Exciton Localization and Transfer in Azobenzene Aggregates: A Surface Hopping Molecular Dynamics Study

Abstract

Molecular photoswitches are a promising class of molecules for the development of new functional, light-controlled materials. In complex systems composed of multiple photoswitchable units, photophysical and photochemical properties may be altered as compared to isolated chromophores. And phenomena such as excitation energy transfer may arise in the aggregated state. In the present work, using nonadiabatic molecular dynamics simulations in conjunction with transition density matrix analysis, we study exciton dynamics in H-type tetramers of azobenzene, a prototypical molecular switch. We consider "free" and "constrained" (embedded in an environment of additional azobenzene molecules) models with different intermolecular distances (3.5 and 5.5 Å). Our simulations reveal ultrafast exciton localization upon ππ* excitation, occurring on a sub-100 fs timescale and proceeding faster for the longer separation distance than for the shorter one. We also find that exciton transfer takes place during excited-state dynamics in the ππ* manifold, but it is strongly inhibited in the nπ* manifold. Moreover, we find that the ππ* trans → cis isomerization quantum yields are lower by a factor of about three for free/not strongly constrained tetramers than for the monomer, and no switching is observed for the most tightly packed model.

Visible Light Induced Exciton Dynamics and trans-to-cis Isomerization in Azobenzene Aggregates: Insights from Surface Hopping/Semiempirical Configuration Interaction Molecular Dynamics Simulations

“…There are only three trajectories demonstrating the opposite (see Figure S2). The very rare occurrence of the nπ* exciton transfer is in agreement with our previous simulations of the ππ* dynamics (where the nπ* states are reached via internal conversion from the ππ* states) . Moreover, it agrees with the previous simulations of Sangiogo Gil et al.…”

“…Using the elements of the F matrix, we compute the participation number (PN) following the prescription of ref [there the quantity is called “delocalization length” (DL)] P N = 1 ∑ X true( ∑ Y F X Y + F Y X 2 true) 2 PN is scalar ranging from 1 (complete exciton localization) to 4 (complete exciton delocalization). We note that the quantity given by eq (or equations of a similar form) is sometimes called “inverse participation ratio” (IPR), ,, as was also done in our previous works. , However, as recently highlighted by Herbert, the term IPR may lead to a confusion (see further discussion in the Supplementary Note in the Supporting Information). Therefore, following Alfonso Hernandez et al , we will call the quantity defined by eq “participation number” (or “PN”) to avoid possible confusions associated with the “IPR”.…”

“…To model nonadiabatic dynamics induced by visible light (nπ*) excitation, we use the approach applied in ref to study relaxation after UV (ππ*) excitation. In this approach, the electronic structure of the QM tetramer is modeled with configuration interaction singles (CIS) based on molecular orbitals (MO) obtained from a self-consistent field calculation with floating occupation (FO) numbers using the Austin model 1 (AM1) that was reparameterized (r) for azobenzene .…”

“…The vertical absorption spectra were then calculated for the selected geometries using rAM1/FOMO–CIS, and the obtained stick spectra were broadened as I false( E false) = 1 N s n ∑ α = 1 N normals normaln ∑ i = 1 N s t f i , α exp ( − 1 2 γ 2 ( E − E i , α ) 2 ) Here, I is the intensity, E is the excitation energy, N sn = 151 is the number of selected snapshots, N st = 20 is the number of excited singlet states, and E i ,α and f i ,α are the excitation energy and oscillator strength, respectively, for the S 0 → S i transition, for snapshot α, and γ = 0.18598 eV (1500 cm –1 ) is a broadening parameter (the same value as used in ref ). The brightest state among the nπ* states (S 1 –S 4 ) was selected as the initial state for SH calculations.…”

“…Sangiogo Gil et al used SH combined with an exciton model to simulate Frenkel exciton dynamics in an azobenzene dimer (azobenzenophane) and an azobiphenyl monolayer . Recently, we have employed a supermolecule approach to investigate exciton dynamics in several azobenzene tetramers (both free and embedded in a SAM-like environment) after ππ* excitation . The exciton evolution was explored using a transition density matrix (TDM) analysis, allowing one to judge on the spatial localization of excitons during SH dynamics. ,,,,− …”