Influence of chirality on fluorescence and resonance energy transfer

Abstract

Electronically excited molecules frequently exhibit two distinctive decay mechanisms that rapidly generate optical emission: one is direct fluorescence; the other, energy transfer to a neighboring component. In the latter, the process leading to the ensuing 'indirect' fluorescence is known as FRET: fluorescence resonance energy transfer. For chiral molecules, both fluorescence and FRET exhibit discriminatory behavior with respect to optical and material handedness. While chiral effects such as circular dichroism are well known, as too is chiral discrimination for FRET in isolation, this article presents a study on a step-wise mechanism that involves both. Chirally sensitive processes follow excitation through the absorption of circularly polarized light, and are manifest in either direct or indirect fluorescence. Following recent studies setting down the symmetry principles, this analysis provides a rigorous, quantum outlook that complements and expands on these works. Circumventing expressions that contain complicated tensorial components, our results are amenable for determining representative numerical values for the relative importance of the various coupling processes. We discover that circular dichroism exerts a major influence on both fluorescence and FRET, and resolving the engagement of chirality in each component reveals the distinct roles of absorption and emission by, and between, donor and acceptor pairs. It emerges that chiral discrimination in the FRET stage is not, as might have been expected, the main arbiter in the step-wise mechanism. In the concluding discussion on various concepts, attention is focused on the validity of helicity transfer in FRET.

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

“…Whilst the discussion thus far has focused on the coupling of electric dipole allowed transitions (μ-μ), the interplay between the μ-μ and μ-m coupling of a donor-acceptor system is also of relevance in energy transfer mechanisms involving chiral species. 56,89 For example, we recently demonstrated a 500-fold amplification of the |g lum | of a π-extended superhelicene, 21, when embedded in an achiral conjugated polymer matrix. 90 We proposed that the amplification arises through electrodynamic coupling between the electric and magnetic transition dipoles of the polymer donor and superhelicene acceptor, resulting in CP fluorescence resonance energy transfer (Figure 13).…”

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

“…Couplings involving at least one magnetic dipole e.g. (M1E1 3 )U typically have an enhanced CP emission compared to the pure electric dipole coupling, and do not have the same distance‐dependence as V, such that chiral energy transfer does not only occur between nearest neighbours [31, 32] . The FRET process has been previously shown to generate reasonably weak CPL (| g PL | ≈3×10 −3 ) from achiral small molecule acceptors when incorporated into chiral supramolecular structures such as nanohelices [33–36] …”

“…(M1E1 3 )U typically have an enhanced CP emission compared to the pure electric dipole coupling, and do not have the same distance-dependence as V, such that chiral energy transfer does not only occur between nearest neighbours. [31,32] The FRET process has been previously shown to generate reasonably weak CPL (j g PL j % 3 10 À3 ) from achiral small molecule acceptors when incorporated into chiral supramolecular structures such as nanohelices. [33][34][35][36] Here, we describe the highly dissymmetric photoluminescence (j g PL j > 0.1) of an enantiopure chiral p-conjugated small molecule acceptor (oxa [7]superhelicene, hereafter oxa- [7]H) embedded within a range of ACP donors (PFO, F8BT, F8PFB, Figure 1 and Supporting Information, SI).…”

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

“…It is proposed that E1−M1 interactions and the interference between E1−E1 and M1−M1 couplings are dependent on the handedness of the energy donor and acceptor, which contribute greatly to chirality-sensitive FRET. 42 On the basis of that point, Fuchter and Andrews et al demonstrated the CPL amplification in another FRET process. The reported twisted polymer fibril here plays the role of chiral donor with high g lum , which can transmit energy to the oxa [7]superhelicene as an acceptor with poor intrinsic CPL performance.…”

“…When it comes to the CPL in FRET, terms E1 and M1 are coined to describe this process, which arises from the electric (μ) and magnetic transition dipole moments ( m ), respectively. It is proposed that E1–M1 interactions and the interference between E1–E1 and M1–M1 couplings are dependent on the handedness of the energy donor and acceptor, which contribute greatly to chirality-sensitive FRET . On the basis of that point, Fuchter and Andrews et al demonstrated the CPL amplification in another FRET process.…”