presence of a local electron–proton transfer (LEPT) state associated with both electron and proton transfer from the phenol to the pyridine (i.e., within the (pyridinyl)­phenol moiety).…”