Development of visible-to-NIR light absorbing and emitting polymers incorporating hypervalent germafluorene-fused π-conjugated systems

Abstract

Novel molecular designs for creating near-infrared (NIR) light absorbing and emitting materials are required to meet the growing demand for NIR light-based technologies. Herein, we demonstrate that the five-coordinate hypervalent germanium (Ge) compounds composed of germafluorene-fused azomethine and azo scaffolds exhibit efficient light absorption and emission in the visible-to-NIR region (λ_abs = 505–902 nm, λ_PL = 667–1014 nm, and Φ_PL up to 18.1%). These excellent optical properties are attributed to the hypervalent Ge-centered spiroconjugation and their planar π-conjugated systems. Experimental and theoretical data indicate that spiroconjugation plays a significant role in the elevation of the HOMO (highest occupied molecular orbital) energy level. Moreover, the introduction of bulky substituents at the germafluorene moiety contributes to enhancing the planarity of the π-conjugated systems, which further narrows their energy gaps. Since the bulky substituents not only improved processability but also suppressed intermolecular interactions, efficient NIR emission was also observed in polymer films. These results suggest that the integration of hypervalent states with π-conjugated systems can be one of the effective strategies for tuning the electronic properties of NIR light absorbing and emitting materials.

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Masayuki Gon

Masayuki Gon received his PhD degree in 2016 from Kyoto University. He worked as a visiting research fellow at Virginia Commonwealth University in USA in 2014. In 2016, he has been an Assistant Professor in the Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University. His present research theme is development of functional π-conjugated materials focused on unique nature of heteroatoms and organic–inorganic polymer hybrids.

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Introduction

Near-infrared (NIR) light has attracted considerable attention because of its unique features, such as invisibility to the human eye, high biotissue permeability, and smaller light scattering compared with visible light.^1,2 Therefore, the development of NIR light absorbing and emitting materials has been actively pursued, and a variety of chemical scaffolds have been proposed, such as cyanine, squaraine, rhodamine, boron dipyrromethene (BODIPY), and donor–acceptor (D–A) dyes (Fig. 1a).^3,4 However, wide π-conjugated systems are generally needed to achieve narrow energy gaps for NIR light absorption and emission. Consequently, it is often challenging to tune the optical properties of such π-conjugated systems through chemical modification and polymerization due to their low solubility and intrinsic structural constraints of their π-conjugated frameworks. Moreover, solid-state luminescence is often diminished by concentration quenching originating from non-specific intermolecular interactions in the condensed state.⁵ Due to the above reasons, the development of compact and versatile molecular structures to afford NIR light absorption and emission remains highly desirable.

Fig. 1 Research background and concept of this work. (a) General chemical structures of NIR light absorbing and emitting dyes. (b) Molecular orbital (MO) energy modulation via aza-substitution in a vinylene-based π-conjugated system. (c) General chemical structures of element-fused azomethine and azo scaffolds and MO energy modulation via 3c–4e bonds in hypervalent element-fused π-conjugated systems. (d) MO energy modulation via spiroconjugation in spirobifluorene. (e) A molecular design strategy for narrowing the energy gap of a π-conjugated system based on a hypervalent Ge compound to develop visible-to-NIR light absorbing and emitting materials.

The introduction of heteroatoms into chemical backbones is an effective strategy for tuning optical properties without altering the molecular skeletons.^6–19 We have recently proposed that the replacement of the skeletal carbon atom with a nitrogen atom at the “isolated frontier molecular orbital (FMO)” position, where only one of the FMOs is distributed in the π-conjugated system, provides a facile protocol for selectively modulating the energy level of one of the FMOs.^20,21 Based on this concept, π-conjugated systems incorporating azomethine (Ar–CH=N–Ar) and azo (Ar–N=N–Ar) scaffolds represent promising candidates for lowering the lowest unoccupied molecular orbital (LUMO) energy levels from those including vinylene (Ar–CH=CH–Ar) scaffolds (Fig. 1b).^22–24 Since the vinylene unit is one of the famous π-conjugated linkers for showing excellent optical properties,^25,26 the utilization of azomethine and azo scaffolds is expected to provide both structural versatility and superior optical properties in the field of NIR chemistry.

It is known that π-conjugated systems containing azomethine and azo moieties tend to be non-emissive due to fast internal conversion of the excited energy via non-radiative deactivation pathways often accompanied by cis–trans photoisomerization.^27–32 Recently, it has been reported that boron coordination at these units markedly alters their electronic properties, thereby inducing bright emission.^33,34 Our research group has also found that boron-fused azomethine and azo (BAm and BAz) scaffolds exhibit aggregation-induced emission (AIE) and crystallization-induced emission (CIE) (Fig. 1c).^35,36 In AIE and CIE-active materials, the emission is quenched in solution, while emission enhancement occurs in aggregate and crystalline formation, respectively.^37–39 BAm and BAz scaffolds can also be used as electron-accepting comonomers for highly emissive D–A type π-conjugated polymers in the yellow-to-NIR region.^40,41 This is because boron–nitrogen (B–N) coordination additionally lowers the LUMO energy levels of the azomethine and azo moieties owing to the Lewis acidity of the boron center.

More recently, we have revealed that hypervalent heavy element-fused azomethine and azo compounds such as gallium (Ga),⁴² tin (Sn),^43–45 germanium (Ge),^46,47 silicon (Si),^48,49 bismuth (Bi),⁵⁰ and antimony (Sb)⁵¹ show absorption and emission bands in the longer wavelength region than their boron (B) analogues (Fig. 1c). This unique bathochromic shift was attributed to a polarized hypervalent bond consisting of a three-center four-electron (3c–4e) bond around the hypervalent heavy element center.^52–61 Among these elements, hypervalent Ge compounds possess narrower energy gaps owing to their less distorted trigonal bipyramidal geometry, in which the electronic perturbation from the 3c–4e bond to the π-conjugated system operates effectively.^46,60–63 Consequently, we previously observed efficient NIR emission reaching 800 nm from hypervalent Ge-fused compounds incorporating azobenzene-based π-conjugated systems.^46,47 However, it has been still challenging to realize structural diversity because hypervalent Ge compounds often decompose under prolonged and harsh reaction conditions, such as those used in polymerization.⁴⁶ Although there is such a synthetic difficulty, we considered that the development of novel hypervalent Ge compounds has the potential to create materials exhibiting further bathochromically shifted NIR absorption and emission.

Herein, to achieve a further narrowing of the energy gaps in hypervalent Ge compounds while maintaining sufficient chemical stability, we introduced germafluorene scaffolds exhibiting hypervalent Ge-centered spiroconjugation (Fig. 1e). Spiroconjugation refers to the interaction between two orthogonally oriented π-conjugated systems that share a single spiro atom (Fig. 1d).^64–66 Unlike typical π-conjugation where double bonds and single bonds alternate, delocalization of π-electrons occurs even in a stereochemically twisted arrangement. In addition, by introducing a heavy element as a spiro center, it is known to further modulate the energy levels of the π-conjugated systems.^67,68 Although hypervalent elements can be spiro centers with higher coordination states, their electronic effects on π-conjugated systems have been hardly evaluated.^69–75 In our system, it was revealed that the hypervalent Ge spiro center within the germafluorene scaffold plays a significant role in narrowing the energy gap of the hypervalent Ge-fused azomethine and azo (GAm and GAz) compounds. This spiroconjugation is mainly attributed to an increase in HOMO energy levels of the azomethine- and azo-based π-conjugated system. In addition, we found that bulky substituents on the germafluorene unit can enhance the planarity of the π-conjugated system, leading to a further narrowing of the energy gaps between FMOs. Moreover, the bulky substituents also improve the chemical stability of the hypervalent Ge compounds. Thanks to these advantageous properties, we can synthesize various types of π-conjugated copolymers and show tuning of the optical properties in the visible-to-NIR region. We also demonstrate that a polymer nanoparticle (NP) exhibited the second NIR (NIR-II) emission (λ_PL = 1014 nm) in deionized H₂O owing to the high chemical stability and good solubility of the hypervalent germafluorene-fused π-conjugated materials. NIR-II emission (λ > 1000 nm) has been recognized as a powerful tool for deep-tissue biological imaging.^3,4 From these findings, we can say that the hypervalent Ge-fused compounds can be versatile units for constructing NIR light absorbing and emitting materials.

Results and discussion

Synthesis

Scheme 1 shows the syntheses of hypervalent germafluorene-fused azomethine and azo monomers (GAmFl-Br and GAzFl-Br). By reacting each brominated tridentate ligand AmOH-Br and AzOH-Br with 2,4,6,8-tetra-tert-butyl-5,5-dichloro-5H-dibenzo[b,d]germole (FlGeCl2)⁶⁷ in the presence of triethylamine (Et₃N), the corresponding compounds GAmFl-Br and GAzFl-Br were obtained. The tert-butyl groups on the germafluorene moiety were introduced not only for improving the chemical stability of the hypervalent compounds by sterically shielding the reactive Ge center from nucleophilic attack^46,63 but also for enhancing the solubility and planarity of the resulting π-conjugated systems. As model compounds, Ge-fused azomethine and azo compounds with phenyl substituents on the Ge atom (GAmPh-Br and GAzPh-Br) were also prepared (Scheme 1). GAzPh-Br was synthesized according to the method described in our previous literature.⁴⁷

Scheme 1 Syntheses of the hypervalent Ge-fused azomethine and azo compounds.

Next, we synthesized π-conjugated alternating copolymers containing the germafluorene-fused azomethine and azo units in the main chain (Scheme 2). The Migita–Kosugi–Stille cross-coupling^76,77 polymerizations with GAmFl-Br or GAzFl-Br and fluorene (FL) or bithiophene (BT), or cyclopentadithiophene (CDT) comonomers were carried out under catalytic conditions using Pd₂(dba)₃ (dba = dibenzylideneacetone) and 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos) to provide six types of copolymers, P-GAmFl-Ar and P-GAzFl-Ar (Ar = FL, BT, CDT). The relative molecular weights of the products were determined by gel permeation chromatography (GPC) with polystyrene standards by using chloroform (CHCl₃) as an eluent (Fig. S1 and Table 1). The resulting polymeric compounds showed good solubility in common organic solvents such as toluene, CHCl₃, dichloromethane and tetrahydrofuran (THF). In addition, all synthesized compounds were characterized by ¹H and ¹³C{¹H} NMR spectroscopy and high-resolution mass spectrometry (HRMS) (see the SI) and had enough stability to use them under ambient conditions. From the characterization data, we concluded that the samples had the target structures and enough purity for further analyses.

Scheme 2 Syntheses of the π-conjugated alternating copolymers containing hypervalent Ge moieties in the main chain. Regiorandom polymers are obtained and only a certain structure is shown here.

Table 1 Results of polymerizations

	Mn^a	Mw^a	Mw/Mn	DPn^b
a Determined by gel permeation chromatography (GPC) with polystyrene standards in CHCl3 as an eluent, Mn: number-average molecular weight, Mw: weight-average molecular weight.b Number-average degree of polymerization.
P-GAmFl-FL	7400	12 200	1.65	6.4
P-GAmFl-BT	13 000	37 500	2.89	11.2
P-GAmFl-CDT	17 400	43 200	2.49	14.9
P-GAzFl-FL	5800	8600	1.49	5.0
P-GAzFl-BT	13 700	30 500	2.22	11.8
P-GAzFl-CDT	9600	19 200	2.00	8.2

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Crystal structure

We successfully obtained the single crystals of GAmPh-Br and GAmFl-Br, and their structures were investigated through single crystal X-ray diffraction (SC-XRD) analysis (Fig. 2 and Fig. S2, S3 and Tables S1–S3). The results clearly indicate not only the formation of five-coordinate hypervalent Ge compounds with distorted trigonal bipyramidal geometries^46,47,62,63 but also the presence of the germafluorene structure. GAmFl-Br exhibits a more planar azomethine-based π-conjugated system than GAmPh-Br, although their overall structural features are largely similar. The O(1)–Ge(1)–O(2) angles of GAmPh-Br and GAmFl-Br were 167.7° and 167.2°, respectively. This means there is little difference in the linearity of the 3c–4e bonds.⁴⁶ The N(1)–C(13) bond lengths of GAmPh-Br and GAmFl-Br were 1.298 Å and 1.295 Å, respectively. This indicates that these bonds can be denoted as C=N bonds. The Ge(1)–N(1) bond lengths of GAmPh-Br and GAmFl-Br were 2.004 Å and 2.008 Å, respectively. These data suggest that the introduction of the germafluorene unit hardly affected either the bond lengths or the Ge–N interaction. On the other hand, the absolute values of dihedral angles of C(1)–N(1)–C(13)–C(7) and C(2)–O(1)–O(2)–C(8) were 178.3° and 33.22° for GAmPh-Br, 179.6° and 11.96° for GAmFl-Br, respectively. These results suggest that bulky substituents on the germafluorene moiety suppress distortion of the π-conjugated system by sterically protecting both sides of the π-surface. Thus, effective π-conjugation can be maintained without altering other structural parameters. Considering the orthogonal arrangement of the germafluorene moiety and the azomethine-based π-conjugated system in GAmFl-Br, the occurrence of spiroconjugation is anticipated.

Fig. 2 ORTEP drawings of (a) GAmPh-Br and (b) GAmFl-Br (50% probability for thermal ellipsoids). Hydrogen atoms and minor conformations are omitted for clarity. All crystallographic data are shown in the SI.

Optical properties

To investigate the optical properties of the hypervalent Ge compounds, we performed UV-vis-NIR absorption and photoluminescence (PL) measurements with the diluted solutions (1.0 × 10⁻⁵ M in CHCl₃) and films. The results are summarized in Fig. 3 and Table 2. Accordingly, the hypervalent Ge compounds exhibited broad absorption and emission bands in the wide wavelength region from the visible-to-NIR region depending on their chemical structures. In particular, the introduction of the germafluorene skeleton induced the distinct bathochromic shifts in the absorption and emission bands. These data indicate that the electronic perturbation of the azomethine- and azo-based π-conjugated systems through spiroconjugation and the resulting planar π-conjugated system contribute to narrowing the energy gaps. Additionally, the further bathochromic shifts in absorption and emission bands were observed by polymerization accompanied by improving emission efficiency. To comprehend emission mechanisms, radiative rate constants (k_r) and non-radiative rate constants (k_nr) were estimated from PL lifetime measurements (Fig. S4 and S5). In the monomers, the k_r values were on the order of 10⁷ s⁻¹. In contrast, in the polymers, the k_r values were increased to the order of 10⁸ s⁻¹, which is the main factor responsible for the improved emission efficiency. Efficient emission was maintained in the NIR region beyond 700 nm despite the gradual increase in the k_nr values. Interestingly, in the polymer films, bathochromic shifts in the absorption and emission wavelengths were hardly observed compared to those in solution. This suggests that polymer aggregation is suppressed owing to steric protection provided by the bulky substituents on the germafluorene moiety. This effect is also favorable for improving solubility and processability, which are advantageous for polymer materials.

Fig. 3 (a), (c), (e), and (g) UV-vis-NIR absorption and (b), (d), (f) and (h) PL spectra of hypervalent Ge compounds in CHCl₃ (1.0 × 10⁻⁵ M for monomers and 1.0 × 10⁻⁵ M per repeating unit for polymers) and in film, excited at wavelengths of absorption maxima. Some signal noise was observed due to the weak emission intensity of the samples and the low sensitivity of the photomultiplier tube (PMT) sensor in the long-wavelength region. Because of detector switching, PL spectra with maxima below 810 nm do not include the longer-wavelength region (>850 nm), which cannot be detected using the PMT. Photographs of the hypervalent Ge compounds under room light and UV light (365 nm) are also shown in each figure.

Table 2 Spectroscopic data of hypervalent Ge compounds

	λabs^b/nm	λPL^b/nm	ΦPL^b^c^d/%	τav^b^e/ns	kr^f/10^8 s^−1	knr^f/10^8 s^−1
a From ref. 47.b In CHCl3 (1.0 × 10^−5 M for monomers and 1.0 × 10^−5 M per repeating unit for polymers).c Excited at λabs.d Determined as an absolute value in the integrating sphere.e PL lifetimes monitored at λPL, excited at 504 nm with a diode laser.f kr = ΦPL/τav, knr = (1 − ΦPL)/τav, τav; average PL lifetime.g In film.h Not measured or not calculated.
GAmPh-Br	473	634	9.6	1.2	0.78	7.3
GAmFl-Br	505	663	6.7	1.0	0.66	9.2
P-GAmFl-FL	553 (557)^g	684 (691)^g	13.3 (4.6)^g	0.81 (0.44)^g	1.7 (1.0)^g	11 (22)^g
P-GAmFl-BT	575 (577)^g	667 (690)^g	18.0 (5.6)^g	0.80 (0.43)^g	2.2 (1.3)^g	10 (22)^g
P-GAmFl-CDT	700 (687)^g	744 (743)^g	8.5 (2.0)^g	0.26 (<0.1)^g	3.3 (—^h)^g	35 (—^h)^g
GAzPh-Br^a	570	715	2.6	0.37	0.70	26
GAzFl-Br	606	747	1.2	0.28	0.43	35
P-GAzFl-FL	678 (682)^g	776 (797)^g	3.2 (2.0)^g	0.27 (0.19)^g	1.2 (1.1)^g	36 (52)^g
P-GAzFl-BT	723 (731)^g	819 (845)^g	4.4 (2.6)^g	0.25 (<0.1)^g	1.8 (—^h)^g	38 (—^h)^g
P-GAzFl-CDT	902 (898)^g	982 (974)^g	0.6 (0.2)^g	—^h (—^h)^g	—^h (—^h)^g	—^h (—^h)^g

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Cyclic voltammetry

Cyclic voltammetry (CV) was performed to experimentally investigate the origin of the energy-gap differences in the hypervalent Ge compounds. We estimated the HOMO and LUMO energy levels from onset potentials of oxidation and reduction curves in the voltammogram, respectively (Fig. 4 and Fig. S6 and Table S4).^78,79 Accordingly, only minor changes in the HOMO and LUMO energy levels were observed upon introduction of the germafluorene structure (GAmFl-Br and GAmFl-Br) compared to the diphenylgermane compounds (GAmPh-Br and GAmPh-Br) (Fig. 4). This result is inconsistent with that obtained from the optical measurements, suggesting that the effect of spiroconjugation may be difficult to detect by CV. In this point, further investigation was conducted in the next section through theoretical calculations. The LUMO energy levels were significantly lowered by employing the azo-based π-conjugated system compared to the azomethine-based one, which is the main factor responsible for narrowing the energy gap of the hypervalent Ge compounds. This effect originates from replacement of the methine carbon with a more electronegative nitrogen atom, and such selective control of molecular orbital (MO) energy levels has been reported by our group as the concept of the aza-substitution.^20,21 In the polymers, the azomethine- and azo-based hypervalent Ge compounds were found to exhibit similar trends in their HOMO energy levels depending on the electron-donating abilities of the comonomers FL, BT, and CDT. These results suggest that the D–A systems are formed within the polymers, in which the hypervalent Ge moiety acts as the acceptor, and the FL, BT, and CDT units act as the donors.

Fig. 4 The summary of the HOMO and LUMO energy levels estimated from the results of CV data. The details are described in the SI.

We compared the electronic energy gaps obtained from CV data (E_g,CV) with the optical energy ones derived from the onset wavelengths in the UV-vis-NIR absorption spectra (E_g,opt). As a result, good agreement between E_g,CV and E_g,opt was observed for GAmPh-Br, GAzPh-Br, P-GAmCDT, and P-GAzCDT, whereas slight discrepancies were obtained for the other compounds (Table S4). Furthermore, these deviations were much clearer in the compounds with spiroconjugation and in weak D–A interaction. This tendency is consistent with the previous observations that the effect of spiroconjugation may be difficult to detect by CV. In addition, when the roles of the donor and acceptor units are not clearly separated, discrepancies between redox and optical properties are likely to arise.

Theoretical calculations

To obtain further insight into optical properties of the hypervalent Ge compounds, quantum chemical calculations with density functional theory (DFT) and time-dependent (TD)-DFT were performed (Fig. 5 and Fig. S7–S17 and Tables S5–S16). As a result, the effect of electronic perturbation induced by spiroconjugation on the azomethine- and azo-based π-conjugated systems was clearly revealed. The optimized structure of GAmPh-Br exhibits a partially distorted π-conjugated structure compared to that of GAzPh-Br because of steric hindrance from the methine proton (Fig. 5). In contrast, both GAmFl-Br and GAzFl-Br exhibit highly planar π-conjugated systems as a result of steric effects from the tert-butyl groups on the germafluorene units, which is in good agreement with the results of the SC-XRD analyses. In contrast to the CV results, the HOMO energy levels are elevated and the LUMO energy levels are lowered in GAmFl-Br and GAzFl-Br compared to GAmPh-Br and GAzPh-Br, respectively (Fig. 6 and Fig. S7). Therefore, the corresponding transition energies (S₀ → S₁ for GAmPh-Br and GAzPh-Br, S₀ → S₁ and S₂ for GAmFl-Br and GAzFl-Br) are reduced upon introduction of the germafluorene units (Table S5). Based on the HOMO distribution, the significant increase in the HOMO energy level can be attributed to spiroconjugation in addition to σ–π conjugation between the azomethine- and azo-based π-conjugated systems and the germafluorene units (Fig. 6). In addition, the reduction of the LUMO energy levels is likely attributable to effective π-conjugation originating from the planar π-conjugated system, rather than to spiroconjugation. This is because orbital interactions between the azomethine- and azo-based π-conjugated systems and the germafluorene units are hardly observed (Fig. S9 and S11). The discrepancy between the CV and theoretical calculation results for the germafluorene-fused compounds may stem from the small energy difference between the HOMO and HOMO−1 (Fig. S8–S11). Indeed, broadening at the onset of the oxidation curve was observed in the voltammogram of GAmFl-Br and GAzFl-Br (Fig. S6). This should lead to underestimation of the HOMO energy levels.

Fig. 5 Optimized structures of hypervalent Ge monomers by quantum chemical calculations using DFT. Selected bond lengths (d), angles (θ), and dihedral angles (φ) are shown.

Fig. 6 Kohn–Sham orbitals (isovalue = 0.02) with HOMO and LUMO energy levels of hypervalent Ge monomers. Hydrogen atoms are omitted for clarity.

Next, for theoretically estimating the electronic structures of polymers, the model compounds (M-GAmFl-Ar and M-GAzFl-Ar (Ar = FL, BT, and CDT)) were used for the reduction of the calculation cost. The results for M-GAmFl-FL are shown in Fig. 7 as the representative example, and all results are described in the SI (Fig. S7, S12–S17 and Table S6). In the model compounds, the tendency of the changes in HOMO and LUMO energy levels is in good agreement with the experimental results from CV (Fig. S7). This means that the constructed π-conjugated system can be categorized as a D–A π-conjugated system. Interestingly, spiroconjugation was still observed in the HOMO of the π-conjugated system which extends over the hypervalent germafluorene-fused azomethine-based π-conjugated moiety as well as the electron-donating fluorene units (Fig. 7). This effect likely contributes to an increase in the HOMO energy level and the consequent reduction of the energy gap of the π-conjugated polymer.

Fig. 7 A chemical structure and Kohn–Sham orbitals (isovalue = 0.02) with HOMO and LUMO energy levels of M-GAmFl-FL as a model compound of P-GAmFl-FL.

Applications of water-dispersible nanoparticles

NIR-light absorbing and emitting π-conjugated polymers are promising candidates for bioimaging applications owing to their high brightness, which originates from their large molar extinction coefficients as well as good emission efficiencies of π-conjugated systems.⁸⁰ Therefore, to demonstrate the utility of our NIR light absorbing and emitting polymer for biological applications, we prepared a polymer NP dispersible in water.⁴¹ The NP was fabricated using the micellizing agent Pluronic F-127, an amphiphilic molecule commonly employed for the biocompatible encapsulation of hydrophobic compounds.^81,82 We selected P-GAzFl-CDT as the NIR light absorbing and emitting polymer because it can exhibit the closest emission wavelength to the NIR-II region. The detailed procedure for the polymer NP preparation is shown in the SI. As a result, the water-dispersible polymer NPs with an average size of 98.5 nm were obtained (Fig. 8a). The polymer NPs showed maximum absorption and emission wavelengths of 807 and 1014 nm (Φ_PL < 0.1%), respectively (Fig. 8b). The hypsochromic shift in the maximum absorption wavelength relative to those in solution and in the film state suggests the formation of H-aggregate-like structures.⁸³ It is known that the formation of H-aggregates reduces the transition probability of emissive states, leading to decreased luminescence intensity.⁸⁴ In addition, the presence of water in the surrounding environment likely affects the aggregates through polarity change, resulting in a bathochromic shift of the emission and a decrease in luminescence efficiency compared to the film state.⁸⁵ Thus, by creating hypervalent germanium compounds incorporating a germafluorene skeleton, we successfully developed NIR emissive materials that exhibit strong absorption and emission properties along with good processability. Although further studies, such as toxicity evaluation, cellular uptake behavior, and metabolic pathways of the prepared nanoparticles, are necessary for practical biological applications, we have successfully developed a novel polymer with advantageous optical properties.

Fig. 8 (a) UV-vis-NIR absorption (dotted line) and PL spectra (solid line), (b) a dynamic light scattering (DLS) profile of the polymer NP with P-GAzFl-CDT and Pluronic F-127 in deionized H₂O. The photograph of the polymer NP solution and the average particle size are denoted in each figure.

Conclusion

We synthesized novel five-coordinate hypervalent Ge compounds featuring germafluorene-fused azomethine and azo scaffolds. The monomers and polymers exhibited efficient light absorption and emission in the visible-to-NIR region (λ_abs = 505–902 nm, λ_PL = 667–1014 nm, and Φ_PL up to 18.1%), owing to hypervalent Ge-centered spiroconjugation as well as the resulting planar and extended π-conjugated systems. In addition, efficient emission was also observed from polymer films because of suppression of interchain interactions by the bulky substituents on the germafluorene moiety. Furthermore, the high processability of the hypervalent Ge compounds enabled the fabrication of the water-dispersible NP exhibiting NIR-II emission. Our studies revealed the structural versatility of five-coordinate hypervalent Ge compounds incorporating germafluorene-fused azomethine and azo frameworks, and this versatility can be applied to the development of processable NIR light absorbing and emitting materials. These polymer materials can be used not only for bioimaging probes but also for enhanced security applications because of their optical invisibility in the NIR region to the human eye. Moreover, the hypervalent germafluorene-fused π-conjugated system is also expected to be applicable as a charge-transporting material by exploiting its narrow energy-gap properties. This can be achieved by reducing the steric substituents near the hypervalent germanium atom to enhance interchain π–π interactions.

Conflicts of interest

There are no conflicts to declare.

Data availability

All experimental and characterization data and detailed experimental procedures are available in the published article and supplementary information (SI). Supplementary information: instrumentation, materials, synthetic procedures, characterization data (¹H, ¹³C{¹H} NMR spectra, and HRMS), and experimental data (GPC profiles, single crystal X-ray structure analysis, PL decay curves, cyclic voltammograms, computational details for quantum chemical calculations, preparation methods of NPs, and coordinates for optimized structures with theoretical calculations). See DOI: https://doi.org/10.1039/d6tc00672h.

CCDC 2534786 and 2534788 contain the supplementary crystallographic data for this paper.^86a,b

Acknowledgements

This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2024-00406152), The Asahi Glass Foundation (for M. G.), Japan Society for the Promotion of Science (JSPS), a Grant-in-Aid for Scientific Research (B) (JP25K01818) (for M. G.) and (JP24K01570) (for K. T.).

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