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Circularly Polarized Persistent Room-Temperature Phosphorescence from Metal-Free Chiral Aromatics in Air Shuzo Hirata, and Martin Vacha J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00554 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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The Journal of Physical Chemistry Letters

Circularly Polarized Persistent Room-temperature Phosphorescence from Metal-free Chiral Aromatics in Air Shuzo Hirata*, Martin Vacha Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan Corresponding Author * Correspondence and requests for material should be addressed to S. H. (email: [email protected])

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ABSTRACT

Circularly polarized room-temperature phosphorescence (RTP) with persistent emission characteristics was observed from metal-free chiral binaphthyl structures. Enantiomers of the binaphthyl compounds doped into an amorphous hydroxylated steroid matrix produced blue fluorescence and yellow persistent RTP in air. The lifetime and quantum yield of the yellow persistent RTP were 0.67 s and 2.3%, respectively. The dissymmetry factors of circular dichroism (CD) in the first absorption band, circularly polarized fluorescence (CPF), and circularly polarized persistent RTP were |1.1×10−3|, |4.5×10−4|, and |2.3×10−3|, respectively. A comparison between the experimental data and calculations by time-dependent density functional theory for transient CD spectra confirmed that the binaphthyl conformations in the lowest singlet excited state (S1) and the lowest triplet state (T1) were different. The large difference in the dissymmetry factors for the CPF and the circularly polarized persistent RTP was likely caused by this conformational change between S1 and T1.

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Molecules and materials showing circularly polarized luminescence (CPL) are potentially useful for a variety of applications, such as in high contrast flat panel displays,1-3 three dimensional displays,4 optical data storage,5 information storage and processing,6,7 spintronics devices,8 sensors,9 security inks, and biological probes.10 CPL is typically observed as fluorescence from chiral aromatic structures,11 including optically active conjugated polymers,1,12-14 and supramolecular aggregates of chiral aromatics.14-20 By contrast, chiral metal-complexes with heavy atoms show circularly polarized room-temperature phosphorescence (RTP), which is generated by large spin orbital coupling because of the heavy atom effect.21-24 Chiral lanthanoid complexes often produce RTP with very high dissymmetry factors.23,24 While circularly polarized RTP has been observed from many metal-complexes, it has not been observed from metal-free aromatics. In 2010, RTP of metal-free aromatics has been reported from the crystals of some twisted aromatic compounds including heavy-atom free benzophenone derivatives.25 However, the phosphorescence rate of heavy atom-free aromatics with a planer aromatic structure is often smaller than 100 s-1, which is small compared with that of twisted aromatic compounds. Because the slow phosphorescence rate is much smaller than the rate of the nonradiative process from the lowest excited triplet state (T1) at room-temperature in air,26 it was very difficult to obtain efficient RTP from the planer aromatic structures. In 2010, we demonstrated RTP with a phosphorescence lifetime longer than 1 s from various aromatic compounds with small phosphorescence rate in a rigid non-conjugated amorphous host,27 and then summarized a design protocol for these materials in 2013.28 In these materials, the high rigidity of the host matrix largely suppressed the nonradiative deactivation process from T1 for a variety of aromatic guests, resulting in generation of slow RTP from the metal-free aromatics. The quantum yield of the persistent RTP from the aromatic hydrocarbons increased to 8 % and


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the lifetime was longer than 1 s. Deuteration of the aromatics was useful for increasing the quantum yield of the persistent RTP so that it was larger than 10% for red-green-blue colors. Persistent RTP has also been observed from several molecular crystals of aromatic chargetransfer structures.29-35 Because of its very long lifetime, persistent RTP of a material remains after the excitation light has ceased. With molecule-based persistent RTP materials,28-35 a variety of functions are possible, including mechanoresponsiveness,30-32 thermoresponsiveness,36,37 pH responsiveness,38 and photoresponsiveness39. These functions are difficult to achieve with conventional inorganic metal oxide-based persistent RTP materials.40 Molecule-based persistent RTP characteristics could be useful in many applications, including background independent emission probes,39 stimuli sensors,30-32 thermometers,36,37 and security inks.38,39 However, persistent RTP with circularly polarized luminescent characteristics has so far not been reported. Here, we report circularly polarized RTP with afterglow emission characteristics in air from metal-free and heavy atom-free chiral aromatics. Enantiomers of N,N′-dimethyl-1,1′binaphthyldiamine (DMBDA) doped into β-estradiol produced blue fluorescence and yellow persistent RTP in air. The lifetime and quantum yield of the yellow persistent RTP were 0.67 s and 2.3% in air, respectively. (S)- and (R)-DMBDAs showed blue CPF with a dissymmetry factor of ±4.5×10−4 and yellow circularly polarized persistent RTP with a dissymmetric factor of ±2.3×10−3, respectively. Good agreement between transient circular dichroism (CD) characteristics of triplet–triplet (T–T) absorption and theoretical results obtained by timedependent density functional theory (TD-DFT) indicated that the conformation at T1 was largely different from that at the lowest excited singlet state (S1). This large conformation difference is proposed as the origin of the different values of the dissymmetry factors for the CPF and the circularly polarized persistent RTP.


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The structures of (R)- and (S)-DMBDA and β-estradiol are shown in Figure 1a. Figure 1b shows spectral characteristics of amorphous β-estradiol doped with (R)-DMBDA at a mass fraction of 0.3% (sample R). The sample was held between two quartz substrates. Because βestradiol does not absorb at wavelengths longer than 320 nm,28 the black line in Figure 1b represents the absorption spectrum of (R)-DMBDA. The molar absorption coefficient (ε) was (6.3±0.3)×103 M−1 cm−1 at 361 nm. The blue line in Figure 1b represents the emission spectrum

Figure 1. (a) Structures of (R)-DMBDA, (S)-DMBDA, and β-estradiol. (b) Absorption spectrum (black), emission spectrum under excitation at 360 nm (blue), and emission spectrum soon after ceasing excitation at 360 nm (yellow) for a 0.3 % (mass fraction) (R)-DMBDA-doped β-estradiol film. (c) Changes in the luminescence of 0.3 % (mass fraction) (R)-DMBDA-doped β-estradiol powders under excitation and after ceasing the excitation.


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of sample R under excitation at 360 nm. The emission, with the main peak occurring at 420 nm, was ascribed to fluorescence of (R)-DMBDA because the lifetime of the emission was 6.0 ns (Figure S1). The quantum yield of the blue fluorescence (Φf) was 30%. The values of ΦF and the peak wavelength of the fluorescence for (R)-DMBDA changed little when the environment was changed from the β-estradiol matrix to a toluene solution (Figure S2). In contrast, the phosphorescence characteristics of (R)-DMBDA were largely different in the two environments. (R)-DMBDA in β-estradiol showed a small shoulder between 500–650 nm under excitation at 360 nm (blue line in Figure 1b and red line in Figure S2). This emission was ascribed to RTP from (R)-DMBDA because an emission spectrum from sample R soon after ceasing the excitation at 360 nm was located at 500–600 nm (yellow line of Fig. 1b), and the lifetime of the yellow persistent RTP (τp) was 0.67 s (inset of Figure 1b). Because of the long RTP lifetime, clear yellow emission remained after ceasing the excitation (Figure 1c). The RTP yield (Φp) of sample R in air was determined as 2.3% using an integrating sphere (see Supporting Information in earlier study28). No large changes in the values of Φp and τp were observed when the temperature was altered from room-temperature to that of liquid nitrogen (Figure S3), and this agreed with our previous report using other achiral aromatic guests in amorphous hydroxylated steroid hosts.28 The emergence of persistent RTP at room-temperature is caused by the rigidity and high gas barrier characteristics of β-estradiol, which minimize nonradiative deactivation from T1 for DMBDA doped into the hydroxylated steroid host, as discussed in our earlier report.28 By contrast, RTP of (R)-DMBDA was not observed in toluene at all, even when oxygen was removed by freeze-pump-thaw degassing (blue line in Figure S2). The CD characteristics of sample R and 0.3 % (mass fraction) (S)-DMBDA-doped βestradiol film (sample S) are shown in Figure 2a. The dissymmetry factors for the CD (gabs) in


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Figure 2. (a,b) CD (a) and CPF (b) spectra of 0.3 % (mass fraction) (R)-DMBDA-doped βestradiol film (red) and 0.3 % (mass fraction) (S)-DMBDA-doped β-estradiol film (blue). (c) Three types of conformations (1–3) of (R)-DMBDA optimized at S0 and S1. The structures were determined by conformation analysis and TD-DFT (CONFLEX 7 and Gaussian 09, ωB97XD functional, cc-pvtz basis set). Figures in green show the angle between two methylamino naphthalenes. The R values in red and blue represent the rotatory strength of the lowest singlet transition in the S0 or the S1 geometries.

the first absorption band of (S)- and (R)-DMBDAs were (+1.1±0.1)×10−3 and (−1.1±0.1)×10−3, respectively. The CPF characteristics of samples R and S under excitation at 360 nm are shown in Figure 2b. The dissymmetry factors for the CPF (gf) of (S)- and (R)-DMBDAs were (+4.6±


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1.6)×10−4 and (−4.5±1.7)×10−4, respectively. These results show that gf was smaller than gabs in the first absorption band. Although absorption and fluorescence radiation processes are intrinsically the same,41 the molecular conformation in absorption is different from that in fluorescence because of conformation changes due to relaxation process in S1 after absorption.2 This conformation change potentially causes the difference between |gf| and |gabs| in the first absorption band. To discuss the relationship between conformation and the magnitude and sign of the CD in the first absorption band and those of CPF, local minimum structures of (R)-DMBDA at S0 and S1 were explored using CONFLEX 7 and DFT calculations (Gaussian 09, ωB97XD functional, cc-pvtz basis set). There were three local minimum structures for (R)-DMBDA (1–3, Figure 2c), which had similar total energies. As shown by the rotatory strength values (R)42 in Figure 2c, the TD-DFT (Gaussian 09, ωB97XD functional, cc-pvtz basis set) calculation indicated that conformations 1 and 3 of (R)-DMBDA had negative Cotton effects both for CD in the first absorption band and for CPF. By contrast, conformation 2 of (R)-DMBDA had a negative Cotton effect for CD in the first absorption band and a positive Cotton effect for CPF. Experimentally, the signs of both the CD in the first absorption band and that of CPF were negative. Therefore, (R)-DMBDA must be either in conformation 1 or 3 in amorphous β-estradiol. However, it is difficult to distinguish between the two conformations using only the information from S0 and S1. This is because these two conformations showed similar absorption and fluorescence energies in the calculations (Table S1). The relationship between |gabs| in the first absorption band and |gf| was also investigated in the calculations. However, the experimentally observed relationship of |gf|

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