Acid/Base-Responsive Circularly Polarized Luminescence Emitters with Configurationally Stable Nitrogen Stereogenic Centers

Abstract

A way to prevent the fast configurational interconversion of tertiary amines is to invoke Tröger's base analogs, which display methano- or ethano-bridged diazocine cores fused to aromatic rings. These derivatives are configurationally stable, even in acidic media when their structures bear ethylene bridges. Here, a two- to three-step synthesis is presented of methano- and ethano-bridged Tröger's base analogs with two peripheral fluorophores, i.e., anthracene, pyrene, and 9,9-dimethylfluorene units. These compounds, possessing two nitrogen stereogenic centers, exhibit good circularly polarized luminescence (CPL) dissymmetry factors (|g_lum| up to 1.2 × 10^-3) and brightnesses (B_CPL up to 26.3 M^-1 cm^-1), as well as excellent fluorescence quantum yields, demonstrating the Tröger´s base core to be a convenient scaffold to prepare CPL emitters upon functionalization with simple achiral fluorophores. Furthermore, the configurationally stable ethano-bridged Tröger's base analogs are employed to modulate their CPL response, generating a CPL switch through their protonation/deprotonation by consecutive additions of acid and base. The reversibility of the switching process is demonstrated for two cycles without altering the CPL performance of the molecule. It is believed that this straightforward and efficient approach to building CPL emitters employing the Tröger's base core could lead to its incorporation in CPL-based sensors and materials.

Keywords: Tröger's Base; chiroptical switch; circularly polarized luminescence; configurationally stable nitrogens; fluorophores.

Figure 1

Structural formulae of a) (RR) and (SS) methano‐ and b) ethano‐bridged Tröger's base cores displaying the enantiomers and the point chirality of the nitrogen atoms. c) Structural formulae of the photoluminescent point‐chiral methano‐ and ethano‐bridged Tröger's base analogs 1a,b‐3a,b investigated in this research, bearing two emissive anthracene, pyrene, or 9,9‐dimethylfluorene units. The Tröger's base analogs 1b‐3b possess configurationally stable stereogenic nitrogen centers in acid media on account of the ethano bridge.

Scheme 1

Synthesis of the fluorescent point chiral methano‐ and ethano‐bridged Tröger's base analogs 1a,b‐3a,b.

Figure 2

UV–vis absorption (solid lines) and normalized emission (dashed lines) spectra of methano‐ and ethano‐bridged Tröger's base analogs a) 1a,b (λ_exc = 340 nm), b) 2a,b (λ_exc = 340 nm), and c) 3a,b (λ_exc = 320 nm) recorded at r.t. in CH₂Cl₂ at concentrations of ca. 6 × 10⁻⁵ m (for 1a,b) and 2 × 10⁻⁵ m (for 2a,b‐3a,b).

Figure 3

Chiroptical properties of the photoluminescent methano‐ and ethano‐bridged Tröger's base analogs (RR)/(SS)‐1a,b‐3a,b. ECD (solid lines) and CPL (dashed lines) spectra of: a) (RR)/(SS)‐1a (λ_exc = 403 nm), b) (RR)/(SS)‐1b (λ_exc = 403 nm), c) (RR)/(SS)‐2a (λ_exc = 372 nm), d) (RR)/(SS)‐2b (λ_exc = 372 nm), e) (RR)/(SS)‐3a (λ_exc = 340 nm), and f) (RR)/(SS)‐3b (λ_exc = 342 nm) recorded at r.t. in CH₂Cl₂ at concentrations of ca. 6 × 10⁻⁵ m (for 1a,b) and 2 × 10⁻⁵ m (for 2a,b‐3a,b).

Figure 4

a) Structural formulae of methano‐bridged Tröger's base analogs (RR)‐1a‐3a showing the two dihedral angles α and β. b) Representation of the dihedral couple (α;β) of (RR)‐1a (left), (RR)‐2a (middle), and (RR)‐3a (right) along the 20 ns MD simulation, where each point represents a snapshot of the simulation. For 1a,b‐2a,b, because of steric hindrance, only the half‐rotation is possible and the dihedral angles extend over a 180° range (either [0;180] or [−180;0]). Note that for 3a,b, since the complete rotation of the fluorene moieties is possible, the dihedral angles range from −180 to +180°. Averaged calculated c) UV–vis absorption and d) ECD spectra of (RR)‐1a (green lines, left), (RR)‐2a (red lines, middle), and (RR)‐3a (blue lines, right) for 50 extracted snapshots of MD simulation (full width at half maximum (FWHM) = 0.3 eV). The results for (SS)‐1b‐3b are presented in the Supporting Information.

Figure 5

Switching of the fluorescence and CPL signals of ethano‐bridged Tröger's base analog (RR)/(SS)‐2b upon the addition of acid and base. a) Fluorescence emission spectra (λ_exc = 340 nm) of 2b in CH₂Cl₂ after the addition of increasing quantities of a CF₃CO₂H solution (0 equiv. – 50 equiv.) and b) evolution of the fluorescence intensity at different wavelengths. c) CPL (λ_exc = 372 nm) spectra of (RR)/(SS)‐2b in CH₂Cl₂ in the presence (orange lines) and the absence (pink lines) of 100 equiv. of CF₃CO₂H. d) In situ switching of the CPL signal (λ = 431 nm) of (RR)‐2b after consecutive addition of CF₃CO₂H (cycles 0.5 and 1.5, orange squares) and Et₃N (cycles 0, 1 and 2, pink squares).