C–N Bond Rotation Controls Photoinduced Electron Transfer in an Aminostyrene–Stilbene Donor–Acceptor System

Abstract

We investigate energy transfer and electron transfer in a dimethylsilylene-spaced aminostyrene−stilbene donor−acceptor dimer using time-dependent density functional theory calculations. Our results confirm that the vertical S 3 , S 2 , and S 1 excited states are, respectively, a local excitation on the aminostyrene, local excitation on the stilbene, and the charge-transferred (CT) excited state with electron transfer from aminostyrene to stilbene. In addition, an energy minimum with the C−N bond of the amino group twisted at about 90°is also identified on the S 1 potential energy surface. This S 1 state exhibits a twisted intramolecular charge transfer (TICT) character. A potential energy scan along the C−N bond torsional angle reveals a conical intersection between the S 2 stilbene local excitation and the S 1 CT/TICT state at a torsional angle of ∼60°. We thus propose that the conical intersection dominates the electron transfer dynamics in the donor−acceptor dimer and copolymers alike, and the energy barrier along the C−N bond rotation controls the efficiency of such a process. Moreover, we show that despite the zero oscillator strength of the S 1 excited states in the CT and TICT minima, an emissive S 1 state with a V-shaped conformational structure can be located. The energy of this V-shape CT structure is thermally accessible; therefore, it is expected to be responsible for the CT emission band of the dimer observed in polar solvents. Our data provide a clear explanation of the complex solvent-dependent dual emission and photoinduced electron transfer properties observed experimentally in the dimer and copolymer systems. More importantly, the identifications of the conical intersection and energy barrier along the C−N bond rotation provide a novel synthetic route for controlling emissive properties and electron transfer dynamics in similar systems, which might be useful in the design of novel organic optoelectronic materials.

Excited-State Dynamics in 4-[4′(Dimethylamino)styryl]pyridine, a Photobase: Role of Photoinitiated Proton-Coupled Electron Transfer

“…In the case of dimethylaminobenzonitrile, photoexcitation promotes intramolecular charge transfer (ICT). Subsequently, the excited state leads to the formation of a twisted intramolecular charge transfer (TICT) state in which the donor and acceptor groups are oriented perpendicular to each other. , Such systems are typically characterized by dual emission that is attributed to locally excited (LE) and TICT states. − It is indeed challenging to unravel the reaction dynamics when the proton transfer event is also coupled to such ICT processes. Thus, the objective of the current work is to explore the extent of the photoinitiated PCET process and associated dynamics in a molecular adduct of a photobase with HFIP.…”

Accurate Molecular Geometries in Complex Excited-State Potential Energy Surfaces from Time-Dependent Density Functional Theory

“…Theoretical and computational calculations aid photochemistry research in many different ways. In combination with experiments, they allow for a deeper understanding of the nature and structure of excited states − or of reaction pathways and deactivation mechanisms. − Beyond complementing experiments, excited-state calculations are employed in the design of new devices and materials for optoelectronic applications, − can lead to the discovery of new mechanisms, and allow studying potential energy surfaces which may reveal new reaction pathways and structures. , …”

“…15−18 Beyond complementing experiments, excited-state calculations are employed in the design of new devices and materials for optoelectronic applications, 19−21 can lead to the discovery of new mechanisms, 22 and allow studying potential energy surfaces which may reveal new reaction pathways and structures. 23,24 Commonly, calculations of excited-state properties employ either wave-function-based methods [e.g., coupled cluster (CC), algebraic diagrammatic construction to second order (ADC(2)), complete active space second-order perturbation theory (CASPT2), etc.] or time-dependent density functional theory (TD-DFT).…”