Synergistic Approach of Ultrafast Spectroscopy and Molecular Simulations in the Characterization of Intramolecular Charge Transfer in Push-Pull Molecules

Abstract

The comprehensive characterization of Intramolecular Charge Transfer (ICT) stemming in push-pull molecules with a delocalized π-system of electrons is noteworthy for a bespoke design of organic materials, spanning widespread applications from photovoltaics to nanomedicine imaging devices. Photo-induced ICT is characterized by structural reorganizations, which allows the molecule to adapt to the new electronic density distribution. Herein, we discuss recent photophysical advances combined with recent progresses in the computational chemistry of photoactive molecular ensembles. We focus the discussion on femtosecond Transient Absorption Spectroscopy (TAS) enabling us to follow the transition from a Locally Excited (LE) state to the ICT and to understand how the environment polarity influences radiative and non-radiative decay mechanisms. In many cases, the charge transfer transition is accompanied by structural rearrangements, such as the twisting or molecule planarization. The possibility of an accurate prediction of the charge-transfer occurring in complex molecules and molecular materials represents an enormous advantage in guiding new molecular and materials design. We briefly report on recent advances in ultrafast multidimensional spectroscopy, in particular, Two-Dimensional Electronic Spectroscopy (2DES), in unraveling the ICT nature of push-pull molecular systems. A theoretical description at the atomistic level of photo-induced molecular transitions can predict with reasonable accuracy the properties of photoactive molecules. In this framework, the review includes a discussion on the advances from simulation and modeling, which have provided, over the years, significant information on photoexcitation, emission, charge-transport, and decay pathways. Density Functional Theory (DFT) coupled with the Time-Dependent (TD) framework can describe electronic properties and dynamics for a limited system size. More recently, Machine Learning (ML) or deep learning approaches, as well as free-energy simulations containing excited state potentials, can speed up the calculations with transferable accuracy to more complex molecules with extended system size. A perspective on combining ultrafast spectroscopy with molecular simulations is foreseen for optimizing the design of photoactive compounds with tunable properties.

Keywords: DFT; ICT; TD-DFT; machine learning; molecular simulations; push-pull molecules; transient absorption spectroscopy; two-dimensional electronic spectroscopy; ultrafast spectroscopy.

Figure 1

Scheme depicting the evolution of the Intramolecular Charge Transfer (ICT) in push-pull molecules. S₀ is represented by the black curve located at the bottom. (a) Vertical excitation from S₀ to a Locally-Excited S1 state (S₁LE). (b) ICT from S₁LE to an unrelaxed S₁ICT state. (c) The unrelaxed S₁ICT state can be stabilized by geometrical rearrangements (PICT or TICT) and by solvation. (d) Decay of S₁ICT state through fluorescence emission. (e) Inter System Crossing (ISC) from S₁ICT state to a Triplet state (T₁). (f) Relaxation of T₁ state to S_0.

Figure 2

Scheme of a typical Transient Absorption Spectroscopy (TAS) set-up.

Figure 3

Molecular structures of push-pull systems analyzed in Section 2 with the relative references numbers. In the molecular structures of [28] R—dodecyl.

Figure 4

Illustration of a 2D map. The signals are characterized by diagonal peaks and cross peaks with both positive (GSB and SE) and negative Photo-Induced Absorption (P-IA) signals.

Figure 5

Timescale length governing the choice of the simulation technique used to predict a given chemical phenomenon. All the techniques can be in principle accelerated through learning methods, as machine or deep learning which includes neural networks, reported in the figure.