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Letter
Dicyanomethyl Radical-Based Near-Infrared Thermochromic Dyes with High Transparency in the Visible Region Kohei Okino, Daisuke Sakamaki, and Shu Seki ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00049 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 31, 2019
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Kohei Okino, Daisuke Sakamaki,*† and Shu Seki* Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT: In this work, we prepared novel three dicyanomethyl radicals having a triphenylamine skeleton, which show thermochromism in the near-infrared (NIR) region based on the reversible dimerization–dissociation reactions. We demonstrated that the coplanar arrangement of the amino group to the benzene ring bearing the dicyanomethyl radical by O or S bridges causes the drastic bathochromic shift of the radical absorption bands toward the NIR region. In particular, the radical with two bridging O atoms has the most red-shifted radical band of max = 1059 nm compared to that of the radical with no bridging O atoms (max = 712 nm). Due to the significant bathochromic shift of the radical bands, these radicals have a wide optical window in the visible region. In particular, the radical with the two O bridge shows almost no perceptible color change to the naked eye and keeps high transparency over all temperature ranges, whereas the intensity of the NIR absorption drastically changes. These radicals maintained their absorption properties in a polycarbonate matrix and high stability.
Organic radicals that undergo dimerization and dissociation reversibly have been of growing interest both from fundamental chemistry relating to the nature of chemical bonding and practical applications in dynamic covalent chemistry (DCC).1–2 Since 2016, we have reported some para-amino substituted dicyanobenzyl radicals3 showing reversible thermochromism based on the equilibrium between the radical form with strong absorption in the visible range and the almost colorless -bonded dimer form (Figure 1).4,5 For example, the radical with a diphenylamino group (DP·) showed reversible color change in the solution phase between vivid green and faint yellow by temperature change resulting from the radical absorption band around 700 nm.4 The effective spin-delocalization over the amino substituent at the para-position plays a key role in the increased thermodynamic stability of these radicals compared with the original dicyanobenzyl radical.3 We also demonstrated that the dicyanomethyl radical with a julolidine skeleton (Jul·) dimerized to form a -dimer instead of a -dimer (Figure 1).5 Very recently, other research groups also reported the syntheses and the dynamic behaviors of the dicyanomethyl radicals introduced in various -systems, such as para-substituted benzenes,6–8 naphthodithiopnene,9 hexa-peri-hexabenzocoronene,10 carbazole,11 and subporphyrin.12 These are suggestive of uniqueness of dicyanomethyl radicals representing not only tunable inter-radical interaction but also a variety of dimerization behavior by modifying the structure. In particular, the coplanar arrangement of the amino group to the benzene ring with the dicyanomethyl group may have a profound influence on the dimerization behavior judging from the fact that only the radicals with a julolidine skeleton undergo -dimerization among the para-amino substituted dicyanobenzyl radicals.5,7 Therefore, in this work, we prepared novel three dicyanomethyl radicals with the rotationally fixed diphenyl amino group bound to the benzene bearing the dicyanomethyl group by the insertion of a sulfur atom (SDP·), an oxygen atom (ODP·), and two oxygen atoms (O2DP·). We investigated the DCC properties of thee radicals
Figure 1. Structures of dicyanomethyl radicals with a para-amino group. and found that these radicals are very rare examples of NIR thermochromic dyes with a wide spectral window in the visible region (Figure 1).13–15 The three radicals were synthesized according to Scheme S1. The precursors of SDP· and ODP· were synthesized by bromination of 10-phenylphenoxiazine or 10-phenylphenothiazine followed by palladium-catalyzed coupling with malononitrile, respectively. The 2,2’:6’,2’’-dioxatriphenylamine16 skeleton of O2DP· was synthesized by the improved procedure reported by Wakamiya et al. 17 The target radicals were obtained by the oxidation of three precursors, SDPH, ODPH, and O2DPH, with PbO2 in CH2Cl2. The obtained products were red-brown solids for the oxidation of SDPH and ODPH, and a yellow solid for the oxidation of O2DPH. The toluene solutions of the solids showed intense ESR and no NMR signals at room temperature due to the existence of the dissociated radicals. The ESR intensities of these solutions decreased as the temperature decreased, indicating the formation of the dimeric species (Figure S13–S15). The 1HNMR signals of these solutions began to appear by lowering the temperature similarly to the dicyanomethyl radicals forming the -dimers (Figure S10–S12). When compared at the same temperature, SDP· gave the clearest 1HNMR signals and O2DP· gave the vaguest signals among the three radicals, suggesting that the order of the tendency of the dimerization will be SDP· > ODP· > O2DP·. We measured the temperature dependence of the electronic absorption spectra of SDP·, ODP·, and O2DP· in toluene (Figure 2). All the solutions showed the prominent longer-wavelength absorption bands with vibronic progressions attributable to the dissociated radical species, and the intensities of the radical bands increased upon temperature upshift. As the temperature decreased, the radical
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Table 1. Experimental dimerization parameters and the calculated spin density on the central carbon of the dicyanomethyl group (c) of SDP·, ODP· and O2DP· (UB3LYP/6-31G(d)).
Sdim / J K−1 mol−1
DP·a
−55
−68
0.463
SDP·
−56
−77
0.455
ODP·
−47
−72
0.427
O2DP·
−43
−50
0.426
a Taken
Figure 2. Variable temperature UV-Vis-NIR absorption spectra of a) SDP·, b) ODP·, and c) O2DP· (1.0×10−4 M in toluene). The jagged spectral shapes around 1150 nm are due to the artifacts (See Figure S19). bands disappeared, and no new absorption bands attributable to the , -dimer were observed, unlike the case of Jul·.5 The temperature dependences of the electronic absorption and NMR spectra are strongly suggestive of conversion into corresponding -dimers. The most interesting features of the radicals are significant bathochromic shifts of the radical bands compared with the previously reported dicyanomethyl radicals. Whereas the dicyanobenzyl radicals have generally the characteristic peak in visible region, these three radicals have the peak in the NIR region. The 0–0 transitions of the radical bands of SDP· and ODP· (max) were observed at 1050 and 934 nm, respectively, and these peaks were redshifted by
c
Hdim / kJ mol−1
from ref. 4.
338 nm (4521 cm−1) and 222 nm (3338 cm−1) compared to that of DP· (max = 712 nm).4 The 0–0 transition of O2DP· was observed at 1059 nm, showing a further redshift of 125 nm (1264 cm−1) compared to that of ODP· resulting from the insertion of another oxygen bridge. Besides the NIR bands, SDP· and ODP· also showed absorption bands in the visible region (around 390 ~ 600 nm) and due to these visible absorption bands, the solutions of SDP· and ODP· became faint red-brown as the temperature increased. On the other hand, the solution of O2DP· did not show remarkable spectral change in the visible region except for an increase of absorption at 440 nm, which is the longer wavelength shoulder of the temperature-independent band at 400 nm. Therefore, the solution of O2DP· has no obvious absorption in the visible region longer than 460 nm. In consequence, the solution maintained its faintyellow color and high transparency regardless of temperature, whereas the absorption in the NIR region changes drastically by temperature variation. Note that the jagged spectral shapes around 1150 nm for all the spectra were due to an artifact originated from the static absorption of cells used the spectroscopy (Figure S16). Encouraged by the unique optical properties in solution, we examined the feasibility of these radicals as solid-plastic NIR thermochromic dyes. Plastic films dispersed with these radicals were successively prepared and their electronic absorption spectra were recorded under variable temperature. Polycarbonate, the choice of plastic matrix, containing these radicals in a 5 wt% was drop-casted onto quartz plates from the toluene solutions, and dried under an ambient conditions. The colors of the as-prepared films were light brown or orange (Figure S17–S19). At 293 K, these films showed similar spectral shapes to the solutions at the same temperature, except for the slight red-shifts of the radical bands. As the temperature increased, these films also showed the similar increase of the radical bands as in the solutions, suggesting that the dimers could dissociate to the radicals in the polymer matrix. The color of the films of ODP· and O2DP· did not change after heated up to 363 K, whereas the color of SDP· became slightly deeper. The spectral shapes of the films once heated and then cooled to room temperature remain unchanged even after 14 days under ambient conditions, suggesting the restricted recombination and the high stability of the dissociated radicals in the polymer matrix. We simulated the absorption spectra of SDP·, ODP·, and O2DP· by the time-dependent density functional theory (TD-DFT) calculations. The calculations at the UB3LYP/6-31G(d) level qualitatively reproduced the observed spectra for all the radicals, while the lowest-energy bands were systematically overestimated by 0.1–0.2 eV (Figure S23–S25). The lowest- and the second lowest-energy transitions were summarized in Figure 3. For all the radicals, the lowest-energy transitions were attributed to the transitions from the -HOMO to the -LUMO. For SDP· and ODP·, the small absorption bands around 550 nm, which increased upon temperature elevation, were attributed to the second lowest-energy transitions from
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Figure 3. TD-DFT calculated transitions of SDP·, ODP· and O2DP· (UB3LYP/6-31G(d)). the -(HO−1)MO to the -LUMO. For these two radicals, the oscillator strengths of the second-lowest-energy transitions were not negligible (0.0858 for SDP· and 0.0548 for ODP·) compared with those of the lowest-energy bands (0.1464 for SDP· and 0.1950 for ODP·), and the observed gradual color changes of the solutions of SDP· and ODP· from faint-yellow to red-brown upon temperature elevation could be ascribed to the increase of the second-lowestenergy bands. On the other hand, for O2DP·, the oscillator strength of the second-lowest-energy transition was calculated to be only 0.0009, whereas that of the lowest-energy transition was calculated to be 0.2085. This could be understood by the MO distributions and the C2 symmetric structure of O2DP·. As shown in Figure 3, the transition from the -(HO−1)MO to the -LUMO is regarded as a charge transfer type transition from the 2,2’:6’,2’’-dioxatriphenylamine skeleton to the dicyanobenzyl skeleton, but however, the transition from the -(HO−1)MO (A) to the -LUMO (B) along with the C2 axis is symmetry forbidden. The transitions from the -(HO−1)MO to the -LUMO are not canceled for SDP· and ODP· with asymmetric substitution patterns (Table S1–S3). The calculations with other functionals and basis sets gave qualitatively same results as obtained using the UB3LYP/6-31G(d) level , and among the tested conditions, the UB97D3/6-31+G(d) level gave the best fits to the observed NIR absorption bands (Table S4). As explained above, the suppression of the second-lowest-energy transition for O2DP· would be the key factor for the little color change of O2DP· in solution compared with SDP· and ODP·. The enthalpy and entropy changes at the dimerization of SDP·, ODP· and O2DP· were calculated from the temperature dependence of the dissociation equilibrium constant in the same way as our previous studies (Table1).4,5 The value of Hdim of SDP· was calculated to be −56 kJ mol−1, and this value was close to that of DP· (−55 kJ mol−1).4 The |Hdim| values of ODP· was 47 kJ mol−1, which is smaller than that of SDP·. The decreased |Hdim| of ODP· could be attributed to the increase of the molecular planarity, which facilitates spin delocalization, of ODP· with a phenoxazine skeleton compared with SDP· with a bent phenothiazine skeleton. The Hdim value of O2DP· with two bridging oxygen atoms was −43 kJ mol−1. The results of our previous works strongly suggest that Hdim of the -dimers of these types of radicals correlate with the Mulliken atomic spin density on the central carbon of the dicyanomethyl group (c) of the radical; a radical with a lower c tend to make a weaker inter-monomer C—C bond.4,5 The values of c were calculated to be 0.455 for SDP·, 0.427 for ODP·, and 0.426 for O2DP·, respectively (at the UB3LYP/6-31G(d) level). The comparison of SDP· and ODP· suggests that the oxygen linker facilitate more spin delocalization than the sulfur linker. Comparing ODP· with O2DP· there was no significant difference in spin density due to the additional crosslinking. As shown in Figure 4, there is a correlation between the experimental Hdim values and the
Figure 4. A plot of the experimental enthalpy changes at the dimerization versus calculated c of para-amino substituted dicyanomethyl radicals (UB3LYP/6-31G(d)). DFT-calculated c values of the para-amino substituted dicyanomethyl radicals forming the -dimer. The c values of ODP· and O2DP· were even smaller than that of Jul· forming the -dimer (0.438). The theoretical evaluation of the stabilization energies on the - and -dimerizations according to Kertesz et al.18-21 also supported the -dimerization of ODP· and O2DP· (Table S5). These results show that the c values could be a good measure of the strength of the inter-monomer C—C bond of the -dimer. For the further accurate prediction of which type of dimer will be formed, some other factors should be considered such as steric hindrance of peripheries. So far, -dimers of the series of radicals have been uniquely observed in the case of Jul·. It is likely that the suppressed steric hindrance in the small and rigid julolidine skeleton facilitates the -dimerization of the radicals. The carbazole derivative (CZ)4 has the largest c value and forms the most robust dimer in spite of having the rotationally fixed nitrogen atom, and this could be understood by the weaker electron-donor ability of carbazole than triarylamines. In summary, we have prepared novel three para-amino substituted dicyanobenzyl radicals with a partially planarized triphenylamine skeleton by insertion of sulfur or oxygen atoms. The variable temperature measurements of NMR, ESR, and electronic absorption spectra showed that these radicals are in equilibrium with the dimers. It turned out that the fixing of the amino group to the benzene ring with the dicyanomethyl radical by sulfur or oxygen atoms results in the remarkable redshift of the radical bands of this class of radicals even to the NIR region, and the solutions of these radicals showed reversible thermochromism in the NIR region. The doubly oxygen-bridged radical exhibited most interesting thermochromic behavior showing the drastic change of the NIR absorption and little change in the visible region assuring high transparency over all temperature ranges. These radicals bound and dispersed into polymer matrices represented almost identical absorption properties with long enough stability. This work provided rare examples of the NIR thermochromic dyes, and the ease of synthesis and the unique photophysical properties of these radicals may lead to new optical materials such as smart windows that regulate the heat from sunlight.
The Supporting Information is available free of charge on the ACS Publications website.
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Details of synthesis and characterization (NMR, HRMS), ESR, DFT calculations
*
[email protected] *
[email protected] Daisuke Sakamaki: 0000-0001-6503-1607 Shu Seki: 0000-0001-7851-4405
†Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Naka-ku, Sakai-shi, Osaka 599-8531, Japan The authors declare no competing financial interests.
This work was supported by a Grant-in-Aid for Young Scientists (A) (17H04874) from the Japan Society for the Promotion of Science (JSPS) and a Grant-in-Aid for Scientific Research on Innovative Areas (“-System Figuration” Area, 26102011). The theoretical calculations were performed using Research Center for Computational Science, Okazaki, Japan.
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