Solution‐processed D‐A‐π‐A‐D radicals for highly efficient photothermal conversion

Organic donor‐acceptor semiconductors exhibit great potential in photothermal conversion. However, it is still challenging to achieve pure organic materials with broad absorption comparable with inorganic materials such as graphene. Herein, two D‐A‐D type DPA‐BT‐O4 and NDI‐TPA‐O4 and three D‐A‐π‐A‐D type Th‐O4, Th2‐O4, and IDT‐O4 were readily prepared via two high‐yield steps and simple air oxidization. The stability can be attributed to their multiple resonance structures based on the aromatic nitric acid radical mechanism. Compared with the D‐A‐D radicals, the conjugation extension of the D‐A‐π‐A‐D radicals endows them with a narrowed band gap and broad absorption in powder. Interestingly, the IDT‐O4 powder with aggregation‐induced radical effect exhibits broad absorption between 300 and 2500 nm, which is comparable with graphene and other inorganic materials. Under irradiation of 0.9 W/cm2 (808 nm), the temperature of IDT‐O4 powder rises to 250°C within 60 s. The water evaporation conversion efficiency of 94.38 % and an evaporation rate of 1.365 kg/m2 h−1 under one sun illumination were achieved. IDT‐O4 stands as one of the most efficient photothermal conversion materials among pure organic materials via a rational design strategy.


INTRODUCTION
The global freshwater scarcity has led to an urgent need to explore the treatment and purification of seawater and wastewater. [1,2]][27] Functionalized carbon materials with poor solubility are difficult to perform solution processing compared with flexible organic materials. [28,29]For other inorganic materials, they may give rise to human health problems due to the potential long-term toxicity caused by their direct contact with seawater. [30]Meanwhile, the inorganic photothermal materials usually contain heavy metal elements (such as gold and platinum), which leads to a high cost of commercialization. [31]Organic polymer materials, such as polypyrrole, exhibit advantages such as broadband light absorption and sufficient contact with the water surface through various interface engineering techniques. [31,32]ompared with the polymer photothermal conversion materials, the organic small molecule materials with well-defined structures show high reproducibility.However, their limited absorption range compared with inorganic materials hampers their further enhancement of performance and application for photothermal conversion. [28,33,34]In particular, solutionprocessable organic small molecules with absorption between 300 and 2500 nm are rarely reported.[35][36][37][38] Different from previously reported strategies for introducing steric hindrance groups to stabilize phenol radicals, [39,40] a series of star-shaped aromatic nitric acid radicals (ANARs) without bulky substituents have been constructed. [41,42]hese open-shell phenoxy radicals with D-π-D configuration have unpaired electrons that can be rationally stabilized via multiple resonance structures. [43]In Figure 1A, the stability of phenoxy radicals can be well understood from their resonance structures (such as TPA-O 3 ) and various derivatives have been reported in previous work. [43]Moreover, the 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) radicals have been studied for several decades and shown wide practical application in many fields due to their good stability from the protection of bulky groups. [44]Compared with the widely-studied TEMPO type radicals, the D-π-D type radicals with conjugated π bridges are more conducive to electron delocalization on phenol group and display absorption between 300 and 2500 nm in powder state, which is challenging to achieve with traditional open-shell radicals and closed-shell molecules. [43,45]Despite the photothermal conversion of previous radicals having been improved, their insufficient absorption and molar extinction coefficient still limit their photothermal properties.It is worth considering that intramolecular charge transfer (ICT) interactions in donor-acceptor (D-A) skeletons usually result in a pronounced reduction in the bandgap, an intense red shift in absorption as well as an enhanced molar extinction coefficient. [46]Therefore, the introduction of D-A units will have a significant promoting effect on broadening the absorption spectra of conjugated radicals.
Herein, we designed and prepared DPA-BT-O 4 and NDI-TPA-O 4 with extremely high yields from their methoxy precursors DPA-BT-OMe and NDI-TPA-OMe, respectively (Figure 1B).The strong electron donor bis(4-methoxyphenyl)amine (DPA-OMe) and an electron acceptor benzothiadiazole (BT) were introduced to construct DPA-BT-O 4 , however, it still showed limited absorption in the near-infrared (NIR) region in powder state.Then, we replaced the donor and acceptor unit with 4-methoxy-N-(4-methoxyphenyl)-N-phenylaniline (TPA-OMe) and naphthalene-diimide (NDI) to prepare NDI-TPA-O 4 .Despite the significant redshift of NDI-TPA-OMe compared to DPA-BT-OMe, the absorption spectrum of the corresponding radical NDI-TPA-O 4 was still not broad enough to efficiently capture sunlight photons.To address this, we synthesized three π-OMe precursors with D-Aπ-A-D configuration, combining the conjugate strategy of D-π-D and the ICT effect of D-A molecules, with two simple high-yield steps.Among them, the different π-bridge cores monothiophene (Th), dithiophene (Th 2 ), and indacenodithiophene (IDT) were introduced to regulate the degree of conjugation and molar extinction coefficient (Figure 1C).All methoxy-substituted precursors can be efficiently demethylated to synthesize open-shell π-O 4 compounds (Th-O 4 , Th 2 -O 4, and IDT-O 4 ).As a result, these radical materials exhibit obviously broader absorption range and photothermal properties compared to precursor materials.Especially, the temperature of IDT-O 4 can rise to 250 • C within 60 s under 0.9 W cm −2 irradiation power.Furthermore, the IDT-O 4 powder was doped into polyurethane (PU) foams for solardriven interfacial water evaporation.This absorber showed a high water evaporation rate of 1.365 kg m −2 h −1 and a 94.38% solar energy-to-vapor efficiency upon one sun irradiation.It is worth mentioning that IDT-O 4 shows enhanced photothermal conversion properties than previously reported organic small molecules, providing a new design strategy for solution-processable pure organic photothermal materials with superior photothermal properties.

Preparation and characterization of TPA-O 3 radical
In order to visualize the conversion process and confirm the purity of the radical products, we prepared tris(4methoxyphenyl)amine (TPA-OMe 3 ) and open-shell radical TPA-O 3 via heating 4,4′,4′′-nitrilotriphenol (TPA-OH 3 ) in air (250 • C).Interestingly, the white TPA-OH 3 turns into totally black TPA-O 3 power after being heated in air, and the 1 H nuclear magnetic resonance (NMR) spectrum of TPA-O 3 still shows the characteristic peak of TPA-OH 3 (Figure 2C).However, TPA-O 3 exhibits completely different photophysical properties from TPA-OH 3 no matter in the ultraviolet (UV) spectrum (the characteristic absorption peak around 800 nm, Figure 2A) or in the electron spin resonance (ESR) spectrum (significantly enhanced ESR signal, Figure 2B).These results, especially the obvious color change in the heating process, prove that the heated TPA-O 3 black sample has intrinsic radical characteristics rather than a hydroxyl compound, which further proves that the TPA-O 3 are pure radical species without polymer impurity or other structure.Moreover, it is noteworthy to mention that the TPA-OH 3 was carefully purified via silica gel column chromatography and this means that the TPA-O 3 is not bromine ion-doped-TPA-OH 3 .
In Figure 2C, the 1 H-NMR spectra of TPA-O 3 radicals show clear signals, which should be silent.This phenomenon is attributed to the partial TPA-O 3 radicals reacting with trace water in DMSO-d 6 and converting into hydroxyl compound TPA-OH 3 and then exhibiting the same characteristic peaks of hydroxyl compounds.Therefore, it is necessary to heat and oxidize the demethylated products to obtain radicals.The TPA-O 3 radical is black in color due to the narrow bandgap confirmed from the film UV-visible-NIR (UV-vis-NIR) absorption spectrum, making it clear to judge a complete transformation of hydroxyl precursor into radical.Similar results were also observed in the following D-A-D and other radical compounds. [43,47]Due to the narrow bandgaps and the elevated highest occupied molecular orbital (HOMO), the conjugated D-A-D and D-A-π-A-D type hydroxyl intermediate compounds can be directly oxidized by O 2 in air at room temperature to form radicals (Figure 1B,C).

UV-Vis-NIR absorption of D-A-D and D-A-π-A-D type compounds
The optical properties were investigated by UV-Vis-NIR absorption spectra.The spectra of DPA-BT-OMe and DPA-BT-O 4 in the DMSO are very consistent (Figure S28), which means the DPA-BT-O 4 radicals can react with the trace water and convert into hydroxyl compound (DPA-BT-OH 4 ), so it does not show the characteristic absorption peak of radicals.In Figure S29, the NDI-TPA-O 4 shows the typical absorption peak at 720 nm in film and a large red shift compared with the film of NDI-TPA-OMe.For the same reason as DPA-BT-O 4 , the π-O 4 shows similar absorption behavior to that of corresponding π-OMe in DMSO (Figure 3A and Figures S30 and S31), which can also be observed in the reported work. [32,41,47]The characteristic absorption peaks of Th-O 4 , Th 2 -O 4, and IDT-O 4 are 550, 540, and 570 nm, corresponding to molar absorption coefficient values of 7420, 8261, and 12962 M −1 cm −1 , respectively.
The gradual redshift is attributed to the extension of the conjugate π-bridge.Remarkably, the redshift is further extended upon aggregation in films, which can be explained by the π-π interaction. [48]For the red curves in Figure 3A and Figures S29-S31, the absorption tails of the four radicals typically reach nearly 1000 nm in the film, suggesting their typical radical characteristics in the solid state, which is similar to the reported work. [45,49]The clear and long absorption tails of radical species in the film can be attributed to the typical absorption of phenoxy radicals, generated from the oxidative dehydrogenation process of phenolic hydroxyl groups in the solid state. [41,43,47]Compared with the π-O 4 in solution without absorption in 800 nm, the π-O 4 in film with radical characteristic peaks are related to the aggregation-induced radical (AIR) effect, that is, the radicals are easily combined with trace water in solution to form π-OH 4 , while the quinone radicals are conducive to produce in film or powder state due to the aggregation effect and π-π stacking. [32,45,50]Among them, the absorption edge of IDT-O 4 is even near 1100 nm, indicating the IDT-O 4 possesses a  S3).In addition, the color of IDT-O 4 in the DMSO solution is totally black (Figure 4A), which is in good agreement with the wide absorption covering the whole visible region (Figure 4D) and guarantees the harvest of more photons from sunlight.In short, the radicals display band tails near 1000 nm in the films due to the synergistic effect of D-A, aggregation, and conjugation, causing the small singlet-triplet energy gap (ΔE S-T ) of radicals. [45]

Simulations on absorption spectra
Additionally, the density functional theory (DFT) calculation was conducted to study the absorption behaviors.The optimized coordinates for π-OMe and π-O 4 are listed in Table S8.In Table S7, the theoretical absorptions are in good accordance with the experimental results.According to our calculated results in  (Figure 3D). [32,45,50]The vertical excitation energies calculated based on OS and T t optimized geometries show that they have nearly the same absorption positions (see Table S7), which means that both the OS and T t together contribute to the absorption spectra.

Cyclic voltammetry measurements
Cyclic voltammetry (CV) test of the samples was conducted to study the electrochemical characteristics and energy  [28,34,35,38,43,69,70]. levels (Figures S38-S42).The highest occupied molecular orbital (HOMO) energy levels were obtained based on the onset potential and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated based on the HOMO and bandgap values (see Table S3).

Photoluminescence measurements
The photoluminescence (PL) spectra are shown in Figures S32-S34 and the PL quantum yields (PLQYs) of films were detected with relevant values listed in Table S3.3D). [37]The PLQY of radicals are much lower than their precursor and can be almost ignored (<0.1%) indicating that the photo-excited state can be readily dissipated through the nonradiative transitions.These results mean that the radiative channel for fluorescence emission is almost forbidden and the channel for heat dissipation is allowed for radicals, which is in good agreement with previous work that radical materials usually display extremely low PLQYs.The remarkable fluorescence emission forbiddance of radicals contributes to the enhancement of photothermal conversion. [28,32,35]

Magnetic field-dependent measurements
To reveal the structure of radicals, ESR spectroscopy was carried out to research the electronic structures and spin  3D).The existence of a thermally accessible triplet state can be evidenced by the triplet state spin density in DFT (Figure S55).The result is consistent with the wide absorption over 1100 nm in film and good photothermal properties of IDT-O 4 . [28,32,35]

Photostability measurements
Moreover, the photostability test was carried out to characterize molecular stability under the irradiation of a 100 W tungsten bulb.The materials were dissolved in DMSO and then tested.After being irradiated for 2 h, 4 mL solution was transferred to perform the UV test.The operation is repeated to evaluate the photostability of each material.In Figure 6A and Figure S35, the intensity change of all the spectra is negligible, implying the radicals and their methoxy precursors possess superior photostability.

Photothermal-dependent measurements
Based on the result that these ANAR radicals exhibit photostability, relatively wide absorption, and negligible PLQY, these materials might show great potential in photothermal applications.The photothermal conversion properties and IR thermal images of DPA-BT-O 4 and NDI-TPA-O 4 powders (15 mg for both samples) are shown in Figure 4B [32] the relatively narrow absorption in powder and poor photothermal response at the beginning/end limit the further enhancement of photothermal performance.
On the contrary, when introducing a π-bridge to construct radicals with a D-A-π-A-D skeleton, the photothermal behavior can be efficiently optimized due to the broadened absorption spectra and improved molar extinction coefficient. [46]The superior photothermal properties of π-O 4 , especially IDT-O 4 , imply that an extended π-bridge favors intensive ESR signal, broadened absorption, and enhanced non-radiative transition.
Then, the photothermal conversion performance of IDT-OMe and IDT-O 4 was tested under different irradiation power.In Figure 6B, the maximum temperatures of IDT-OMe and IDT-O 4 samples rise with the growth of irradiation power.For IDT-O 4 , the temperature rises sharply within 5 s and falls rapidly after turning off the laser, showing rapid photothermal response and excellent photothermal conversion characteristics.It is evident from Figure S44 that NDI-TPA-O 4 exhibits relatively high photothermal temperature (ca.225 • C in 1.0 W/cm 2 ), however, its photothermal response is still not as good as IDT-O 4 .Surprisingly, under the irradiation of 0.9 W/cm 2 , IDT-O 4 can rise to 248.6 • C, which is more than three times higher than that of IDT-OMe (which merely reaches 73 • C).The infrared imager recorded the temperature variation images of IDT-O 4 at 0.9 W cm −2 power and the images are listed in Figure 6D.The reported photothermal conversion materials are summarized in Figure 6E and the detailed values are listed in Table S5.Markedly, the photothermal property of IDT-O 4 stands out among these previously reported materials.For NDI-TPA-O 4 , IDT-OMe, and IDT-O 4 , the maximum temperature values and corresponding irradiation power were fitted and showed a linear growth trend (Figure S46).The maximum temperature is positively associated with the irradiation power (R 2 = 0.998 for IDT-OMe/O 4 and 0.996 for NDI-TPA-O 4 ), hinting that the temperature can be precisely controlled by power regulation.
In addition, we conducted duplicate heating/cooling for ten cycles to research the photobleaching effect under 0.8 W/cm 2 of 808 nm laser.In Figure S45 and Figure 6C, NDI-TPA-O 4 , IDT-OMe, and IDT-O 4 show negligible photobleaching effects in their respective cycles.The maximum temperatures are maintained at about 197, 70, and 230 • C during 10 cycles, respectively.There is no significant temperature loss, which implies their high optical and thermal durability.

Mechanistic study
An ultraviolet-visible-near infrared spectrophotometer experiment was conducted to clarify the reason for the better photothermal performance of radical materials than methoxy precursors.All materials were tested in the powder state from 300 to 2500 nm.In Figure 4D, IDT-O 4 shows a broader light absorption range than Th-O 4 and Th 2 -O 4 , owing to the low band gap caused by the extension of the conjugated chain.Interestingly, the IDT-O 4 still shows strong absorption between 300 and 2500 nm, covering the whole solar spectrum, which guarantees harvesting more photons from sunlight for solar-driven water evaporation.We also researched the typical inorganic materials graphite (G), carbon nanotubes (CNT), ferric oxide (Fe 3 O 4 ), and carbon black (CB) for comparison.Surprisingly, the NIR absorption of IDT-O 4 and inorganic materials is comparable, which can greatly improve the photothermal performance.The absorption spectra of reported organic small molecule powders are shown in Figure S53, and their absorption edges are summarized in Figure 4E.Obviously, the absorption range of IDT-O 4 is broader than that of other reported molecules, which ensures the ideal capture of low-energy photons in sunlight.In Figures S36 and S37, the absorption edges of DPA-BT-O 4 and NDI-TPA-O 4 powder are approximately between 1500 and 1600 nm but have not reached the ultrawide absorption of D-A-π-A-D type radicals.The main difference between methoxy precursors and radicals is the presence or absence of radical electrons in their molecules.It has been reported that strong electronic (tunneling) coupling between neighboring molecules promotes the redshift of the spectrum. [53]At the same time, the DFT calculations also confirmed that there is a pronounced and high spin density on OS state on the terminal groups of the radical materials, which is very different from the spin-density distribution of π-OMe precursors (Figure 5 and Figure S54).Thus, we attributed the broad absorption and intense red shift in the powder state to a reduced optical gap caused by electronic coupling (see Figure 4C).The multi-electron-radical interaction in the ANAR system contributes to their outstanding photothermal properties.

Water evaporation-dependent measurements
Considering the efficient sunlight harvesting of IDT-O 4 powder, solar-driven water evaporation was carried out to investigate the photothermal properties of radicals.The preparation of the material-loaded sponge and test method are shown in Figure 7A.The temperatures of blank PU, NDI-TPA-O4+PU, Th-O4+PU, Th2-O4+PU, and IDT-O4+PU were recorded and presented in Figure S48 S4).Wet NDI-TPA-O 4 +PU exhibits a higher temperature than IDT-O 4 , which is attributed to its larger water contact angle causing the heat on the sponge surface to not be fully transmitted to water for dissipation (Figure S52).The water evaporation performances of various photothermal functional materials are collected in Figure 7F, and detailed values are listed in Table S6.Notably, the IDT-O 4 shows outstanding comprehensive properties, ranking as the top organic small molecule.Compared with inorganic and polymer materials, IDT-O 4 has certain advantages in readily synthetic routes and high reproducibility.56][57] The seawater evaporation experiment of NDI-TPA-O 4 and IDT-O 4 was simulated to confirm the feasibility of seawater desalination.The simulated experimental device is drawn in Figure 7A.We collected the desalinated water and detected the concentration variation of Ca 2+ , Na + , K + , Mg 2+ , and Cl − .After distillation, it is obvious from Figure 7E and Figure S51 that the concentrations of these ions are significantly reduced.Compared with NDI-TPA-O 4 , IDT-O 4 with better water wettability and photothermal property shows promising practical application potential in seawater desalination.Traditional inorganic photothermal conversion materials usually contain non-degradable precious metals, [58] which will cause potential safety problems in application. [59]onventional organic photothermal conversion materials possess the disadvantages of easy photobleaching and relatively narrow absorption. [60]Even if they demonstrate excellent photothermal conversion effects, performance attenuation might be their fatal shortcomings.In short, by introducing extended conjugated fragments and facilely demethylating, we successfully synthesized open-shell radical IDT-O 4 with wide absorption covering 300-2500 nm and high photothermal conversion characteristics.3][64][65][66]

CONCLUSION
In IDT-O 4 powder with the power of 0.9 W/cm 2 can elevate its temperature to nearly 250 • C within 60 s.Furthermore, under one solar irradiation power, IDT-O 4 shows a remarkable water evaporation rate of 1.365 kg m −2 h −1 and a water evaporation conversion efficiency of 94.38%, which represents one of the best pure organic photothermal materials.All these results indicate that D-A-π-A-D radicals exhibit tremendous potential for photothermal conversion compared to the D-π-D and D-A-D configurations.With high photostability and superior photothermal conversion performance, more radical species are in progress to be developed and will present promising application prospects in the field of interface evaporation, biomedical, and so on. [67,68]Based on ANAR and aromatic inorganic acid radical (AIAR) with large conjugated structure, planar radicals may achieve higher conductivity and superconductivity in the future.Our group is currently making substantial progress in these endeavors.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
The D-A-D and D-A-π-A-D type molecules with closed-shell or open-shell structures in this work and synthetic procedures of radicals.(A) The resonance structures of nitric acid and oxidation/reduction process of TEMPO radical (up).The synthetic procedure of TPA-OH 3 and the transformation to TPA-O 3 with the resonance structures (down).(B) The synthetic procedures of DPA-BT-O 4 and NDI-TPA-O 4 with D-A-D configuration.The molecule structures in the dotted box represent the intermediate hydroxyl compounds, which are easily oxidized to radical species in the air at room temperature.(C) The work is based on D-A-π-A-D closed-shell molecules and open-shell radicals.The synthesis and resonance structure of aromatic nitric acid radical (π-O 4 ) with obviously narrowed bandgap and the closed-shell molecules (π-OMe) with large bandgap.The molecule structure (π-OH 4 ) in the dotted box is easily oxidized to radical species in air at room temperature.F I G U R E 2 The optical and magnetic characterization of TPA-O 3 radical.(A) The UV-Vis-NIR absorption spectra of TPA-OH 3 and TPA-O 3 in solution and film, respectively.(B) ESR spectra of TPA-OH 3 and TPA-O 3 with the same mole amount (0.02 mmol) of solid samples at room temperature.(C) 1 H nuclear magnetic resonance of TPA-OH 3 and TPA-O 3 .There is almost no change in the characteristic peaks of 1 H-NMR due to the active reactivity of TPA-O 3 to H 2 O and conversion to TPA-OH 3 .The TPA-O 3 black powder was obtained from heating TPA-OH 3 in air, and the solution state in the nuclear magnetic resonance (NMR) tube, powder state, and reaction formula of the sample can be found in the inset image.

F I G U R E 3
The optical and magnetic characterization and the transition mechanism of IDT-O 4 radical.(A) The ultraviolet-visible-near-infrared (UV-Vis-NIR) absorption spectra of IDT-OMe and IDT-O 4 in solution and film, respectively.The IDT-O 4 can react with the trace water in DMSO and convert it to IDT-OH 4 .(B) Variable temperature ESR (VT-ESR) spectra of IDT-O 4 from 160 to 300 K.The inset represents the spin inversion to form a thermally excited triplet state.(C) Fitted IT−T curves of IDT-O 4 by the Bleaney-Bowers equation.ΔE S-T is an energy difference between the thermally excited triplet state and the singlet ground state.I is the ESR signal intensity at different temperatures.(D) The Jablonski diagram shows the difference in energy dissipation of excited states to understand the different photothermal conversion efficiency of IDT-OMe and IDT-O 4 .The inserted molecular structure represents that one of the resonant structures of IDT-O 4 undergoes spin inversion from the singlet ground state to the thermally excited triplet state.narrower optical band gap (1.59, 1.24, 1.33, 1.28, and 1.13 eV for DPA-BT-O 4 , NDI-TPA-O 4 , Th-O 4 , Th 2 -O 4, and IDT-O 4 , respectively, in Table

F I G U R E 4
The relationship between photothermal performance and absorption.(A) The diagrams of blank polyurethane (PU) and radical-loaded PU foam (up).Powder and solution of precursors and radicals (down), respectively.(B) Photothermal conversion behavior of radicals and precursors (20 mg) under 808 nm laser (0.8 W/cm 2 ) and then turned off in powder.(C) Molecular arrangement model of IDT-OMe and IDT-O 4 with electronic coupling and conjugated interactions.(D) The normalized absorption spectra of Th-O 4 , Th 2 -O 4, and IDT-O 4 in powder state compared with precursors and inorganic materials.(E) The comparison of the powder absorption edge of the previously reported pure organic materials and the open-shell radicals in this work.

F I G U R E 5
The density functional theory (DFT) calculation of π-OMe and π-O 4 .Electron spin density distributions of π-OMe (closed-shell singlet state, isovalue = 0.001 a.u.) and π-O 4 (open-shell singlet state, isovalue = 0.003 a.u.).concentration.With the same amount of 0.02 mmol materials, it is apparent from Figure S43 that almost no paramagnetic signals are measured from methoxy precursors, while the radicals show pronounced spin signals.The DFT calculation was conducted to disclose the electron spin density distributions of methoxy precursors and corresponding radicals (Figure 5 and Figures S54 and S55).The precursors showed spin density on the whole conjugated backbone on their closed-shell singlet states.In contrast, with shallow HOMO energy levels, the phenolic hydroxyl groups can readily react with O 2 in air to produce large amounts of phenolic radicals.Consequently, all the terminal groups of radical species display significantly high electron spin density on their open-shell singlet states, which indicates that the ESR signal comes from the contribution of both carbon radicals and phenolic radicals and is in good accordance with the high spin signal in ESR spectra.It is interesting that the ESR signal of IDT-O 4 powder is the highest compared with those of DPA-BT-O 4 , NDI-TPA-O 4 , Th-O 4 , and Th 2 -O 4 under the same test condition.Subsequently, 20 mg IDT-O 4 was further characterized via VT-ESR.In Figure 3B, the spin signal intensifies significantly with the increasing temperature, revealing its open-shell singlet ground state.The DFT calculation was executed to confirm the ground state electronic structure of π-OMe and π-O 4 .The diradical character index (y 0 ) and tetraradical character index (y 1 ) are used to define the radical character, where y i (i = 0, 1) can have values as y i = 0 (closed-shell), 0 < y i < 1 (intermediate open-shell), and y i = 1 (pure open-shell).In Table 1, the values of y i , ΔE (OS-CS) , and ΔE (OS-Tt) of π-O 4 are about 0.31, −17 kcal mol −1 , and −4.5 kcal mol −1 , respectively.Interestingly, all these radicals exhibit relatively large tetraradical character indexes (y 1 = 0.31) with relatively narrow bandgap, which is related to the excellent photothermal properties of radicals and rarely reported in previous work.In contrast, π-OMe displays a closed-shell singlet ground state.As represented in Figure 3C, the fitting of the VT-ESR data by the Bleaney-Bowers equation (Equation S3) gives ΔE S-T of −2.594 kcal mol −1 for IDT-O 4 .Both the fitting and DFT results give a relatively small ΔE S-T for IDT-O 4 , which is more conducive to the electronic transition from the open-shell singlet ground state to the thermally accessible triplet state (Figure and Figure S47.The maximum temperature values of DPA-BT-O 4 and NDI-TPA-O 4 reach 95 and 197 • C, respectively, under 0.8 W/cm 2 of 808 nm laser.For π-O 4 in Figure 4B, the

F I G U R E 6
The photostability and photothermal performance of IDT-OMe and IDT-O 4 .(A) Light stability test of IDT-OMe and IDT-OH 4 in DMSO solution under the irradiation of a 100 W power bulb.(B) Photothermal conversion behavior of IDT-O 4 and IDT-OMe powder (20 mg) under 808 nm laser irradiation at different laser powers (0.2-0.9 W/cm 2 ).(C) Anti-photobleaching properties of IDT-O 4 and IDT-OMe powder during ten cycles of heating and cooling processes.(D) Infrared (IR) thermal images of IDT-O 4 powder under 808 nm laser irradiation (0.9 W/cm 2 ) and then turned off.(E) The comparison of photothermal conversion performance of reported materials under different laser power.The cited references can be found in Table S5.temperatures of Th-O 4 , Th 2 -O 4, and IDT-O 4 rise rapidly with faster photothermal response than DPA-BT-O 4 and NDI-TPA-O 4 .After 60 seconds of irradiation, their temperatures reach 190, 216, and 231 • C, respectively, under the same powder density.Then, the temperature rapidly drops to the ambient temperature after turning off the laser.In obvious contrast, the maximum temperatures of all the precursors are below 70 • C, which are in good agreement with PL spectra and high PLQYs with low non-radiative transition.Although the NDI-TPA-O 4 with D-A-D configuration exhibits relatively enhanced photothermal performance compared with those of D-π-D radicals, and Figure 7B.After 10 minutes of irradiating the dry material-loaded sponge, the temperatures reach 36.2 • C, 72.3 • C, 74.1 • C, 81.2 • C, and 86.4 • C, respectively.In Figure S49 and Figure 7C, the temperatures of wet PU, NDI-TPA-O 4 +PU, Th-O 4 +PU, Th 2 -O 4 +PU, and IDT-O 4 +PU reached 32.4 • C, 44.9 • C, 37.3 • C, 42.4 • C, and 44.5 • C, respectively, after one hour of irradiation.The water evaporation curves were recorded to assess solar-driven interfacial water evaporation efficiency.In Figure S50 and Figure 7D, the IDT-O 4 demonstrates greater mass change and a sharper slope than the other radicals.The water evaporation rates/solar-driven water evaporation conversion efficiency η of NDI-TPA-O 4 +PU, Th-O 4 +PU, Th 2 -O 4 +PU, and IDT-O 4 +PU are calculated to be 1.271 kg m −2 h −1 /87.71 %, 1.118 kg m −2 h −1 /76.83%, 1.204 kg m −2 h −1 /82.70% and 1.365 kg m −2 h −1 /94.38%, respectively (the calculation method is listed in Table summary, we reported several open-shell ANAR radicals of D-A-D type and D-A-π-A-D type with wide absorption and high photothermal conversion performances.Due to the intense ICT and π-π conjugation, the IDT-O 4 black powder with AIR effect shows broad absorption ranging from 300 to 2500 nm and even exceeds 2500 nm.Irradiation of the F I G U R E 7 The preparation of material-loaded evaporator and water evaporation performance.(A) The preparation of a radical-loaded sponge (up).Test devices for water evaporation rate and seawater distillation (down) under one sun irradiation.(B,C) Temperature-time curves of polyurethane (PU) and radical-loaded PU in the air/water.(D) Water evaporation curves of PU and radical-loaded PU under simulated sunlight with an intensity of 1 kW/m 2 (one sun).The PU (Dark) in (C,D) represents the water evaporation without irradiation.(E) The ion concentration variation before and after seawater distillation of the IDT-O 4 -loaded sponge.(F) The water evaporation rate and conversion efficiency of reported work.
Yuan Li conceived the idea and supervised the overall project.Yuan Li, Jiaxing Huang, Zejun Wang, and Weiya Zhu synthesized the new compounds in the manuscript and drafted the manuscript.Yuan Li and Jiaxing Huang contributed to all measurements and data analysis.Yuan Li designed the experiments and revised the whole manuscript.A C K N O W L E D G M E N T SWe thank Prof. Yong Cao, Prof. Yuguang Ma and Prof. Xuhui Zhu in south china university of technology for their great support and helpful discussion.We thank Prof. Li Shen (College of Chemical Engineering and Environmental Chemistry, Weifang University, Weifang 261061 (China)) for their contribution and guidance in the DFT calculation.The work was financially supported by the Natural Science Foundation of China (51973063), the Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2019TQ05C890), the Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University (JDGD-202010), the Pearl River S&T Nova Program of Guangzhou (201710010194), and the Fund of Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates (2019B030301003).

Materials y 0 y 1 ΔE (OS-CS) (kcal/mol) ΔE (OS-Tt) (kcal/mol)
1, π-O 4 has an open-shell singlet ground state (OS) and a rather small ΔE S-T around 4.50 kcal/mol.It indicates that π-O 4 can be readily thermally excited to a triplet state (T t ) at room temperature and then dissipate the energy in the form of heat from T 1 to S 0 TA B L E 1 The density functional theory (DFT) calculation results of π-OMe and π-O 4 .