Shedding light on thermally-activated delayed fluorescence.

Thermally activated delayed fluorescence (TADF) is a hot research topic in view of its impressive applications in a wide variety of fields from organic LEDs to photodynamic therapy and metal-free photocatalysis. TADF is a rare and fragile phenomenon that requires a delicate equilibrium between tiny singlet-triplet gaps, sizable spin-orbit couplings, conformational flexibility and a balanced contribution of charge transfer and local excited states. To make the picture more complex, this precarious equilibrium is non-trivially affected by the interaction of the TADF dye with its local environment. The concurrent optimization of the dye and of the embedding medium is therefore of paramount importance to boost practical applications of TADF. Towards this aim, refined theoretical and computational approaches must be cleverly exploited, paying attention to the reliability of adopted approximations. In this perspective, we will address some of the most important issues in the field. Specifically, we will critically review theoretical and computational approaches to TADF rates, highlighting the limits of widespread approaches. Environmental effects on the TADF photophysics are discussed in detail, focusing on the major role played by dielectric and conformational disorder in liquid solutions and amorphous matrices.

Solid state solvation: a fresh view.

The design of efficient organic electronic devices, including OLEDs, OPVs, luminescent solar concentrators, etc., relies on the optimization of relevant materials, often constituted by an active (functional) dye embedded in a matrix. Understanding solid state solvation (SSS), i.e. how the properties of the active dye are affected by the matrix, is therefore an issue of fundamental and technological relevance. Here an extensive experimental and theoretical investigation is presented shedding light on this, somewhat controversial, topic. The spectral properties of the dye at equilibrium, i.e. absorption and Raman spectra, are not affected by the matrix dynamics. Reliable estimates of the matrix polarity are then obtained from an analysis of the micro-Raman spectra of polar dyes. Specifically, to establish a reliable polarity scale, the spectra of DCM or NR dispersed in amorphous matrices are compared with the spectra of the same dyes in liquid solvents with known polarity. On the other hand, steady-state emission spectra obtained in solid matrices depend in a highly non-trivial way on the matrix polarity and its dynamics. An extensive experimental and theoretical analysis of the time-resolved emission spectra of NR in a very large time window (15 fs-15 ns) allows us to validate this dye as a good probe of the dielectric dynamics of the surrounding medium. We provide a first assessment of the relaxation dynamics of two matrices (mCBPCN and DPEPO) of interest for OLED application, unambiguously demonstrating that the matrix readjusts for at least 15 ns after the dye photoexcitation.

From symmetry breaking to symmetry swapping: is Kasha's rule violated in multibranched phenyleneethynylenes?

The phenomenon of excited-state symmetry breaking is often observed in multipolar molecular systems, significantly affecting their photophysical and charge separation behavior. As a result of this phenomenon, the electronic excitation is partially localized in one of the molecular branches. However, the intrinsic structural and electronic factors that regulate excited-state symmetry breaking in multibranched systems have hardly been investigated. Herein, we explore these aspects by adopting a joint experimental and theoretical investigation for a class of phenyleneethynylenes, one of the most widely used molecular building blocks for optoelectronic applications. The large Stokes shifts observed for highly symmetric phenyleneethynylenes are explained by the presence of low-lying dark states, as also established by two-photon absorption measurements and TDDFT calculations. In spite of the presence of low-lying dark states, these systems show an intense fluorescence in striking contrast to Kasha's rule. This intriguing behavior is explained in terms of a novel phenomenon, dubbed "symmetry swapping" that describes the inversion of the energy order of excited states, i.e., the swapping of excited states occurring as a consequence of symmetry breaking. Thus, symmetry swapping explains quite naturally the observation of an intense fluorescence emission in molecular systems whose lowest vertical excited state is a dark state. In short, symmetry swapping is observed in highly symmetric molecules having multiple degenerate or quasi-degenerate excited states that are prone to symmetry breaking.

Thermally Activated Delayed Fluorescence: Polarity, Rigidity, and Disorder in Condensed Phases.

We present a detailed and comprehensive picture of the photophysics of thermally activated delayed fluorescence (TADF). The approach relies on a few-state model, parametrized ab initio on a prototypical TADF dye, that explicitly accounts for the nonadiabatic coupling between electrons and vibrational and conformational motion, crucial to properly address (reverse) intersystem crossing rates. The Onsager model is exploited to account for the medium polarity and polarizability, with careful consideration of the different time scales of relevant degrees of freedom. TADF photophysics is then quantitatively addressed in a coherent and exhaustive approach that accurately reproduces the complex temporal evolution of emission spectra in liquid solvents as well as in solid organic matrices. The different rigidity of the two environments is responsible for the appearance in matrices of important inhomogeneous broadening phenomena that are ascribed to the intertwined contribution from (quasi)static conformational and dielectric disorder.

Thermally activated delayed fluorescence: A critical assessment of environmental effects on the singlet-triplet energy gap.

The effective design of dyes optimized for thermally activated delayed fluorescence (TADF) requires the precise control of two tiny energies: the singlet-triplet gap, which has to be maintained within thermal energy, and the strength of spin-orbit coupling. A subtle interplay among low-energy excited states having dominant charge-transfer and local character then governs TADF efficiency, making models for environmental effects both crucial and challenging. The main message of this paper is a warning to the community of chemists, physicists, and material scientists working in the field: the adiabatic approximation implicitly imposed to the treatment of fast environmental degrees of freedom in quantum-classical and continuum solvation models leads to uncontrolled results. Several approximation schemes were proposed to mitigate the issue, but we underline that the adiabatic approximation to fast solvation is inadequate and cannot be improved; rather, it must be abandoned in favor of an antiadiabatic approach.

Understanding TADF: a joint experimental and theoretical study of DMAC-TRZ.

Thermally-activated delayed fluorescence (TADF) is a promising strategy to harvest triplets in OLED towards improved efficiency, but several issues must be addressed to fully exploit its potential, including the nature of involved excited singlet and triplet states and their response to the local environment in order to concurrently optimize the dye inside the matrix. Towards this ambitious aim, we present an extensive spectroscopic study of a typical TADF dye in liquid and glassy solvents. TD-DFT results for the same molecule in gas-phase and under an applied electric field are exploited to build a reliable model for the dye, rigorously validated against experiment. The model, accounting for charge transfer and local singlet and triplet states, spin-orbit coupling, conformational and vibrational degrees of freedom, sets the basis for a sound understanding of the photophysics of TADF dyes in different environments. The charge-transfer nature of the fluorescent state and of the almost degenerate phosphorescent state is unambiguously demonstrated. The concurrent role played by conformational degrees of freedom and the matrix polarizability in governing TADF is addressed.

Optical spectra of organic dyes in condensed phases: the role of the medium polarizability.

When designing molecular functional materials, the properties of the active specie, the dye, must be optimized fully accounting for the presence of a surrounding medium (a solvent, a polymeric matrix, etc.) that may largely alter the dye behavior. Here we present an effective model to account for the effects of the medium electronic polarizability on the spectral properties of charge-transfer dyes. Different classes of molecules are considered and the proposed antiadiabatic approach to solvation is contrasted with the adiabatic approach, currently adopted in all quantum chemical approaches to solvation. Transition frequencies and band-shapes are addressed, and the role of the medium polarizability on symmetry-breaking phenomena is also discussed.

Antiadiabatic View of Fast Environmental Effects on Optical Spectra.

An antiadiabatic approach is proposed to model how the refractive index of the surrounding medium affects optical spectra of molecular systems in condensed phases. The approach solves some of the issues affecting current implementations of continuum solvation models and more generally of effective models where a classical description is adopted for the molecular environment.