Quantum Dynamics Simulations of Excited State Energy Transfer in a Zinc–Free-Base Porphyrin Dyad

Rational design of artificial light-harvesting molecular architectures entails building systems that absorb strongly in the visible and near-IR region of the electromagnetic spectrum and also funnel excited state energy to a single site. The ability to model nonadiabatic processes, such as excited-state energy transfer (EET), that occur on a picosecond time scale can aid in the development of novel artificial light-harvesting arrays. A combination of density functional theory (DFT), time-dependent DFT, tight-binding molecular dynamics, and quantum dynamics is employed here to simulate EET in the ZnFbΦ dyad, a model artificial light-harvesting array that undergoes EET with an experimentally measured rate constant of (3.5 ps)⁻¹ upon excitation at 550 nm in toluene [Yang et al. J. Phys. Chem. B 1998, 102, 9426−9436]. We find that to successfully simulate the EET process, it is important to (1) include coupling between nuclear and electronic degrees of freedom in the QD simulation, (2) account for Coulomb coupling between the electron and hole wavepackets, and (3) parametrize the extended Hückel model Hamiltonian employed in the QD simulations with respect to the DFT

Coupled Electron–Hole Quantum Dynamics on D−π–A Dye-Sensitized TiO₂ Semiconductors

A quantum-mechanical description of the electron–hole charge separation and interfacial electron transfer, including the underlying nuclear motion and solvation dynamics effects, is presented for a prototypical class of D−π–A (electron donor−π conjugated bridge–electron acceptor) organic dyes sensitizing the TiO₂ (101) surface. The dyes are comprised of a triphenylamine donor group and a cyanoacrylic acceptor anchoring group, connected by varying thiophene unit lengths (TPA<i>n</i>, <i>n</i> = 1, 2, 3). Simulations show that the electron–hole Coulomb coupling considerably delays charge separation and interfacial electron transfer. A comparative analysis demonstrates that the relevance of thermal nuclear motion, in this case, is secondary relative to electron–hole coupling during the ultrafast transfer from the D−π–A dyes. A dynamic atomistic method is used to describe the solvation effects and the optical properties of the dye-sensitized interface. The solvation dynamics screens the dye molecule, decreasing its complexation with the semiconductor and delaying the electron injection

A Nonadiabatic Excited State Molecular Mechanics/Extended Hückel Ehrenfest Method

In this work we study intramolecular vibronic relaxation effects and photoisomerization dynamics in atomistic molecular systems by means of a mixed self-consistent quantum–classical Ehrenfest formalism for nonadiabatic molecular dynamics (NA-MD) in the excited state. The quantum mechanical part of the method is based on the extended Hückel formalism, whereas the nuclei are treated with the molecular mechanics method. The self-consistent coupling between quantum and classical degrees of freedom is achieved by nonadiabatic Hellmann–Feynman–Pulay forces that conserve the total (quantum–classical) energy. Moreover, this work demonstrates the capabilities of a method that combines two simple though efficient computational schemes to describe complex excited state effects such as photoisomerization dynamics of stilbene and azobenzene molecules. In particular, for stilbene we point out the key importance of hydrogen out-of-plane (HOOP) modes to trigger the isomerization of the phenyl rings. For azobenzene, we identify the inversion-assisted rotation and the associated pedal-cycle motion of the CNNC dihedral as the driving isomerization mechanism. The method can also be applied for NA-MD studies of molecules adsorbed on extended solid surfaces and charge transfer processes

Intramolecular Polarization Induces Electron–Hole Charge Separation in Light-Harvesting Molecular Triads

Artificial light-harvesting supramolecular structures reproduce the light-to-electrochemical energy transduction mechanisms observed in natural photosynthesis. Among them the prototypical carotenoid­(C)–porphyrin­(P)–fullerene­(C₆₀) type of structures have been the most studied. Several experiments performed in such structures, and others alike, have shown that the photoexcited state <i>C</i>– ¹<i>P</i>–<i>C</i>₆₀ decays to the metastable charge-separated state <i>C</i>– <i>P</i>^•+ −<i>C</i>₆₀^•– within a few picoseconds, whereas the final charge-separated state, <i>C</i>^•+– <i>P</i> −<i>C</i>₆₀^•–, is obtained within hundreds of picoseconds. This paper introduces a nonlinear polarizable extended Hückel Hamiltonian that describes the charge dynamics and charge-separation effects in such triads by means of quantum dynamics simulations performed on the photoexcited electron–hole pair. The results are interpreted on the basis of the discrete self-trapping equation and enlighten the role played by the polarizability on charge-separation phenomena

A Nonadiabatic Excited State Molecular Mechanics/Extended Hückel Ehrenfest Method

In this work we study intramolecular vibronic relaxation effects and photoisomerization dynamics in atomistic molecular systems by means of a mixed self-consistent quantum–classical Ehrenfest formalism for nonadiabatic molecular dynamics (NA-MD) in the excited state. The quantum mechanical part of the method is based on the extended Hückel formalism, whereas the nuclei are treated with the molecular mechanics method. The self-consistent coupling between quantum and classical degrees of freedom is achieved by nonadiabatic Hellmann–Feynman–Pulay forces that conserve the total (quantum–classical) energy. Moreover, this work demonstrates the capabilities of a method that combines two simple though efficient computational schemes to describe complex excited state effects such as photoisomerization dynamics of stilbene and azobenzene molecules. In particular, for stilbene we point out the key importance of hydrogen out-of-plane (HOOP) modes to trigger the isomerization of the phenyl rings. For azobenzene, we identify the inversion-assisted rotation and the associated pedal-cycle motion of the CNNC dihedral as the driving isomerization mechanism. The method can also be applied for NA-MD studies of molecules adsorbed on extended solid surfaces and charge transfer processes

Crucial Role of Nuclear Dynamics for Electron Injection in a Dye–Semiconductor Complex

We investigate the electron injection from a terrylene-based chromophore to the TiO₂ semiconductor bridged by a recently proposed phenyl-amide-phenyl molecular rectifier. The mechanism of electron transfer is studied by means of quantum dynamics simulations using an extended Hückel Hamiltonian. It is found that the inclusion of the nuclear motion is necessary to observe the photoinduced electron transfer. In particular, the fluctuations of the dihedral angle between the terrylene and the phenyl ring modulate the localization and thus the electronic coupling between the donor and acceptor states involved in the injection process. The electron propagation shows characteristic oscillatory features that correlate with interatomic distance fluctuations in the bridge, which are associated with the vibrational modes driving the process. The understanding of such effects is important for the design of functional dyes with optimal injection and rectification properties

Ultrafast Interfacial Charge-Transfer Dynamics in a Donor-π-Acceptor Chromophore Sensitized TiO₂ Nanocomposite

The dynamics of interfacial charge transfer across (<i>E</i>)-3-(5-((4-(9H-carbazol-9-yl)­phenyl)­ethynyl)­thiophen-2-yl)-2-cyanoacrylic acid (CT-CA) and TiO₂ nanocomposites was studied with femtosecond transient absorption, fluorescence upconversion, and molecular quantum dynamics simulations. The investigated dye, CT-CA is a push–pull chromophore that has an intramolecular charge-transfer (ICT) excited state and binds strongly with the surface of TiO₂ nanoparticles. Ultrafast transient absorption and fluorescence measurements, in both solution and thin film samples, were carried out to probe the dynamics of electron injection and charge recombination. Multiexponential electron injection with time constants of <150 fs, 850 fs, and 8.5 ps were observed from femtosecond fluorescence measurements in solution and on thin films. Femtosecond transient absorption measurements show similar multiexponential electron injection and confirm that the picosecond electron injection component arises from the excited ICT state of the CT-CA/TiO₂ complex. Quantum dynamics calculations also show the presence of a slow component (30%) in the electron injection dynamics although most of the electron injection (70%) takes place in less than 20 fs. The slow component of electron injection, from the local ICT state, is attributed to the energetic position of the excited state, which is close to, or slightly below, the conduction band edge. In addition, the transient bleach of CT-CA on the TiO₂ surface is shifted to longer wavelengths when compared to its absorption spectrum and the transient bleach is further shifted to longer wavelengths with charge recombination. These features are attributed to transient Stark shifts that arise from the local electric fields generated at the dye/TiO₂ interface due to charge-transfer interactions