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. 2024 Dec 17;64(6):e202418751. doi: 10.1002/anie.202418751

Metal‐Bridging Cyclic Bilatriene Analogue Affords Stable π‐Radicaloid Dyes with Near‐Infrared II Absorption

Aninda Ghosh 1, Shigeki Mori 2, Yuki Ide 3, Jun Tae Song 4, Yoshihisa Yamaoka 5, Tatsumi Ishihara 4, Takahisa Ikeue 6,✉, Hiroyuki Furuta 4,✉, Masatoshi Ishida 1,✉
PMCID: PMC11795725  PMID: 39655504

Abstract

Stable neutral metal radicaloid complexes have been synthesized from a modified tetrapyrrolic pigment, bilatriene, with iridium(I) and rhodium(I) cyclooctadiene (COD) synthons. The bilatriene skeleton contains α‐linked conjugated pyrrole units, whereas an N‐confused analogue used in this work possesses β‐linked pyrrole moieties at the terminal, demonstrating a unique metal binding capability. Unprecedentedly, the metal‐COD cations are accommodated at the outer nitrogen sites, which induced the formation of open‐shell metal‐radicaloid species. The resulting compounds are highly stable under ambient conditions and demonstrated facile redox conversion to afford the corresponding cation and anion species. Furthermore, the radicaloid complexes showed a distinct second near‐infrared absorption (NIR‐II) capability extending up to 1500 nm along with high photostability. These features emphasized that the complexes can be potential NIR‐II light‐responsible photothermal and photoacoustic imaging contrast agents based on the metal‐radicaloid dye platform.

Keywords: porphyrinoid, metal coordination, radical, near-infrared light, photoacoustic

Unique redox non‐innocent metal‐radicaloid species with an acyclic porphyrinoid structure have been synthesized upon bridging rhodium/iridium‐COD cations at outer nitrogen sites. The resulting complexes exhibit high stability under ambient conditions, redox versatility, and distinct near‐infrared absorption up to 1500 nm, and are promising candidates for NIR‐II light‐responsive photothermal and photoacoustic imaging contrast agents.

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Introduction

The development of stable second near‐infrared (NIR‐II) dyes has garnered significant interest in materials science due to their unique physicochemical properties and potential applications in light‐emitting diodes, [1] energy conversion devices, [2] and as photosensitizers for photodynamic and photothermal (PT) therapies,[ 3 , 4 , 5 ] as well as contrast agents for photoacoustic imaging (PA) in cancer diagnosis.[ 6 , 7 ] For effective PA imaging,[ 8 , 9 ] contrast agents (e.g., dyes) must exhibit high efficiency in generating local heat from photoexcited dye molecules via nonradiative decay processes. This heat induces ultrasonic emissions through thermal expansion in local tissues upon pulsed laser irradiation. In addition, the absorption in the NIR‐II window offers clear advantages over the conventional NIR−I window (700–1000 nm),[ 10 , 11 , 12 ] including higher maximum permissible exposures (MPEs) (e.g., at 1064 nm, the MPE is 1000 mW/cm2 per laser pulse) and reduced scattering of photoacoustic signals in tissues (Figure 1c).[ 13 , 14 ] Consequently, NIR‐II light‐responsive contrast agents for PA imaging are crucial for visualizing target tissues at centimeter‐level penetration depths.[ 15 , 16 ]

Figure 1.

Figure 1

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(a) Representative examples of the reported porphyrinoid‐based neutral π‐radicaloids; 2–7. (b) Designed strategy for NIR‐II radicaloid dyes based on pseudo‐metalla‐porphyrinoid through peripheral metal coordination of 1. (c) Working principle of photothermal (PT) conversion and photoacoustic (PA) imaging using a NIR‐II excitation. The modified Jablonski diagram is shown along with the spectra. Dx stands for the doublet states. (d) Schematic representation of the MO diagrams for porphyrin‐based radicaloids (left) and open‐chain bilatriene radicaloid complexes (right) used in this work. The later species is expected to demonstrate the partially allowed transitions in the NIR‐II region.

Designing NIR‐absorbing small dyes with narrow HOMO–LUMO gaps and efficient nonradiative decay characteristics can be achieved using porphyrin‐related macrocycles. [17] These macrocycles offer flexible structural tunability, high photostability, and large molar absorption coefficients (ϵ). Various approaches, such as peripheral fusion,[ 18 , 19 , 20 ] core expansion,[ 21 , 22 , 23 ] and utilizing open‐shell electronic structures,[ 24 , 25 , 26 ] have been explored for NIR‐II dye applications. Open‐shell radicaloid systems composed of porphyrin‐based skeletons exhibit narrow energy gaps due to SOMO‐based transitions, resulting in significant bathochromic shifts in their absorptions (Figure 1a).[ 27 , 28 , 29 ] However, the frontier molecular orbitals (MOs) of odd‐numbered [4n+1]π‐conjugated porphyrinoid radicals (e.g., 17π, 19π, 25π cyclic systems) demonstrate hybrid [4n+2]/[4n]π MO patterns, displaying distinct MO behaviors of aromatic and antiaromatic porphyrinic transitions for α‐ and β‐spins, respectively (Figure 1d).[ 30 , 31 , 32 , 33 ] Considering Goutermann's orbital theory, the lowest energy transitions of [4n+1]π‐radicaloid porphyrin analogues often involve optically forbidden HOMO‐SUMO transitions coupled with [4n]π antiaromatic electronic structures because of its intrashell transition nature, which resulted in negligibly weak absorptions in the NIR‐II region. [34] Since the optical spectroscopy of typical porphyrins is relatively insensitive to electronic structure changes from peripheral substitutions and central metal ion exchanges, disrupting the globally π‐conjugated porphyrin macrocyclic skeleton such as open‐chain bilatriene, bilatrienone, and bilindione, as observed in natural pigments can induce partial frontier MO degeneracy with partially allowed transitions from both α‐ and β‐spin orbitals using nonaromatic electronic structures (Figure 1d). [35]

Based on the design principle of nonporphyrin‐based structure, we recently developed novel NIR‐II absorbing metalla‐carbaporphyrinoid dyes (i.e., Pd1 and Pt1) by incorporating Pd(II) and Pt(II) ions into the inner “NNCC” organometallic environment of an open‐chain bilatriene analogue (Figure 1b). [36] Through the indolic C−H bond activation, the resulting pseudo‐metalla species exhibited broken degeneracy of the frontier MOs with distinct nonaromatic electronic structures. Consistently, intense NIR‐II absorption was emerged due to strong metal dπ‐ligand pπ interactions. Despite their narrow HOMO–LUMO gaps, the tetrapyrrolic skeletons in Pd1 and Pt1 demonstrated high chemical and photostability, making them suitable for photothermal conversion materials.

In this work, we expanded the coordination chemistry of skeletally modified bilatriene analogue (1) through an N‐confusion approach. [37] We hypothesized that incorporating larger monovalent metal ions (e.g., Ir+, Rh+) into a doubly N‐confused dibenzo‐bilatriene core would yield open‐shell pseudo‐metalla‐porphyrinoid radical species due to its mismatch of ligand valence states. Unexpectedly, novel metal radicaloid species (i.e., Ir1 and Rh1), where the terminal indole nitrogen sites support metal ions, were predominantly formed under mild conditions (Figure 1b). Additionally, the subsequent metal complexation generated bis‐metal complexes (i.e., Rh21 and IrRh1) with square planar coordination modes. These complexes exhibited broad NIR‐II absorption bands extending up to 1600 nm, attributed to narrow SOMO/SUMO‐based transitions. This unique optical profile offers potential for photoacoustic response in the deep NIR‐II region, making these dyes promising candidates for biological imaging and PT applications.

Results and Discussion

Synthesis and Characterization: The doubly N‐confused dibenzo‐bilatriene 1 was synthesized following a previously reported methodology. [38] Diverging from the conditions employed for the syntheses of 1Pd and 1Pt, a facile complexation was achieved at ambient temperature using 1.4 equivalents of [Ir(COD)(OMe)]2 in toluene under aerobic conditions (Figure 2a). The resultant green product, Ir1, was successfully isolated via Al2O3 column chromatography, yielding 64 %. High‐resolution electron spray ionization (HR‐ESI) mass spectrometry corroborated the complexation, presenting a peak at m/z=1197.1645 (calculated for C53H27F15N4Ir (M+) in positive mode=1197.1624) (Figure S1 in the Supporting Information). No reaction occurred under analogous conditions using 1.1 equivalents of [Rh(COD)Cl]2 at room temperature. In contrast, employing K2CO3 as a base resulted in the formation of a bis‐Rh(COD) complex, Rh21, with a yield of 63 %. The HR‐ESI‐MS analysis of Rh21 displayed a peak consistent with bis‐complexation (Figure S3). To further elucidate the metal coordination behaviour of 1, a low‐temperature reaction at −30 °C under identical conditions was conducted. This reaction yielded a minor amount of the mononuclear complex Rh1; however, it rapidly proceeded to the second Rh coordination (Rh21) upon returning to room temperature. [39]

Figure 2.

Figure 2

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(a) Synthesis of Rh/Ir‐COD complexes (Rh1, Ir1, Rh21, and IrRh1) of a doubly N‐confused bilatriene 1; ORTEP diagrams of radicaloid complexes, (b) Ir1, (c) Rh1, (d) IrRh1, and (d) Rh21 obtained using single‐crystal X‐ray diffraction with thermal ellipsoids at 50 % probability. Solvent molecules are omitted for clarity. Selected bond lengths (Å) between the peripheral metal centers and the ligands are shown aside from the structures (f) MO energy diagram of Ir1 obtained by UB3LYP/6‐31G(d,p)‐SDD level calculations. (g) Samples (e.g., Rh21) are highly stable and can be isolated by column chromatography with resistance to water.

For Ir1, attempts to form the corresponding bis‐iridium complex using higher equivalents of Ir(COD) salt were unsuccessful, presumably due to the distinct lower Lewis acidic nature of Ir(I) ion compared to that of Rh(I) ion. [40] On this basis, further metalation of Ir1 with a Rh(COD) fragment led to the formation of the hetero‐metallated species IrRh1 with a yield of 36 %.This outcome suggests a stepwise metal complexation at different nitrogen donor sites in 1 (see below).

The solid‐state structures of Ir1, Rh1, Rh21, and IrRh1 were unambiguously elucidated through single‐crystal X‐ray diffraction (XRD) analysis (Figures 2b–e and Table S1). [41] Suitable crystals for XRD were obtained via slow diffusion of n‐hexane into a CH2Cl2 solution. The iridium and rhodium centers were positioned between the terminal indole nitrogen sites, supported by COD ligands in both Ir1 and Rh1 (Figure 2b and 2c). The square‐planar coordination geometry of the Ir(COD) and Rh(COD) units was confirmed by the angles between the nitrogen atoms and the central alkenyl C=C positions. The metal‐nitrogen bond lengths, approximately 2.05–2.06 Å, are relatively shorter than those in other reported metal‐COD complexes (Figures S29–S33).[ 42 , 43 , 44 , 45 ] This bond shortening suggests a significant overlap between the nitrogen 2p orbitals and the 4d/5d orbitals of the Rh and Ir ions, contributing to specific spin‐delocalized electronic structures (see below). The tetrapyrrolic core skeletons of Ir1 and Rh1 exhibit distinct twisted structures. In Ir1, the dihedral angles between the dipyrrin plane and the individual indole rings are 43.77° and 43.38°, respectively. Similarly, Rh1 displays comparable twisting angles of 43.72° and 43.96°, respectively (Figures S25–S26).

The crystal structures of Rh21 and IrRh1 reveal that the additional Rh‐COD units are bonded at the inner dipyrrin nitrogen sites, akin to porphyrin‐supported Rh(COD) complexes (Figure 2d and 2e). [46] Each M–COD (M=Rh/Ir) unit is positioned on the opposite side relative to the mean plane to avoid steric congestion. Both Rh21 and IrRh1 exhibit similar bonding features at the M(COD)‐nitrogen atoms at the inner dipyrrin moiety. The bite angles of the indole ring for Rh21 and IrRh1 relative to the core plane are smaller than those of Ir‐1 and Rh‐1 (Figures S27–S28). Structural analysis indicates that the coordination of 1 proceeds via initial metal(COD) complexation at the deprotonated indolic amine sites, followed by secondary complexation at the inner dipyrrin environment to afford the bis‐M(COD) complexes.

Magnetic properties: Surprisingly, all the complexes, Ir1, Rh1, Rh21, and IrRh1, are not diamagnetic, as their 1H NMR spectra exhibited no distinct resonances in the typical diamagnetic region (δ: −10~+15 ppm). Instead, broad electron paramagnetic resonance (EPR) signals centered at g=2.014, 2.004, 2.004, and 2.004, respectively, were observed for Ir1, Rh1, Rh21, and IrRh1 in CH2Cl2 at 298 K (Figures 3c–d and S11, S13). Spin quantification using 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) as a reference indicated the formation of doublet organic radicaloid complexes. The EPR signals of the solid samples of these complexes remained unchanged at approximately 4 K (Figures S8–S10). The simulation of the EPR spectra demonstrated that the g x=2.03, g y=2.02, g z=2.01 for Ir1.

Figure 3.

Figure 3

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χT–T plots of solid (a) Rh21 and (b) IrRh1 determined by SQUID magnetometer. EPR spectra of (c) Rh21 and (d) IrRh1 determined in CH2Cl2 at 298 K. Spin‐density maps for (e) Rh21 and (f) IrRh1 obtained by the UB3LYP/6‐31G(d,p)+SDD (for Ir and Rh) level calculations.

To understand the temperature‐dependent behavior of the magnetic properties of these radicaloid complexes (i.e., Ir1, Rh21, and IrRh1), variable‐temperature Superconducting Quantum Interference Device (SQUID) measurements were conducted on solid crystalline samples of Ir1, Rh21, and IrRh1 (Figures 3a–3b, S12). The χT values determined at 300 K closely matched the theoretical values C of 0.309, 0.339, and 0.353 emu K mol−1 for Ir1, Rh21, and IrRh1, respectively, supporting the presence of ground‐state doublet species. These values gradually decreased upon temperature lowering from 30 K due to antiferromagnetic coupling. Since the χT values of these complexes were measured in the solid state, these couplings can be interpreted as intermolecular rather than intramolecular interactions. This is also confirmed by the crystal packing structures of these complexes showing weak intermolecular interactions due to bulky metal‐COD substituted nonplanar structures that prevent close packing (Figures S34–37). The extent of the coupling for Ir1 is weak even at low temperatures, as estimated from the Weiss constant, θ=−1.17 K, and a low zJ value of −3.26 cm−1, based on fitting analysis using the Curie–Weiss law: χ=C/(T–θ) (Figures S34–S37). For Rh21 and IrRh1, the Weiss constant and zJ value were slightly higher than those of Ir1.

Density functional theory (DFT) optimized structures indeed are represented to be ground‐state doublet (S=1/2) electronic structures (Figures 3e–f and S11b, S13b). Spin density analysis indicated spin delocalization over the π‐conjugated bilatriene unit, with a small extent of the metal‐centered spin population (e.g., Rh1=2.8 % and Rh2=2.2 % for Rh21, and Ir=3.7 % and Rh=2.5 % for IrRh1). These results indicate that the outer metal(COD) coordination induced the formation of typical π‐radicaloid complexes. Compared to the bis‐metallic system, the spin population on the Ir atom in Ir1 was determined to be 5.0 %, and that of the Rh atom in Rh1 is 2.9 %, which are consistent with the specific EPR spectral features with the relatively large g‐values (Figure S11–13). The bulky meso‐substituents and well‐delocalized π‐system contribute to the overall thermodynamic stability (see below).

Based on the aforementioned analysis, we propose a plausible reaction mechanism for the formation of these complexes (Scheme S1). Upon deprotonation of the indolic amine of 1, the complexation with a cationic Ir(COD) fragment induces the formation of pseudo‐metallacycles (e.g., Ir1 and Rh1). X‐ray photoelectron spectroscopy (XPS) measurements indicated that the local electronic valences of the iridium and rhodium atoms remain unchanged (Figure S14). Comparing the binding energies of the Rh (3d) peaks of [Rh(COD)Cl]2 and RhCl3 salts, used as references for Rh(I) and Rh(III), respectively, the peaks of 3d3/2 and 3d5/2 for Rh21 and IrRh1 are similar to those of [Rh(COD)Cl]2. Likewise, the binding energies of 4f5/2 and 4f7/2 for Ir1 and IrRh1 also showed similar trends, supporting the Ir(I) oxidation state. These results suggest that the potential aerobic oxidation of the deprotonated 1 is stabilized by metal (COD) coordination, resulting in neutral‐radicaloid species. Throughout the reaction, the oxidation states of the metal ions do not change, which is consistent with the preferable square‐planar d8 Ir/Rh(I) geometries observed in the complexes (Figure S33). Furthermore, the radicaloid M1 (M=Ir and Rh) is robust for the second metal COD complexation to yield Rh21 and IrRh1, respectively.

Optical and Redox Properties: These π‐radicaloid complexes exhibit distinctive NIR‐II absorption characteristics, as previously discussed. The UV/Vis/NIR absorption spectra of Ir1, Rh1, Rh21, and IrRh1 display exceptionally low‐energy absorption bands extending up to 1600 nm (Figures 4a, and S23). As reflected in the open‐shell electronic structure, the mono‐nuclear complexes Ir1 and Rh1 exhibit porphyrin‐like sharp absorption bands at 499 nm and 494 nm, respectively, accompanied by broad bands in the NIR‐II region. The spectral features of the bis‐metal species, Rh21 and IrRh1, are analogous to those of Ir1 and Rh1. The characteristic NIR‐II bands are slightly red‐shifted, with λ max=1372 nm for Rh21 and 1415 nm for IrRh1. Furthermore, the emission properties of these radicaloid complexes are suppressed due to the low‐energy doublet excited state nature.

Figure 4.

Figure 4

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(a) UV/Vis‐NIR absorption spectra of Rh21 (blue) and IrRh1 (red) in CH2Cl2 at 298 K. The inset shows photographs of CH2Cl2 solutions containing the complexes under ambient light. (b) Cyclic voltammograms of Rh21 (blue) and IrRh1 (red) in CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate, with a scan rate of 0.1 V s−1. Potentials are calibrated against the ferrocene/ferrocenium redox couple.

Time‐dependent (TD) DFT calculations on Rh21, and IrRh1 revealed that the frontier SOMOs are predominantly ligand‐centered. All calculated low‐energy bands are primarily attributed to π–π* transitions with minimal metal d‐orbital involvement. Although the calculated energies are somewhat overestimated with the functionals used, the transition from SOMOα to LUMOα is estimated at 1248 nm (f=0.0324), corresponding to the experimental NIR‐II absorption band at 1372 nm, and the HOMOβ‐SUMOβ transition at 1074 nm (f=0.0092) corresponds to the NIR‐II absorption (λ=1160 nm) for Rh21 (Figure 4a, Figure S45–S46, and Table S4). [47] Similar theoretical results were obtained for IrRh1, indicating negligible impact of the metal ions in terms of their electronic properties (Figures S39–S43 and S51–S52, Tables S2, S3, S6).

To elucidate the redox properties of Ir1, Rh21, and IrRh1, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte (Figures 4b and S15–S17). The complexes Rh21 and IrRh1 demonstrated facile oxidation and reversible reduction waves, with exceptionally narrow energy gaps of 0.48 and 0.47 eV, respectively, indicative of their open‐shell radicaloid electronic structures. [48] For Rh21, a reversible first oxidation wave at −0.45 V (vs Fc/Fc+) and a reversible first reduction wave at −0.93 V (vs Fc/Fc+) signified facile 1e− oxidation and reduction, respectively. A similar redox behavior was shown in the CV of IrRh1 with that of Rh21. Both oxidation and reduction potentials were cathodically shifted relative to those of Ir1, suggesting that the inner metal(COD) coordination destabilizes the LUMO energy level. Consequently, the increase in the energy gap is consistent with the DFT‐calculated SOMO‐SUMO gap (Figures 1f, S45, S51).

The reversible oxidation and reduction behaviors observed in the CVs suggest potential interconversion through redox reactions. These processes were analyzed via spectroelectrochemistry, where absorption spectral changes were monitored during reactions in a thin‐layer electrochemical cell (Figures S18‐S20). Upon bulk electrolysis of Rh21, one‐electron oxidation at −0.37 V (vs. Fc/Fc+) resulted in the attenuation of the characteristic NIR‐II band and the emergence of an NIR band around 800 nm. Conversely, one‐electron reduction at −1.07 V (vs. Fc/Fc+) led to an intense absorption band at 1050 nm, accompanied by the disappearance of the original board NIR‐II band. These results indicate the redox interconversion of the radicaloid complexes to yield the corresponding cation (IrRh1 +) and anion (IrRh1 −), respectively. A similar spectral change was observed for Rh21, yielding the respective cation and anion. Using TDDFT calculations, the electronic transitions were predicted to be consistent with the experimental spectra (Figures S48–S49, S54–S55, and Tables S5, S7).

To gain further electronic insight into the charged species by chemical reductant (i.e., cobaltocene, CoCp2), the spectral titration of an equivalent of CoCp2 in THF solution gave the identical spectral feature obtained by electrochemistry (Figures 5a, and S21–S22). Along with the disappearance of the broadband at 1372 nm, a new intense NIR‐II band appeared at 1034 nm under the condition for Rh21 (Figure 5b left). Similarly, the chemical reduction of IrRh1 afforded the distinct NIR‐II spectral change (Figure 5b right). The 1H NMR spectral analysis of the one‐electron reduced anion, Rh21 − revealed that the β‐pyrrole CHs appear at δ 6.45 and 6.10 ppm in the typical alkene region, indicating a nonaromatic metalla species. Compared to 1, the specific α‐benzo‐CHs are low‐field shifted at 8.20 ppm due to the proximity of the electronegative metal center.

Figure 5.

Figure 5

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(a) Reaction Scheme for the one‐electron reduction of Rh21 and IrRh1 using a cobaltocene (CoCp2) (b) UV/Vis‐NIR absorption spectral changes of Rh21 (left) and IrRh1 (right) in THF and CH2Cl2 respectively at 298 K upon addition of an equiv. of cobaltocene. (c) Part of 1H NMR spectrum of the reduced Rh21 in THF‐d 8 in the presence of an equiv. of CoCp2.

NIR‐II Photoacoustic and Photothermal Response: Generally, photoexcitation of radicaloid compounds triggers rapid thermal energy dissipation from high‐energy states to the ground doublet state via nonradiative decay mechanisms (Figure 1c). For an influential generation of photoacoustic signals, it is crucial to employ highly photostable complexes with NIR absorptions and minimal fluorescence quantum yields (ΦFL <0.001) as photoacoustic contrast agents.[ 49 , 50 ] All complexes exhibit long‐term stability in both solution and solid states, as evidenced by their UV/Vis/NIR absorption spectra, which remained unchanged under ambient conditions for a month (Figure 6a). Furthermore, under 808 nm laser irradiation in a toluene solution, the compounds demonstrated remarkable photostability over 60 minutes compared to indocyanine green (ICG) (Figure 6b). [51]

Figure 6.

Figure 6

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(a) Thermal stability and (b) photostability plots of Ir1 (black), Rh21 (blue), and IrRh1 (red) in a toluene solution under ambient conditions. The experiment was conducted under 808 nm laser irradiation, with ICG (pink) used as a reference. NIR‐II PA spectra of (c) Rh21 and (d) IrRh1 in DMF (top) alongside absorption spectra (bottom); (e) PA images (excited at 1370 and 1415 nm) of Rh21 and IrRh1 in a silicone capillary (dimensions=20 mm×1 mm) compared to a blank solvent (DMF).

Given the high stability of the bis‐metallated Rh21 and IrRh1 in DMF and DMSO, both generated photoacoustic (PA) waves effectively upon nanosecond pulsed laser irradiation. These PA waves were detected using an ultrasound transducer. The resulting PA spectra of Rh21 and IrRh1 revealed relatively intense PA signals in the NIR‐II energy region (up to 1500 nm) (Figure 6c, 6d). Additionally, the overall PA spectral features reflect their absorption spectra, suggesting that the inherent absorption characteristics, such as the absorption coefficient (ϵ), of these radicaloid dyes dictate the photoacoustic response. The PA response of Rh21 is estimated to be 1.5‐fold higher than that of IrRh1, potentially due to higher absorption efficiency. Furthermore, utilizing Rh21 and IrRh1 as near‐infrared (NIR‐II) photoacoustic dyes yielded wavelength‐dependent signal responses in the NIR‐II range, in contrast to the absence of signals from the blank solution (Figure 6e).

Furthermore, we investigate the photothermal conversion performance of Rh21 and IrRh1 under 940 nm laser irradiation in toluene. Under the laser irradiation, drastic increases in the solution temperature for both Rh21 and IrRh1 were found, and the rate of the temperature rise is concentration‐dependent; for example, the maximum temperatures reached 42.3 °C, 48.8 °C, and 52.5 °C for 25, 50, and 100 μM concentrations, respectively, within 10 minutes (Figure 7a, 7c). Upon repeating heating‐cooling cycles of the photothermal conversion process, Rh21 and IrRh1 demonstrated the reproducibility of temperature increases, suggesting distinct high photo‐ and thermal‐stabilities (Figure 7b). The linear fitting of time (t) as a function of the negative natural logarithm of the driving force temperature −ln θ) yielded a time constant (τ), and the overall photothermal conversion efficiencies η were estimated for 33 % and 38 % for Rh21 and IrRh1, respectively (Figure 7d). These results suggested that, compared to homometallic Rh21, the heterometal IrRh1 shows superior performance to some extent. While investigations into the structure‐photoacoustic property relationships are ongoing, these porphyrinoid dyes represent a promising addition to NIR‐II photoacoustic imaging agents.

Figure 7.

Figure 7

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(a) Temperature variation of Rh21 (left) and IrRh1 (right) at different concentrations in a toluene solution under laser irradiation at 940 nm. (b) Heating‐cooling cycles for Rh21 (left) and IrRh1 (right) under the identical conditions. (c) Thermography images of Rh21 (left) and IrRh1 (right) were obtained by light irradiation for 0, 5, and 10 mins with 100 μM sample solutions. (d) Plots of time versus −ln (θ) during the cooling period for Rh21 (left) and IrRh1 (right).

Conclusions

In summary, we have successfully developed highly stable NIR‐II‐absorbing dyes based on neutral π‐radicaloid metal complexes, comprising an open‐chain dibenzo‐bilatriene analogue with Rh(COD) and Ir(COD) moieties. Unprecedented metal coordination at the periphery of the pseudo‐metallacyclic skeleton induced dπ‐pπ interactions, stabilizing the open‐shell monoradical structures, as inferred from magnetic susceptibility measurements and DFT calculations. The apparent SOMO/SUMO‐based transitions correspond to deep NIR‐II absorptions with high photostability. The specific MO interactions through the non‐porphyrinic electronic structures gave rise to the partially allowed NIR‐II transitions, contributing to theranostic applications based on PA imaging and PT therapy. Consequently, these results suggest that this novel open‐shell porphyrinoid radical system is a promising platform for developing NIR‐II light‐responsive materials for deep tissue imaging and therapies as well as catalysis in the future.

Supporting Information

Supporting Information Available: Synthesis and characterization of the complexes, spectral/magnetic susceptibility data, X‐ray crystallographic structures, DFT calculation results, along with the cartesian coordinates. The authors have cited additional references within the Supporting Information (Ref. [52–64]).

Conflict of Interests

The authors declare no conflict of interest.

1.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supporting Information

ANIE-64-e202418751-s001.pdf (10MB, pdf)

Acknowledgments

The authors acknowledge the support from Grant‐in‐Aid for Scientific Research JP20H00406 (H.F.), JP22 K19937 (Y.Y.) from JSPS, a JST PRESTO (JPMJPR2103; M.I.), and Izumi Science and Technology Foundation (M.I.). The authors acknowledge Prof. H. Shinokubo and Dr. K. Wang (Nagoya Univ.) for HR‐ESI‐MS spectral measurements and Prof. K. Sugiura (TMU) for fruitful discussion. The work for magnetic properties was conducted at the Institute for Molecular Science, supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) – proposal Number JPMXP1222MS1802. This work was also performed under the Cooperative Research Program of “NJRC Mater. & Dev. (MEXT). A.G. is grateful for the Tokyo Global Partner fellowship from TMU.

Ghosh A., Mori S., Ide Y., Song J. T., Yamaoka Y., Ishihara T., Ikeue T., Furuta H., Ishida M., Angew. Chem. Int. Ed. 2025, 64, e202418751. 10.1002/anie.202418751

Contributor Information

Takahisa Ikeue, Email: ikeue@riko.shimane-u.ac.jp.

Prof. Dr. Hiroyuki Furuta, Email: furuta.hiroyuki.165@m.kyushu-u.ac.jp.

Prof. Dr. Masatoshi Ishida, Email: ishidam@tmu.ac.jp.

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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ANIE-64-e202418751-s001.pdf (10MB, pdf)

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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