Additive-Controlled Kinetic Trapping of Quadruple Platinum(II) Stacks with Emergent Photothermal Behaviors

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Oct 2021Additive-Controlled Kinetic Trapping of Quadruple Platinum(II) Stacks with Emergent Photothermal Behaviors Zongchun Gao†, Yukui Tian†, Hung-Kai Hsu, Yifei Han, Yi-Tsu Chan and Feng Wang Zongchun Gao† CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026 , Yukui Tian† Institute of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601 , Hung-Kai Hsu Department of Chemistry, Taiwan University, Taipei 10617 , Yifei Han CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026 , Yi-Tsu Chan Department of Chemistry, Taiwan University, Taipei 10617 and Feng Wang *Corresponding author: E-mail Address: [email protected] CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026 https://doi.org/10.31635/ccschem.021.202000511 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Molecular chaperones are widely employed as additives in nature to trap proteins in the kinetic state, which inspire the development of kinetically trapped artificial supramolecular systems. Till now, such additive-controlled approaches have enabled the stabilization of extended supramolecular structures in the kinetically trapped state, while discrete assemblies with sufficient kinetic persistence are scarce. In this study, a Pt(II)-based discrete supramolecular system has been constructed by taking advantage of Cu+-bridged ions as chaperone-like additives. The resulting quadruple Pt(II) stacking structure possesses high kinetic stability, which survives the column chromatography conditions. Moreover, it takes months for the kinetic-to-thermodynamic transformation to take place at ambient conditions even in dilute solutions. Intriguingly, the kinetically trapped state displays remarkably low-energy absorbance because of close π–π/Pt(II)–Pt(II) distances in the quadruple stacks, resulting in excellent photothermal conversion under red/near-infrared light irradiation, which is unattainable for the thermodynamic state under the same conditions. Therefore, the additive-controlled strategy exemplified in this study provides new avenues toward kinetically trapped assemblies with precise stacking numbers and tailored functionalities. Download figure Download PowerPoint Introduction Kinetic trapping plays a crucial role in protein self-assembly, which avoids deleterious aggregation and provides well-defined supramolecular structures with biological functions.1 Inspired by this fascinating biological phenomenon, kinetic trapping of artificial supramolecular systems has drawn tremendous attention in the last decade.2–7 By manipulating of the transformation from the kinetically trapped state to the thermodynamically stable state, we can endow structural and functional diversity for artificial supramolecular assemblies under the same experimental conditions. Up to now, kinetic trapping has primarily been generated by modulating external parameters such as temperature,8,9 solvent,10–12 vortex stirring,13 ultrasonication,14 and so forth. It is distinct from biological systems in which molecular chaperones or chaperone-like components are employed to hold protein in the kinetically trapped state.1 To mimic this process, some pioneering work has been reported toward kinetically trapped artificial assemblies via the assistance of chaperone-like additives.8,15–17 For example, Meijer and co-workers8 have employed (S)-dibenzoyl tartaric acid to direct the formation of kinetically trapped supramolecular polymers. In addition, Davis's group15 has demonstrated the stiffening of guanosine-based supramolecular hydrogels with the presence of the chaperone-like component, thioflavin T. These studies underscore the potency of chaperone-like additives to stabilize kinetically trapped species in artificial supramolecular assemblies. Despite the progress that has been achieved, these examples are exclusively based on long-range ordered supramolecular polymeric systems. In comparison, discrete supramolecular systems generally exhibit weaker noncovalent complexation strength. It is more likely to escape from the nonequilibrium kinetic state and convert to the thermodynamically stable state. Hence, the question remains: Can such an additive-controlled approach be applied to lock discrete supramolecular systems in the kinetically trapped state? We sought to tackle this issue by employing platinum(II) acetylides as the supramolecular building blocks. Thanks to their square-planar geometry, Pt(II) acetylides show a strong aggregation tendency via the assistance of π–π stacking and metal–metal interactions. They are accompanied by significant spectroscopic changes between the monomeric and self-assembled states.18–25 Moreover, when two Pt(II) acetylides are in close proximity, they facilitate the sandwiching of Cu+ ions between the neighboring η2 acetylene ligands.26–28 By taking full advantage of these properties, herein a novel type of discrete supramolecular system has been constructed in which Cu+ ions serve to bridge and stabilize quadruple Pt(II) acetylide stacks. Thanks to the slow dissociation rate for Cu+-diacetylene bridging complexes, the resulting Pt(II) stacks display sufficient kinetic stability in solution, which convert to a thermodynamic stable state over a prolonged period. Accordingly, this demonstrates the feasibility of kinetically trapping discrete supramolecular assemblies by utilizing Cu+ ions as the chaperone-like additives. Specifically, (−)- 1 was obtained via the CuI-participated coupling reaction between compound 5 and Pt(II)(C^N^N)Cl complex (−)- 4 [(C^N^N) denotes 6-phenyl-2,2′-bipyridine derivative] (Scheme 1). The quadruple Pt(II)(C^N^N) stacks in (−)- 1 represented a unique discrete supramolecular architecture, which were bridged by two Cu+-diacetylene units. Ascribed to the synergistic participation of Cu+-bis(η2-alkyne) coordination bonds and π-stacking interactions between the neighboring Pt(II) pincers, the quadruple Pt(II) stacks in (−)- 1 displayed high activation energy of dissociation (Ea). As a consequence, they were trapped at the local minimum of the Gibbs free energy landscape (Scheme 1), which required months at ambient conditions for the transformation to the global minimum of the energy landscape [thermodynamically stable products: 2 equiv of (−)- 2 and CuI, Scheme 1]. Intriguingly, the kinetically trapped state exhibited remarkably low-energy absorption signals because of close π-distances in the quadruple Pt(II) stacks assisted by Cu+ additives. This induced photothermal conversion under red/near-infrared (NIR) light irradiation, which was unattainable for the thermodynamic stable state under the same conditions (Scheme 1). Hence, with the employment of Cu+ additives as the modulators, this study afforded new avenues toward kinetically trapped supramolecular assemblies with tailored functionalities. Scheme 1 | Schematic representation for kinetic trapping of quadruple Pt(II)(C^N^N) stacks in (−)-1 assisted by Cu+ additives. Download figure Download PowerPoint Experimental Methods Measurements 1H, 13C, and diffusion ordered spectroscopy (DOSY) NMR spectra were collected on Varian Unity INOVA-400 and INOVA-500 spectrometers (Bruker, Switzerland) with tetramethylsilane as the internal standard. UV–vis spectra were recorded on a UV-1800 Shimadzu spectrometer (Shimadzu, Japan). Circular dichroism (CD) measurements were performed on a Jasco J-1500 CD spectrometer (Jasco, Japan). Solution steady-state fluorescence emission spectra were recorded on a FluoroMax-4 spectrofluorometer (slit width: 5 nm; Horiba Scientific, USA). Electrospray ionization interfaced with traveling-wave ion-mobility mass spectrometry (ESI-TWIM-MS) was conducted on a Waters Synapt HDMS G2 instrument (Waters, Milford, USA) with a LockSpray ESI source. Matrix-assisted laser desorption/ionization coupled with a time-of-flight detector mass spectrometry (MALDI-TOF MS) was conducted on a Bruker autoflexTM speed MALDI TOF/TOF mass spectrometer with a 355 nm frequency tripled Nd:YAG SmartBeam laser (Bruker, Switzerland). Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 Spirit BioTWIN electron microscope (acceleration voltage: 120 kV; FEI Company, Hillsboro, USA). Scanning electron microscopy (SEM) images were recorded on a Philips XL30E scanning electron microscope (FEI Company, Hillsboro, USA) at an acceleration voltage of 8 kV. Synthesis of (−)-1 Compounds 5 (85.0 mg, 0.20 mmol), (−)- 4 (235 mg, 0.42 mmol), CuI (15.2 mg, 0.08 mmol), and NEt3 (1.5 mL) were dissolved in 30 mL of CH2Cl2, and stirred at room temperature for 24 h (the reaction mixture displayed dark-green color within 5 min). After the reaction was complete, the solvent was evaporated under reduced pressure, and the residue was extracted with H2O/CH2Cl2. The combined organic extracts were dried over anhydrous Na2SO4. The residues were purified by flash column chromatography (neutral Al2O3, eluent:CH2Cl2/MeOH 50:1, v/v). After concentrating the eluent solution, diethyl ether was added (50 mL × 3) to precipitate the product. It was filtrated and dried in vacuo to afford (−)- 1 as a dark-green colored solid (130 mg, 44%). 1H NMR (400 MHz, CDCl3, 298 K, δ) (ppm): 9.18 (s, 2H), 9.09 (s, 2H), 8.86 (s, 2H), 8.56 (s, 2H), 7.89 (d, J = 7.7 Hz, 4H), 7.75 (d, J = 8.5 Hz, 6H), 7.68 (t, J = 5.3 Hz, 4H), 7.45 (d, J = 7.5 Hz, 2H), 7.38 (dd, J = 14.5, 7.4 Hz, 6H), 7.09 (d, J = 8.6 Hz, 4H), 6.91 (d, J = 9.2 Hz, 4H), 6.84 (d, J = 7.4 Hz, 2H), 6.76 (d, J = 8.0 Hz, 2H), 6.72 (t, J = 7.8 Hz, 2H), 6.68–6.55 (m, 8H), 6.47 (d, J = 7.4 Hz, 2H), 6.40 (t, J = 8.6 Hz, 4H), 6.30 (s, 2H), 5.88 (s, 4H), 4.09 (t, J = 6.5 Hz, 4H), 2.92–2.70 (m, 8H), 2.44 (dd, J = 26.7, 5.1 Hz, 8H), 2.07 (d, J = 24.6 Hz, 4H), 1.95–1.79 (m, 4H), 1.60–1.52 (m, 4H), 1.25 (s, 6H), 1.04 (t, J = 7.4 Hz, 6H), 0.95–0.79 (m, 4H), 0.76 (s, 6H), 0.67 (s, 6H), 0.59 (s, 6H). 13C NMR (100 MHz, CDCl3, δ) (ppm): 163.7, 162.0, 160.4, 156.9, 155.9, 155.0, 154.8, 154.6, 149.2, 148.6, 146.6, 146.5, 146.4, 146.2, 146.1, 144.5, 139.3, 139.1, 137.9, 137.5, 132.6, 131.8, 131.6, 130.8, 130.3, 129.19, 128.52, 128.4, 127.4, 126.5, 125.1, 124.6, 123.3, 122.4, 122.3, 122.0, 121.8, 118.9, 118.0, 116.9, 116.4, 115.5, 114.7, 111.0, 107.0, 105.0, 98.1, 68.3, 66.2, 64.1, 44.4, 44.2, 40.0, 39. 7, 39.4, 38.7, 33.5, 32.3, 31.6, 31.4, 30.0, 29.7, 25.4, 25.2, 23.0, 20.8, 20.4, 19.6, 15.6, 14.5, 14.2. ESI-MS (m/z): [(−)- 1 – 2I]2+ calcd for C154H130Cu2N10O2Pt4, 1529.8760; found, 1529.8938. Density functional theory computation Density functional theory (DFT) computations were performed by the Gaussian 09 D.01 version software package (Gaussian, Inc.). During the optimization, nonmetallic atoms were described by ωb97xd/6-31G computational level, while all metal elements (Pt and Cu) were described by Lanl2dz effective core potential. Considering that the peripheral n-butyloxyphenyl units do not participate in the self-complementary complexation process, they were omitted for the purpose of reducing computational cost during the structural optimization of (−)- 1. Photothermal conversion experiments The samples in centrifuge tubes were irradiated by 660 and 808 nm lasers with the power density of 1.0 W cm−2 (New Industries Optoelectronics, Changchun, China). The real-time temperatures and IR images were recorded using an IR camera (ICI7320, Infrared Cameras Inc., Beaumont, TX). The data were further analyzed by IR Flash thermal imaging analysis software (Infrared Cameras Inc.). Results and Discussion Synthesis and structural determination of the quadruple Pt(II) stacks The key synthetic step toward (−)- 1 involved platinum–carbon coupling reaction29 between compound 5 and 2.1 equiv amount of the cyclometalated Pt(II)(C^N^N)Cl complex (−)- 4 with the presence of CuI and triethylamine (Scheme 1 and Supporting Information Scheme S1, Figures S21–S24). The originally expected product, namely dinuclear Pt(II) compound (−)- 2 (Scheme 1), hardly formed. On the contrary, (−)- 1 was serendipitously obtained as the main product. It was rather stable under flash column chromatography conditions (neutral Al2O3, eluent:CH2Cl2/methanol, 50:1, v/v), with the isolation yield of 44% upon purification. By performing the similar coupling reaction between 5 and 0.2 equiv amount of (−)- 4, it provided the mononuclear Pt(II) compound (−)- 3 (Scheme 1) as the main product (isolation yield: 85%). Notably, the purified sample of (−)- 1 displayed green color in both solid and solution states ( Supporting Information Figure S1). This was strikingly different from the yellow color of (−)- 3 under the same conditions ( Supporting Information Figure S1 and Figures S25 and S26). 1H NMR was employed to differentiate between (−)- 1 and (−)- 3 (Figures 1a and 1b). As can be seen, protons H15 and H16 for both compounds were located at similar positions [H15: 7.75 ppm for (−)- 1 and 7.72 ppm for (−)- 3; H16: 7.09 ppm for (−)- 1 and 7.03 ppm for (−)- 3, Supporting Information Figures S2 and S3]. Nevertheless, the aromatic resonances on Pt(II)(C^N^N) pincers were distinct from each other. Specifically, (−)- 3 displayed only one set of signals (H1: 8.33 ppm; H2: 8.73 ppm), whereas the same H1 and H2 signals split into two peaks each and moved downfield (H1: 9.18 and 9.09 ppm; H2: 8.86 and 8.56 ppm) for (−)- 1 (Figures 1a and 1b and Supporting Information Figures S4 and S5). Hence, reaction between 5 and excessive amounts of (−)- 4 is more likely to form complexed species (−)- 1 rather than the dinuclear Pt(II) structure (−)- 2 as originally anticipated. Figure 1 | (a) Partial 1H NMR spectra (400 MHz, 298 K, CDCl3) of (−)-3; (b) the fresh sample of (−)-1; and (c) the aged sample of (−)-1 after standing for 3 months. Download figure Download PowerPoint Mass spectrometry was employed to confirm the structure of (−)- 1. A signal at 1529.8827 m/z [the isotope spacing (Δm) = 0.5 amu] was detected, indicating the formation of a doubly charged ion [(−)- 1 – 2I]2+, which was confirmed after matching the experimental and calculated theoretical m/z values (Figure 2, inset).30–32 The peak was further analyzed by means of ESI-TWIM-MS.33,34 The analysis revealed that [(−)- 1 – 2I]2+ had a narrow drift time distribution with the peak maximum at 13.01 ms (Figure 2), indicating the absence of structural isomers or conformers. After confirming the absence of Cu+ contamination from the ion source, mass spectrometry unambiguously supported the presence of two Cu+ ions in the structure of (−)- 1, which were introduced during the CuI-participated reaction between 5 and (−)- 4. The additional evidence for the presence of Cu+ in the structure of (−)- 1 came from X-ray photoelectron spectroscopy (XPS). In particular, the binding energies emerged at 932.8 and 952.6 eV (Figure 3a), which are characteristic for Cu 2p3/2 and Cu 2p1/2 of Cu+, respectively.35 Figure 2 | ESI-TWIM-MS plot and drift time distributions for m/z ratio corresponding to the +2 charge state of (−)-1. Inset: isotope pattern of [(−)-1 – 2I]2+ in ESI-MS spectrum. Download figure Download PowerPoint Since we failed to get the single crystals of (−)- 1 suitable for X-ray crystallography, the energy-minimized structure of (−)- 1 was further elucidated via DFT calculation. In the optimized geometry, the two nonadjacent Cu+ ions were coordinated between the neighboring alkynyl ligands (Figure 3b and Supporting Information Figure S6). The noncovalent Cu+-bis(η2-alkyne) coordination mode was further confirmed by IR spectroscopy, through the emergence of triple-bond stretching vibration in the low wavenumber region (2007 cm−1, Supporting Information Figure S7).28 In addition, the Pt(II)(C^N^N) pincers were in close proximity, with the staggered arrangement of bulky pinene units to avoid steric hindrance. The neighboring π-distances in (−)- 1 were determined to be 3.32, 3.25, and 3.34 Å, respectively, leading to the formation of a quadruple Pt(II) stacked structure. The discrete Pt(II) stacking structure in (−)- 1 was further demonstrated via 2D DOSY NMR measurements.36 The diffusion coefficient (D) value of (−)- 1 (2.00 mM in CDCl3) was determined to be 6.60 × 10−10 m2·s−1. According to the Stokes–Einstein equation, the hydrodynamic diameter of (−)- 1 was 1.7-fold larger than that of (−)- 3 (D = 1.12 × 10−9 m2·s−1) under the same conditions ( Supporting Information Figure S8). Figure 3 | (a) Cu 2p XPS spectrum of (−)-1. (b) The optimized structure of (−)-1 via DFT calculation. The dark blue- and orange-colored balls denote platinum(II) and copper(I) atoms, respectively. Download figure Download PowerPoint The mechanism for the formation of quadruple Pt(II) stacks in (−)- 1 was then clarified. Cyclometalated Pt(II)(C^N^N) acetylides were generated in situ during the reaction between 5 and (−)- 4. They served as the η2 ligands, facilitating the sandwiching of Cu+ ions (originating from the reagent CuI) between the neighboring two acetylene units. These findings are consistent with the previous work by Wu and co-workers,28 which demonstrated the capability to form Cu+ bridging complex with two adjacent Pt(II) acetylides. More importantly, the rigid diphenyl pyridine moiety on 5 provided a preorganization effect, thus guaranteeing the formation of quadruple Pt(II) stacks in (−)- 1 with two Cu+-bis(η2-alkyne) units. Spectroscopy of the quadruple Pt(II) stacks Attributed to the presence of two Cu+-bis(η2-alkyne) coordination bonds, the quadruple Pt(C^N^N) pincers in (−)- 1 exhibited close π–π and Pt(II)–Pt(II) distances according to DFT calculations. Orbital overlapping of the neighboring Pt(II) atoms resulted in the appearance of metal–metal-to-ligand charge-transfer (MMLCT) transitions.21 In particular, (−)- 1 displayed a high-energy visible-light absorbance at 400–480 nm in dimethylformamide (DMF) (λmax = 424 nm, Figure 4a), together with a low-energy band at 530–700 nm (λmax = 600 nm). on previous the band was to and charge-transfer of Pt(C^N^N) while the low-energy band was for the Pt(II)–Pt(II) Notably, the absorbance was in the mononuclear compound (−)- 3 (Figure evidence for the of π–π and Pt(II)–Pt(II) interactions in (−)- 1 was from the emission which displayed a band at the region (λmax = nm, Figure As such a low-energy emission band was not for the compound (−)- 3 (Figure Figure 4 | and UV–vis absorption and emission spectra of (−)-1 and in of images of (−)-1 mM in and mM in (c) CD spectra of and in The values of (−)-1 and obtained via CD Download figure Download PowerPoint CD spectroscopy was then employed to (−)- 1 in of the presence of units in structure. (−)- 1 exhibited CD signals in dilute (Figure The two were located at and nm, while the one was at nm = = at = = at = cm−1, = in The from units to the quadruple Pt(II) stacks, to supramolecular in (−)- 1. It is to that dichroism was ( Supporting Information Figure the absence of Accordingly, the CD signal was to supramolecular of (−)- 1. As CD signals were for the 1 with the presence of units (Figure and Supporting Information Scheme On the contrary, CD signals were for the mononuclear compound (−)- 3 under the same in of the presence of in (Figure which was to free of and This the of quadruple Pt(II) stacks in (−)- 1, that and supramolecular For CD experiments of (−)- 1, the values from to mM = Figure and Supporting Information Figure In addition, the absorbance the ( Supporting Information Figure Hence, the quadruple Pt(II) stacks of (−)- 1 were stable under the conditions. Kinetic trapping for the quadruple Pt(II) stacks the color of the fresh solution of (−)- 1 mM in after standing at 298 for 3 from green to yellow (Figure the absorbance of (−)- 1 (Figure with CD signals in the light region (Figure The the of quadruple Pt(II) stacks in (−)- 1, accompanied by the of and Pt(II)–Pt(II) interactions. To get into the color and spectroscopy we performed 1H NMR experiments on the aged (−)- 1 the solvent and the in CDCl3, only one set of 1H NMR aromatic signals were (Figure distinct from of the (−)- 1 fresh solution (Figure 1b). The aged sample was as the dinuclear Pt(II) complex (−)- 2 after of 1H NMR the quadruple Pt(II) stacks represented the kinetically trapped state, as by two Cu+-bis(η2-alkyne) coordination bonds in the structure of (−)- 1. These to 2 equiv amounts of (−)- as the thermodynamic state, to the of alkynyl bonds in solution ( Supporting Information Figure The thermodynamically stable state has Gibbs free energy than that of the kinetic state, the conversion from (−)- 2 and CuI to (−)- 1 not Figure 5 | and visible-light absorption and CD spectra of (−)-1 mM in 298 of images of the fresh and aged of (−)-1 mM in (c) Schematic representation for the formation of kinetically trapped structure during the as as kinetically structure during the The one to upon flash column chromatography (neutral Al2O3, v/v). Download figure Download PowerPoint from kinetically trapped to thermodynamically stable states were further It was that (−)- 1 in displayed a conversion rate than that in under the same conditions. This was by the color of (−)- 1 mM in after it at 298 for 3 months ( Supporting Information Figure The was to the of than that of leading to complexation of (−)- 1 in the Hence, with the of Cu+ the quadruple Pt(II) stacks were as a discrete supramolecular system with sufficient kinetic On the in with the value the kinetic-to-thermodynamic from (−)- 1 to ( Supporting Information Figure standing (−)- 1 in at 298 K, the absorbance within 1 the solution of (−)- 1 at K, the absorption reduced by in h ( Supporting Information Figure further acceleration of kinetic-to-thermodynamic conversion at high It is the of temperature to over the activation energy In to Cu+ the cyclometalated structures crucial on kinetic persistence of quadruple Pt(II) stacks. This was demonstrated by the in which CuI-participated coupling reaction place between 5 and complex (Figure with two steric units. The reaction mixture exhibited a dark-green color in CH2Cl2, similar to that of (−)- 1. In the a new absorbance emerged between and nm, characteristic of Pt(II)–Pt(II) (λmax = nm, Supporting Information Figure This the formation of Cu+-bridged quadruple Pt(II) stacks during the the sample was upon column chromatography (neutral Al2O3, v/v), and to the compound as the purified sample (Figure and Supporting Information Scheme Figures and The was to the steric between the neighboring pincers, leading to the formation of quadruple Pt(II) stacks as the kinetically state (Figure Considering that the kinetically species activation energy of dissociation than that of the kinetically trapped one ( Supporting Information Figure it was to column chromatography and more to to the thermodynamically stable structure. Photothermal conversion for the kinetically trapped Pt(II) assemblies As a of the presence of low-energy absorbance between and nm, the photothermal conversion of quadruple Pt(II) stacks at the kinetically trapped state was further The solution of (−)- 1 mM in was to laser at 660 nm with a power density of 1.0 W of (−)- 1 was in situ by an IR As in Figure temperatures to the within 4 to the maximum temperature of The time of (−)- 1 was determined to be ( Supporting Information with the photothermal conversion of ( Supporting Information Figure Photothermal conversion was at the thus the excellent of (−)- 1 ( Supporting Information Figure The photothermal conversion of (−)- 1 can be as Ascribed to the between the and the (−)- 1 is to state. The state to the state through the photothermal As absorbance is in the thermodynamically stable state, temperature takes place upon [(−)- under the same conditions (Figure Therefore, it is that an for the photothermal conversion capability between the kinetically trapped and thermodynamically stable Figure | temperature changes of the kinetically trapped state [(−)-1 with quadruple Pt(II) stacks, mM in the thermodynamically stable state mM in and the solvent upon laser nm, 1.0 W Inset: temperature of (−)-1 in situ by an IR camera upon 660 nm laser at (a) and (b) Download figure Download PowerPoint we sought to photothermal conversion in an which be for In this the structure of (−)- 1 was by = units in the the structure of (−)- Figure and Supporting Information Figure and (−)- 8 exhibited in Moreover, place for (−)- 8 in because of between Pt(II) stacks and ( Supporting Information Figure The was confirmed via and measurements of