Influence of chirality on fluorescence and resonance energy transfer

Enabling robust blue circularly polarized organic afterglow through self-confining isolated chiral chromophore

Creating circularly polarized organic afterglow system with elevated triplet energy levels, suppressed non-radiative transitions, and effective chirality, which are three critical prerequisites for achieving blue circularly polarized afterglow, has posed a formidable challenge. Herein, a straightforward approach is unveiled to attain blue circularly polarized afterglow materials by covalently self-confining isolated chiral chromophore within polymer matrix. The formation of robust hydrogen bonds within the polymer matrix confers a distinctly isolated and stabilized molecular state of chiral chromophores, endowing a blue emission band at 414 nm, lifetime of 3.0 s, and luminescent dissymmetry factor of ~ 10-2. Utilizing the synergistic afterglow and chirality energy transfer, full-color circularly polarized afterglow systems are endowed by doping colorful fluorescent molecules into designed blue polymers, empowering versatile applications. This work paves the way for the streamlined design of blue circularly polarized afterglow materials, expanding the horizons of circularly polarized afterglow materials into various domains.

Discriminatory fluorescence and FRET in the chiral-perovskite/RhB system

Förster resonance energy transfer (FRET) holds a significant position in various natural and artificial systems, especially within donor-acceptor systems encompassing chiral components. Despite extensive investigations, a clear understanding of the effects of chirality and FRET on discriminatory fluorescence remains elusive. Here, chiral perovskite nanowires (CPNWs) and achiral rhodamine B (RhB) are employed to examine the FRET and discriminatory fluorescence behavior in a donor-acceptor system involving a chiral nanostructure. A notable FRET from the CPNWs to RhB is observed, along with circular dichroism (CD) and circularly polarized luminescence (CPL) activities in RhB. Although the FRET interaction remains consistent over time, a notable inversion in the polarity preference of the CD and CPL of RhB is observed. This reveals that the discriminatory fluorescence of the acceptor arises from the electromagnetic influence of the chiral donor. These findings elucidate that "chirality", as a property related to spatial orientation, cannot accompany the transfer of energy (which is a scalar) from chiral nanostructures to achiral molecules, which helps advance the understanding of the discriminatory fluorescence in the donor-acceptor system with a chiral nanostructure.

Pivotal role of transition density in circularly polarized luminescence

Realizing high luminescence dissymmetry factor (g) in circularly polarized luminescence (CPL) materials remains a big challenge, which necessitates understanding systematically how their molecular structure controls the CPL. Here we investigate representative organic chiral emitters with different transition density distributions and reveal the pivotal role of transition density in CPL. We rationalize that to obtain large g-factors, two conditions should be simultaneously satisfied: (i) the transition density for the S₁ (or T₁)-to-S₀ emission must be delocalized over the entire chromophore; and (ii) the chromophore inter-segment twisting must be restricted and tuned to an optimal value (∼50°). Our findings offer molecular-level insights into the CPL of organic emitters, with potential applications in the design of chiroptical materials and systems with strong CPL effects.

Efficient Circularly Polarized Electroluminescence from Achiral Luminescent Materials

Circularly polarized electroluminescence (CP-EL) is generally produced in organic light-emitting diodes (OLEDs) based on special CP luminescent (CPL) materials, while common achiral luminescent materials are rarely considered to be capable of direct producing CP-EL. Herein, near ultraviolet CPL materials with high photoluminescence quantum yields and good CPL dissymmetry factors are developed, which can induce blue to red CPL for various achiral luminescent materials. Strong near ultraviolet CP-EL with the best external quantum efficiencies (η_ext s) of 9.0 % and small efficiency roll-offs are achieved by using them as emitters for CP-OLEDs. By adopting them as hosts or sensitizers, commercially available yellow-orange achiral phosphorescence, thermally activated delayed fluorescence (TADF) and multi-resonance (MR) TADF materials can generate intense CP-EL, with high dissymmetry factors and outstanding η_ext s (30.8 %), demonstrating a simple and universal avenue towards efficient CP-EL.

Two bridge-particle-mediated RET between chiral molecules

The problem of resonance energy transfer between a pair of chiral molecules mediated by two electrically polarizable bridging particles is solved using molecular QED theory. In this framework a single virtual photon propagates between any two-coupled entities and is responsible for the conveyance of excitation energy from emitter to absorber. Electric dipole and quadrupole, and magnetic dipole couplings linear in the Maxwell fields are employed for donor and acceptor, while each mediator scatters two virtual photons and responds quadratically to the electric displacement field via its electric dipole polarizability. This enables fourth-order diagrammatic perturbation theory to be used to compute the probability amplitude for the process. Individual multipole moment contributions to the Fermi golden rule rate are then extracted for oriented and isotropic systems. Discriminatory transfer rates arise when either the donor or the acceptor are electric-magnetic dipole and the other has a pure multipole moment, or when both are chiral, with mixed electric dipole-quadrupole contributions vanishing in the fluid phase. The bridge-mediated transfer rate is found to be a maximum for a collinear geometry. Moreover, a multi-level model of the mediator is necessary for energy migration. Asymptotically limiting rates for arbitrary and collinear geometries are also obtained for one centre purely electric dipolar and the other purely quadrupolar, or both donor and absorber purely quadrupolar. Understanding is gained of radiationless and radiative transfer mechanisms between chiral moieties in a dielectric medium.

Toward Large Dissymmetry Factor of Circularly Polarized Luminescence in Donor-Acceptor Hybrid Systems

Chiroptical materials with circularly polarized luminescence (CPL) activity have aroused a lot of interest. One essential factor for evaluating the features of CPL-active materials is the dissymmetry factor (g_lum), which represents the circular polarization of emitted light. Essentially, for the practical application of CPL, the most important issue is to greatly improve the g_lum value. Recently, benefiting from the flexible and efficient design in hybrid donor-acceptor systems, more and more examples involving g_lum value amplification have been reported. In this Perspective, we highlight the proposed mechanism for the generation and amplification of CPL in these hybrid systems. We also present the corresponding design principles and potential pitfalls in experimental processes. We hope that this Perspective can shed light on the development of highly efficient CPL-active materials.

Pathways to increase the dissymmetry in the interaction of chiral light and chiral molecules

The dissymmetric interaction between circularly polarised (CP) light and chiral molecules is central to a range of areas, from spectroscopy and imaging to next-generation photonic devices. However, the selectivity in absorption or emission of left-handed versus right-handed CP light is low for many molecular systems. In this perspective, we assess the magnitude of the measured chiroptical response for a variety of chiral systems, ranging from small molecules to large supramolecular assemblies, and highlight the challenges towards enhancing chiroptical activity. We explain the origins of low CP dissymmetry and showcase recent examples in which molecular design, and the modification of light itself, enable larger responses. Our discussion spans spatial extension of the chiral chromophore, manipulation of transition dipole moments, exploitation of forbidden transitions and creation of macroscopic chiral structures; all of which can increase the dissymmetry. Whilst the specific strategy taken to enhance the dissymmetric interaction will depend on the application of interest, these approaches offer hope for the development and advancement of all research fields that involve interactions of chiral molecules and light.

Discriminatory Resonance Energy Transfer Mediated by a Third Molecule

Relay of resonant excitation energy between two chiral molecules by an inert third particle is studied using molecular quantum electrodynamics theory. A single virtual photon propagates between each interacting pair. Fourth-order diagrammatic time-dependent perturbation theory is employed to compute the matrix element. Rate terms dependent upon the chirality of the donor and acceptor species are extracted using the Fermi golden rule. Interestingly, the mediated rate is discriminatory. For freely tumbling particles it exhibits an inverse-square dependence on each interparticle separation distance, indicating a purely radiative exchange mechanism. Furthermore, the isotropic rate is found to be a maximum for a collinear geometry and vanishes when the angle between the donor, mediator, and acceptor is 90°. The indirect rate is compared with direct transfer between two chiral molecules. Insight is gained into discriminatory migration of energy in a dielectric medium.

Resonance energy transfer mediated by a chiral molecule

The problem of resonant energy transfer (RET) between an electric dipole donor, D, and an electric dipole acceptor, A, mediated by a passive, chiral third-body, T, is considered within the framework of molecular quantum electrodynamics theory. To account for the optical activity of the mediator, magnetic dipole and electric quadrupole coupling terms are included in addition to the leading electric dipole interaction term. Fourth-order diagrammatic time-dependent perturbation theory is used to obtain the matrix element. It is found that the Fermi golden rule rate depends on pure multipole moment polarizabilities and susceptibilities of T, as well as on various mixed electric and magnetic multipole moment response functions. The handedness of T manifests through mixed electric-magnetic dipole and mixed electric dipole-quadrupole polarizabilities, which affect the rate and, respectively, require the use of fourth-rank and sixth-rank Cartesian tensor averages over T, yielding non-vanishing isotropic rate formulae applicable to a chiral fluid medium. Terms of a similar order of magnitude proportional to the product of electric dipole polarizability and either magnetic dipole susceptibility or electric quadrupole polarizability of T are also computed for oriented and freely tumbling molecules. Migration rates dependent upon the product of the pure electric dipole or magnetic dipole polarizability with the mixed electric-magnetic or electric dipole-quadrupole analogs, which require fourth- and fifth-rank Cartesian tensor averaging, vanish for randomly oriented systems. Asymptotically limiting rate expressions are also evaluated. Insight is gained into RET occurring in complex media.

500-Fold Amplification of Small Molecule Circularly Polarised Luminescence through Circularly Polarised FRET

Strongly dissymmetric circularly polarised (CP) luminescence from small organic molecules could transform a range of technologies, such as display devices. However, highly dissymmetric emission is usually not possible with small organic molecules, which typically give dissymmetric factors of photoluminescence (gPL ) less than 10-2 . Here we describe an almost 103 -fold chiroptical amplification of a π-extended superhelicene when embedded in an achiral conjugated polymer matrix. This combination increases the |gPL | of the superhelicene from approximately 3×10-4 in solution to 0.15 in a blend film in the solid-state. We propose that the amplification arises not simply through a chiral environment effect, but instead due to electrodynamic coupling between the electric and magnetic transition dipoles of the polymer donor and superhelicene acceptor, and subsequent CP Förster resonance energy transfer. We show that this amplification effect holds across several achiral polymer hosts and thus represents a simple and versatile approach to enhance the g-factors of small organic molecules.