Control of Dynamic Chirality in Donor-Acceptor Fluorophores

Abstract

Recently, the control of dynamic chirality has emerged as a powerful strategy to design chiral functional materials. In this context, we describe herein a molecular design in which a tethered configurationally stable binaphthyl chiral unit efficiently controls the dynamic chirality of donor-acceptor fluorophores, involving diverse indolocarbazoles as electron donors and terephthalonitrile as an electron acceptor. The high conformational discrimination in such a molecular system suggested by density functional theory calculations is experimentally probed using electronic and vibrational circular dichroism and confirmed by the crystallization of these chiral molecules in gel and their single crystal X-ray diffraction analysis. This work also highlights the positive effect of the configurationally stable chiral unit on the magnitude of the dissymmetry factors of the active dynamically chiral fluorophores, both in ground and excited states, through chiral perturbation.

Keywords: Chiral materials; Circularly polarized luminescence; Dissymmetry factor; Electronic circular dichroism; Vibrational circular dichroism.

Figure 1

Illustration of the dynamic chirality control concept with : A. recently described examples; B. the molecular design investigated in this work.

Figure 2

a) Synthesis of ICzTPN(R)‐BN; b) Frontier molecular orbitals distribution of ICzTPN(R)‐BN calculated at the B3LYP/6‐311G(d,p) level with SMD for implicit solvation (CHCl₃), isovalue: 0.2 eV; c) Geometries of the 3 main conformers of ICzTPN(R)‐BN, interconversion barriers and Boltzmann population calculated at the B3LYP/6‐311G(d,p) level with SMD for implicit solvation (here CHCl₃); d) Geometries of the 3 main conformers of the model compound ICzTPN, interconversion barriers and Boltzmann population calculated at the B3LYP/6‐311G(d,p) level with SMD for implicit solvation (here CHCl₃).

Figure 3

a) Simulated ECD spectra (Δϵ=f (λ)) of the 3 conformers of ICzTPN(R)‐BN; A: (P, P)‐ICzTPN(R)‐BN (pink curve), B: (M, P)‐ICzTPN(R)‐BN (grey curve), C: (M, M)‐ICzTPN(R)‐BN (blue curve); b) Comparison between the experimental (red, C=1.2×10⁻⁴ M) and simulated (green, considering the Boltzmann distribution determined based on the DFT calculations) UV and ECD spectra (Δϵ=f (λ)) of ICzTPN(R)‐BN in acetonitrile; c) simulated VCD spectra of the 3 conformers of ICzTPN(R)‐BN. A: (P, P)‐ICzTPN(R)‐BN (pink), B: (M, P)‐ICzTPN(R)‐BN (grey), C:(M, M)‐ICzTPN(R)‐BN (blue); d) Comparison between the experimental (red, C=0.01 M) and simulated (green, considering the Boltzmann distribution determined based on the DFT calculations) VCD spectra in DCM solution.

Figure 4

SCXRD structures of ICzTPN(R)‐BN and ICzTPN(S)‐BN.

Figure 5

A) Synthesis of substituted ICzTPNBN analogues; B.a) UV/Visible spectra and B.b) Fluorescence spectra (excitation wavelength: 470 nm) of ICzTPN(R)‐BN in different solvents (C=10 μM); The inset picture shows ICzTPNBN in cyclohexane, toluene, dichloromethane, and DMSO solutions from left to right under UV light (254 nm); B.c) Table summarizing the photophysical and chiroptical properties at the excited states of the ICzTPNBN derivatives, g_lum are calculated as the average of absolute value for both enantiomers.

Figure 6

a) Optimized geometries of the model compounds (P , P )‐ICzTPN, (P , P )‐ICzTPNNaphthol, and the 3 conformers ((P, P), (M, P), (M, M)) of ICzTPN(R)‐BN, 3,8‐BrICzTPN(R)‐BN, and 3,8‐OMeICzTPN(R)‐BN at the excited state and representation of the electric (red) and magnetic (blue) transition dipole moments (a.u., m magnified by 5); b) Calculated emission dissymmetry factors.