Long-wavelength visible to near infrared photoluminescence from carbon-bridged styrylstilbene and thiadiazole conjugates in organic and aqueous media

Donor–acceptor–donor conjugates composed of electron-donating carbon-bridged styrylstilbene (COPV2) and electron-accepting thiadiazole derivatives equipped with carbazolyl (Cz) terminators, Cz-COPV2-A-COPV2-Cz (A = benzothiadiazole (BTz), naphthobis(thiadiazole) (NTz), or benzobis(thiadiazole) (BBTz)), were newly synthesized and found to serve as efficient and stable long-wavelength photoluminescent dyes in organic and aqueous media. In particular, Cz-COPV2-BBTz-COPV2-Cz showed photoluminescence in the near infrared region (895–927 nm) with a photoluminescence quantum yield (PLQY) of up to 0.19 in cyclohexane and of 0.02–0.03 in THF/water mixtures. Its analogues with weaker acceptors, Cz-COPV2-BTz-COPV2-Cz and Cz-COPV2-NTz-COPV2-Cz, showed yellow to deep-red emission in organic solvents, with PLQYs of up to 0.71 in organic solvents and 0.45 in THF/water mixtures.


Introduction
Long-wavelength visible to near-infrared (NIR) light-emitting materials have found a broad range of applications, in e.g. sensors, electroluminescence, telecommunications, and bioimaging. [1][2][3][4][5] Organic NIR-emitting dyes have attracted increasing attention due to the tunability of their emission wavelength via molecular design, the absence of rare and/or toxic metals in their structures, and their biocompatibility, although their luminescence efficiency still requires further improvement. To achieve long-wavelength emission using such dyes, extending the p-conjugated skeleton and connecting electron-donating and -accepting groups is widely used as a design principle. 1 However, the resulting molecules oen suffer from disadvantages such as conformational disorder arising from free rotation around single bonds within the extended p-conjugated skeleton and strong intermolecular interactions that can cause aggregation, which oen results in low emission efficiency by aggregation-caused quenching (ACQ).
To circumvent the conformational disorder and reduce undesirable molecular motions, physical approach has been adopted in such ways as xing molecular structures in low-temperature matrices for a long time, and more recently, aggregation-induced emission (AIE) [6][7][8] and albumin-induced emission. [9][10][11][12][13] Chemical construction of the structurally xed molecules in terms of laddertype structures that are rigidly planarized by sp 3 -carbon bridges 14,15 and that contain bulky substituents would be an alternative methodology. To this end, we have developed carbon-bridged oligo(p-phenylenevinylene) (COPV), [16][17][18] which features a rigid planar phenylenevinylene skeleton with bulky substituents on the bridging carbon atoms. The strong emission of COPVs in the visible region and their utility as active materials in organic lasers have been demonstrated. [19][20][21][22][23][24][25][26] These compounds also exhibit relatively high HOMO energy levels, which make them suitable as energy donors. 27,28 Herein, we report conjugates of carbon-bridged styrylstilbene (COPV2) and benzo-or naphtho-fused thiadiazole 29-31 ( Fig. 1) that exhibit efficient long-wavelength emission with photoluminescence quantum yields (PLQYs) of up to 0.71 in the visible and 0.19 in the NIR regions.

Results and discussion
This desirable molecular structure was designed based on the time-dependent density functional theory (TD DFT) calculations of several model compounds at the CAM-B3LYP/6-31G* level of theory using COPV2 0 (a model compound without aromatic substituents on the sp 3 -hybridized carbon atoms in the bicyclo [2.2.0]octene moieties to reduce calculation cost) and benzothiadiazole (BTz) as the donor and acceptor, respectively ( Fig. S1 and Table S1 †). The donor-acceptor (D-A) diad COPV2 0 -BTz and the triads BTz-COPV2 0 -BTz (A-D-A) and COPV2 0 -BTz-COPV2 0 (D-A-D) were examined rst. Among these architectures, the D-A-D triad COPV2 0 -BTz-COPV2 0 was expected to show the longest absorption wavelength and the largest transition oscillator strengths for both absorption and emission. The installation of carbazole terminators into this compound [32][33][34][35] to afford Cz-COPV2 0 -BTz-COPV2 0 -Cz with a D 1 -D 2 -A-D 2 -D 1 architecture was expected to result in only a slight shi in the absorption and emission wavelengths but a signicant increase in the transition oscillator strengths. Thus, a series of compounds with the carbazole-terminated architecture Cz-COPV2-A-COPV2-Cz was synthesized using BTz or the more electron-accepting naphthobis(thiadiazole) (NTz) or benzobis(thiadiazole) (BBTz) moieties as the acceptor (A).
The absorption and emission spectra in cyclohexane, THF, chloroform, and THF/water mixtures are shown in Fig. 2, and the corresponding numerical data are summarized in Table 1. Cz-COPV2-A-COPV2-Cz showed two distinct absorption bands (solid lines in Fig. 2). The shorter wavelength one at 406-442 nm with a vibronic ne structure was less sensitive to the acceptor than the other broad band, which exhibited signicant bathochromic shis depending on the electron-withdrawing ability of the acceptor group (Cz-COPV2-BTz-COPV2-Cz: 469 nm; Cz-COPV2-BBTz-COPV2-Cz: 730 nm). As suggested by calculations (vide infra), the former band was assigned to the transition to an excited state localized in the COPV2 moiety (LE band), while the latter was attributed to the lowest excited state with chargetransfer character from the COPV2 moieties to the acceptor (CT band, see also E T (30) plot 44 in Fig. S2 †).
Excitation at the CT band in the organic solvents afforded photoluminescence with a yellow-orange color for Cz-COPV2-BTz-COPV2-Cz (569-619 nm) and orange-red color for Cz-COPV2-NTz-COPV2-Cz (590-668 nm), depending on the solvent polarity. The photoluminescence quantum yields (PLQYs) for the CT emission were 0.60-0.71, and their uorescence lifetimes and decay constants were typical values for uorescence from organic compounds. The emission band of Cz-COPV2-BBTz-COPV2-Cz, which contained a strongly electronwithdrawing benzobis(thiadiazole) moiety, appeared in the NIR region (750-1200 nm). In cyclohexane, the emission peak was observed at 895 nm (PLQY: 0.19), which is relatively high for NIR emission 4,45,46 and much higher than those of previously reported quinoidal COPV derivatives. 47,48 The emission peak was further bathochromically shied to 927 and 925 nm in the polar organic solvents THF and chloroform, with decreased PLQYs of 0.04 and 0.06, respectively. The uorescence quenching in these polar solvents was also obvious in the lifetime measurements and calculated non-radiative decay constants, which obeyed the low-bandgap law. 49 THF/water solutions were prepared by adding water to the THF solutions of the dyes. The resulting solutions of Cz-COPV2-BTz-COPV2-Cz and Cz-COPV2-BBTz-COPV2-Cz appeared to be homogeneous at micromolar concentrations and showed photoluminescence similar to that in organic solvents with a slight spectral shi. The emission peak of Cz-COPV2-BTz-COPV2-Cz was slightly hypsochromically shied compared to that in THF nm] relative to that in THF (664 nm). These emission maxima are similar to that in nonpolar cyclohexane (590 nm), and the uorescence decay prole became biexponential, accompanying a short-lifetime component. These results suggest that Cz-COPV2-NTz-COPV2-Cz forms aggregates in aqueous media and that their local environment is rather hydrophobic and nonpolar, 50,51 which seems to be a plausible origin of the drastic hypsochromic shi of the spectrum.
TD DFT calculations of model compounds at the CAM-B3LYP/6-31G* level supported the assignments of the spectral data (Table S2, Fig. S3 and S4 †). The CT absorption at longer wavelength was assigned to the transition to the lowest excited state (S 1 ), i.e., the HOMO / LUMO transition, where HOMO is  Calculations on the emission wavelengths were also performed using the optimized S 1 geometry. They reproduced the experimental data well (552 nm for BTz, 568 nm for NTz, and 907 nm for BBTz) and suggests that these molecules obey Kasha's rule. The LE absorption at shorter wavelength is attributed to the transition to the S 3 state for Cz-COPV2 0 -BTz-COPV2 0 -Cz and Cz-COPV2 0 -NTz-COPV2 0 -Cz or the S 4 state for Cz-COPV2 0 -BBTz-COPV2 0 -Cz. There are signicant contributions of the orbitals localized in the COPV2 0 parts, such as HOMOÀ1 and LUMO+1/LUMO+2 (for BTz and BBTz) or LUMO+2/LUMO+3 (for NTz), whose orbital energy levels are rather insensitive to the acceptor part. More detailed insight into the S 1 and S 3 states was obtained by the geometry optimization of the excited states of Cz-COPV2 0 -BTz-COPV2 0 -Cz (Table S3 †). In the S 1 state, the bond lengths in the BTz unit (r 11 ) and those adjacent to the BTz unit (r 8 and r 10 ) changed drastically, and the dihedral angle between BTz and COPV2 became much smaller than in the ground state (u 2 : 38.48 at S 0 ; 17.40 at S 1 ). In the S 3 state, the COPV unit exhibited signicant structural change, whereas little change was observed in the BTz unit, suggesting an LE character of the excited state. Specically, the bond lengths in the COPV2 unit of the excited state showed quinoidal bond alternation similar to that observed in the rst excited state of the parent COPV2.

Calculations
Geometry optimization of the ground state of each compound was performed at the CAM-B3LYP/6-31G* level using Gaussian 09. 53 Calculations of the excitation energy and oscillator strength, along with the geometry optimization of the excited states were performed using time-dependent (TD) DFT calculations at the CAM-B3LYP/6-31G* level.

Conclusions
In summary, the efficacy of the use of the rigid planar electrondonating carbon-bridged styrylstilbene moiety (COPV2) to achieve efficient long-wavelength emission especially in NIR region was demonstrated in COPV2-thiadiazole conjugates. A noteworthy feature of the present chemical xation method by intramolecular bridging is that once synthesized, the rigidity of the molecular structure is maintained even in room temperature solution independent of the solvent and without additives. The present work demonstrates that this feature played a key role in the NIR emission, which otherwise oen results in low efficiency due to undesired structural disorder and molecular motion. The application of these dyes to optical devices and attempts to further elongate the spectral bathochromic shi via the use of longer COPV homologues and other types of acceptors are also currently under investigation in our groups, and the results will be reported in due course.

Conflicts of interest
There are no conicts to declare.