Controlling the thermally activated delayed fluorescence of axially chiral organic emitters and their racemate for information encryption

A pair of axially chiral organic enantiomers were facilely prepared through a one-pot sequential synthesis. They exhibit circularly polarized luminescence activities and have thermally activated delayed fluorescence (TADF) and aggregation-induced emission enhancement properties. Meanwhile, these two enantiomers present remarkable and reversible thermochromism in the crystalline state, enabling dual-colour TADF switching between orange and red. However, when they form cocrystals, the resulting racemate shows opposite thermochromic behaviors. These intriguing results probably emanate from their different optical activities, leading to distinct molecular packing modes and molecular conformation variations. Moreover, information encryption based on thermochromism of organic enantiomers and their racemate has been presented for the first time. This work may expand the application scope of chiral organic luminogens and pave a new way to construct intelligent luminescent systems.

carbazole, and super dry N,N-dimethylformamide were purchased from J&K Scientific. 9,9-Dimethyl-9,10-dihydroacridine was purchased from Suzhou Geao Company (China). All the materials noted above were used as received. Other solvents and reagents were purchased from Guangzhou Zeyuan Company (China) with analytical grade and used without further purification.

Measurements
Nuclear magnetic resonance ( 1 H NMR and 13 C NMR) spectra were measured on a Bruker AVANCE 400 MHz spectrometer (in CDCl3 with internal standard of tetramethylsilane). Mass spectra were conducted on DSQ & MAT95XP-HRMS thermospectrometers. Effective diameters of the nanoaggregates in H2O/THF mixtures with 90% water fraction were investigated on a Malvern Zetasizer (Nano ZS90). Thermal transitions of the samples were determined by differential scanning calorimetry (DSC) at a heating rate of 10 ºC/min under N2 atmosphere using a TA thermal analyzer (Q20). Wide-angle XRD patterns were achieved at 293 K by a Bruker X-ray diffractometer (D8 ADVANCE, Germany) in a scan rate of 4° (2θ)/min. PL spectra were obtained on an Ocean Optics spectrophotometer (QE65 Pro) with a 365 nm LLS-LED as excitation source. UV-visible absorption spectra were monitored on a Hitachi U-3900H spectrophotometer. CPL spectra were measured on a JASCO CPL-300 spectrophotometer. Circular dichroism (CD) spectra were characterized by using a CD spectrometer (J-815) from JASCO instrument. Time-resolved emission decay curves and absolute PL quantum yields (PLQYs) were recorded on a spectrometer (FLS980) equipped with a calibrated integrating sphere from Edinburgh Instruments.
Single-crystal X-ray diffraction data for the enantiomers and racemate was obtained from an Agilent Technologies Gemini A Ultra system with Cu-K radiation ( = 1.54178 Å). The structures were solved using direct methods following the difference Fourier syntheses. All non-hydrogen atoms were anisotropically refined through least-squares on F 2 using the Olex2 program suite.
CCDC 1846015, 2092971 and 2092972 contain the supplementary crystallographic data for this paper. Theoretical calculations based on time-dependent density functional theory (TD-DFT) were 3 conducted on Gaussian 16 program with a method similar to previous literature. [1] Ground state (S0) geometries of (S)-CzACN were directly selected from single crystal structures and were used as molecular models without further optimization. On the basis of this, Kohn-Sham frontier orbitals and energy levels of singlet (S1) and triplet (T1) excited states were simulated by using TD-DFT at the B3LYP/6-311G(p, d) level.

Synthesis
Scheme S1. Synthetic routes of the target compounds.

Preparation of (R,S)-CzACN
The phosphorescence spectra for the pristine and annealed samples of the enantiomers and racemate were recorded, and the results were presented in Figs. S32 and S33. In each delayed emission spectrum, there is a broad band at around 555 nm, which can be assigned to the phosphorescence of binaphthalene. [2] It means that the delayed luminescence of the samples at cryogenic temperature was probably composed of phosphorescence from both the binaphthalene unit and the donoracceptor fragment of the involved molecules. In fact, the donor-acceptor fragments of (R)-CzACN and (S)-CzACN emitted very weak phosphorescence even at 77 K. Therefore, the phosphorescence bands of the binaphthalene moieties became observable in the delayed spectra of the samples and partially overlapped with those of the donor-acceptor fragments. As a result, it is difficult to accurately determine the EST values of (R)-CzACN and (S)-CzACN from experimental data.
Herein, by using the onset of steady-state PL spectra and the first emission peak of phosphorescence from the donor-acceptor fragments, the EST values for the pristine and annealed samples of the enantiomers were roughly estimated to be 0.17 eV and 0.11 eV, while those of the racemate were evaluated to be 0.16 eV and 0.18 eV, respectively. All of them are smaller than 0.20 eV, indicating that the samples have potential to produce TADF. However, the experimental result was that no delayed emission could be observed from the pristine cocrystals of racemate. Consequently, the unstable triplet excited states resulted from their lower energy levels and the relatively active molecular motions, which are supported by the reduce of F,s value, may be responsible for the loss of TADF property of the red cocrystal.