Solid-state AIEnh-circularly polarised luminescence of chiral perylene diimide fluorophores

Solid-state organic fluorescent materials are important for the development of electroluminescent sensing devices. Herein, we report that N,N′-bis((R)-1-phenylethyl)perylene-3,4,9,10-tetracarboxylic diimide [(R,R)-BPP] and its antipode [(S,S)-BPP], which contain extended π-electrons through planar perylenes, emit solid-state aggregation-induced-enhanced (AIEnh) circularly polarised luminescence (CPL) in inorganic (KBr) pellets and organic-polymer-film (PMMA- and myo-IPU-film) states; this CPL is difficult to observe in solution. These chiral perylene fluorophores emit AIEnh-CPL with high dissymmetry factors (gCPL) (up to 2.4 × 10−3) and high quantum yields (ΦF, up to 0.43) in the three solid matrices.


Introduction
Because of their potential applications to electroluminescence (EL) and optical-sensing devices, solid-state organic uorescent materials have attracted much attention. 1In particular, optically active organic and inorganic luminophores that emit circularly polarised luminescence (CPL) with high dissymmetry factors (g CPL ) and high quantum yields (F F ) have received signicant attention in the eld of circularly polarised organic light-emitting diodes (CP-OLEDs). 2 Recently, aggregationinduced-enhanced (AIEnh) CPL has also attracted much interest.However, AIEnh-CPL is almost always emitted from highly concentrated solutions. 3Therefore, systems that emit solid-state AIEnh-CPL are highly desired.With the aim of providing a common design approach to novel chiral solid-state AIEnh organic uorophores, in this study we prepared solidstate AIEnh circularly polarised luminophores by doping chiral N,N 0 -bis((R)-1-phenylethyl)perylene-3,4,9,10tetracarboxylic diimide [(R,R)-BPP] and its antipode [(S,S)-BPP] into various types of solid matrix.The two BPPs are chiral bisubstituted perylene uorophores composed of extended pelectron planar perylenes.We then examined the AIEnh-CPL emissions from these imbedded BPP systems as AIEnh-uorophore models. 4KBr was used as the solid inorganic matrix, while poly(methyl methacrylate) (PMMA) and polyurethane (myo-IPU) were chosen as solid organic polymer matrices. 5Myo-IPU is derived from naturally occurring myoinositol (a meso-hexose) and characteristically has a higher glass transition temperature (T g $150 C) than conventional PMMA (T g $105 C).
Herein, we report that (R,R)-and (S,S)-BPP emit solid-state AIEnh-CPL with high quantum efficiencies when imbedded in solid inorganic and organic polymer matrices (Fig. 1).

Unpolarised photoluminescence (PL) quantum yields
Samples were dispersed in KBr pellets in the following way.Microcrystalline powdered samples (10 mg) were nely ground with 120 mg of KBr, and the mixed powder was compressed at 80 MPa cm À2 under vacuum for 25 min to provide a 10 mmdiameter, 0.7 mm-thick transparent disk.Samples dispersed in PMMA or myo-IPU lms were prepared using PMMA or myo-IPU (1.0 Â 10 À2 M) in a spin coater at 3000 rpm (Opticoat MS-A100, Mikasa, Tokyo, Japan).
Absolute unpolarised photoluminescence (PL) quantum yields (F F ) of the KBr-pellet-dispersed states, the PMMA-or myo-IPU-lm-dispersed states, and CHCl 3 solutions were determined using a Hamamatsu Photonics C9920-02 absolute quantum yield spectrometer (Hamamatsu, Japan) in air at room temperature, at excitation wavelengths of 480 nm (KBr) and 450 nm (polymer lm), respectively, and at 450 nm for the CHCl 3 solution.A 10 mm path length was used for solutionphase spectroscopy.

Solid-state PL and CPL spectroscopy
Solid-state PL and CPL spectra of the KBr-pelleted samples were acquired by comprehensive chiroptical spectrophotometry (CCS, JASCO, Tokyo, Japan). 6CPL spectra were recorded with a slit width of 33 mm and a 10 nm spectral bandwidth for the excitation and emission monochromators.The PL and CPL spectra of the PMMA or myo-IPU lms and the CHCl 3 solutions were acquired using a JASCO CPL-300 spectrouoropolarimeter (Tokyo, Japan) at room temperature, at a scattering angle of 0 upon excitation with unpolarised, monochromated incident light with a 10 nm bandwidth.An excitation wavelength of 480 nm was used for the KBr-pelleted samples, while 450 nm was used for the PMMA or myo-IPU lms, and for the CHCl 3 solutions.A 10 mm path length was used for solution-phase spectroscopy.

Circular dichroism (CD) and UV-Vis absorption spectroscopy
Circular dichroism (CD) and UV-Vis absorption spectra of the KBr-pelleted samples were acquired by CCS at room temperature. 6The CD and UV-Vis absorption spectra of PMMA-or myo-IPU-lm-dispersed states and CHCl 3 solutions were recorded with a JASCO J-820 spectropolarimeter at room temperature.The KBr-pelleted and PMMA (or myo-IPU) lms were prepared as described for the samples used for solid-state PL and CPL spectroscopy.A 1 mm path length was used for solution-phase spectroscopy.

Results and discussion
Prior to elucidating the solid-state chiroptical properties of (R,R)-BPP and (S,S)-BPP, we rst investigated their PL, CPL, and CD spectral characteristics in CHCl 3 solution, at room temperature.To determine whether or not the obtained results are due to artefacts introduced by (R,R)-BPP and other factors, the CPL spectrum of (S,S)-BPP was also acquired.The CPL and PL spectra of (R,R)-BPP and (S,S)-BPP are shown in Fig. 2 ((R,R)-BPP in blue; (S,S)-BPP in red).When dissolved in CHCl 3 (1.0Â 10 À4 M), (R,R)-BPP exhibited PL at 549, 576, 623 nm with an absolute photoluminescence quantum yield (F F ) of 0.76.Unfortunately, meaningful CPL spectra of chiral BPP could not be acquired under these conditions.
The CD and UV-Vis absorption spectra of (R,R)-BPP and (S,S)-BPP in CHCl 3 are shown in Fig. 3; as expected, the CD spectra are almost mirror images.
Absorption bands at 459, 490, and 527 nm that originate from p-p* transitions in the perylene units are commonly observed, irrespective of the chirality of the sample.As same as CPL properties in the solution-dissolved state, these corresponding CD bands could not be mainly observed.
For the chiral BPPs to emit CPL, we turned to solid-state AIEnh-CPL spectroscopy.The solid-state PL and CPL spectra of (R,R)-BPP were acquired in the inorganic KBr matrix, the results of which are shown in Fig. 4. In contrast to the results in CHCl 3 solution, solid-state CPL was observed from the KBrpelleted sample; (R,R)-BPP in the KBr pellet exhibited a l CPL of 657 nm with a corresponding F F of 0.09.When the spectrum of the KBr-pellet-dispersed state is compared with that of the CHCl 3 solution, quite different l PL values are observed (576 nm for CHCl 3 and 632 nm for KBr).The l PL of the KBr pellet suggests that CPL from this state is AIEnh CPL derived from the aggregation of (R,R)-BPP in the solid state.Unfortunately, the value of F F in the KBr-dispersed state was smaller than that in CHCl 3 solution, which is ascribable to uorescence quenching under high-density conditions in the solid compared to the lowconcentration conditions in solution.In addition, the external heavy-atom (Br) effect may also be partially responsible.
A negative (À) CPL spectrum was observed for (R,R)-BPP in the solid state.To determine whether or not these results are due to artefacts introduced by (R,R)-BPP and the KBr matrix, the CPL spectrum of (S,S)-BPP was acquired (Fig. 4).(R,R)-BPP and (S,S)-BPP in KBr matrices exhibit almost mirror-image CPL spectra.The magnitude of the circular polarisation in the excited state is dened as g CPL ¼ DI/I ¼ (I L À I R )/(I L + I R ), where I L and I R are the output signals for le and right circularly polarised light under unpolarised photoexcitation conditions.The CPL efficiency, |g CPL |, of BPP in the KBr pellet is of the order of 10 À3 , while the |g CPL | of BPP is $2.0 Â 10 À3 in the KBrdispersed state.
To examine molecular chirality in the ground state, the CD and UV-Vis absorption spectra of (R,R)-BPP and (S,S)-BPP in their KBr-pellet-dispersed states were acquired, the results of which are shown in Fig. 5.
Several UV-Vis bands between 400 and 650 nm, which are characteristic of p-p* transitions in the perylene groups of BPP, can be observed.In this state, the Cotton CD bands of (R,R)-BPP and (S,S)-BPP, are almost mirror images and are due to p-p* transitions, as evidenced by their UV-Vis spectra.The CD spectra of (R,R)-BPP and (S,S)-BPP are almost unaffected by KBrpellet dispersion.Interestingly, the CD spectra of the KBrdispersed states are quite different to those of the CHCl 3 solutions are similar to those of the previously reported nanoscale aggregates in the solution-dispersed state.4g We conclude that the CD spectra of the KBr-dispersed states also result from intermolecular interactions between the perylene units of multiple aggregated BPP molecules within the KBr pellets.To quantitatively evaluate the CD amplitude in the ground state, we evaluated the anisotropy factor.The magnitude of the circular polarisation in the ground state is dened as In order to emit stronger CPL, we next examined CPL from organic polymer-lm states.As an organic polymer matrix, poly(methyl methacrylate) (PMMA) and polyurethane (myo-IPU) derived from naturally occurring myo-inositol were used.The CPL and PL spectra of (R,R)-BPP and (S,S)-BPP dispersed in   PMMA and myo-IPU lms are shown in Fig. 6(a) and (b), respectively.
(R,R)-BPP in PMMA emitted at 654 nm (l CPL ) with a F F of 0.40, while in myo-IPU, it emitted at 637 nm (l CPL ) with a F F of 0.43.As expected, (R,R)-BPP exhibits solid-state AIEnh-CPL in both polymers, with several regressional phonon sidebands originating from the lowest p-p* transitions between perylene groups observed.The CPL spectra of (R,R)-BPP in PMMA and myo-IPU lms mostly exhibited negative (À) CPL bands (Fig. 6(a) and (b), respectively).As expected, (S,S)-BPP exhibited positive (+) CPL spectra in these polymers (Fig. 6 As expected, despite the CPL wavelengths of (R,R)-BPP in the KBr and polymer states being similar, the values of F F in these polymers are signicantly higher than that observed for the KBr state (0.09).We assume that these approximately ve-timeshigher values are due to the suppression of thermal deactivation modes and effective packing arrangements because the molecular geometry of the chiral BPP is effectively and suitably xed by the surrounding solid polymer matrix without quenching.
The CD and UV-Vis absorption spectra of (R,R)-BPP and (S,S)-BPP in the PMMA and myo-IPU lms are shown in Fig. 7(a) and (b), respectively; the CD spectra in these polymers are similar in their long-wavelength tails and are similar to those of the KBrdispersed states.These results suggest that the CD spectra of the polymer-dispersed states are essentially due to intermolecular interactions between multiple molecules rather than intramolecular interactions within individual BPP molecules.The characteristic UV peaks originating from p-p* transitions of the perylene groups are commonly observed between 400 and 600 nm.The CD spectra of (R,R)-BPP and (S,S)-BPP in both matrices are almost mirror images, as expected.The signs of the rst Cotton CD bands of (R,R)-BPP, at 553 nm in PMMA and at 551 nm in myo-IPU, are both positive (+), while the CD signs of the corresponding bands for (S,S)-BPP in PMMS and myo-IPU are both negative (À).The |g CD | value at the rst Cotton CD bands are ~3.1 Â 10 À4 and ~2.9 Â 10 À4 for the PMMA-and myo-IPU-dispersed states, respectively.BPP in these matrices exhibit |g CD |values of similar magnitude.
As was observed in the KBr matrix, the |g CPL | values are higher than the corresponding |g CD | values in the polymer lms.This shows that intermolecular twists between perylene units in the ground state increase upon photo-excitation, as evidenced by the intense excimer CPL bands in spectra of the polymer-matrix imbedded samples.

Conclusions
Chiral (R,R)-and (S,S)-N,N 0 -bis(1-phenylethyl)perylene-3,4,9,10tetracarboxylic diimide (BPP) dispersed in solid KBr and PMMA (or myo-IPU) matrices exhibited solid-state AIEnh-CPL with high F F values at room temperature.The value of F F from the PMMA (F F 0.40) and myo-IPU matrices (F F 0.43) are signicantly higher than that observed from the KBr matrix (F F  View Article Online
where Abs L and Abs R are the absorbances of the le and right circularly polarised light, respectively.The value of |g CD | at the rst Cotton CD band of BPP in KBr was determined to be $5.4 Â 10 À4 at 561 nm.The observed CD results in KBr support the CPL observations mentioned above.Interestingly, a comparison of these anisotropy factors, i.e., the |g CPL | and |g CD | values, reveals that the |g CPL | is larger than the corresponding |g CD | value, indicating that, although the perylene units are stacked and twisted in the ground state, 4f the twists increase in the photoexcited state, as conrmed by the intense excimer CPL bands.
(a) and (b)).Although the CPL spectral shapes of (R,R)-BPP and (S,S)-BPP in the KBr-pellet and PMMA-and myo-IPU-lm states are nearly mirror images, subtle differences in the shapes of the CPL signals are noted.These differences may be due to the effect of humidity in the pellets and lms during the preparation of the pellet and lm samples.The |g CPL | values of BPP in the PMMA and myo-IPU states are $2.4 Â 10 À3 and $1.3 Â 10 À3 , respectively, which are similar to that observed in the KBr state ($2.0 Â 10 À3 ).