Synthesis and Characterization of Quinone Compounds Derived from Doubly and Triply Linked Diporphyrins and Tuning of Their Absorption Properties.

Porphyrins are attracting increasing attention in materials science and photochemistry owing to their unique properties and diverse applications. This study focuses on modifying and tuning the absorption properties of porphyrins, specifically those of quinoidal porphyrins, to extend their spectral range into the near-infrared (NIR) region. We report the synthesis and structural and physical properties of quinone compounds derived from doubly and triply linked diporphyrins and their metal complexes. Doubly linked diporphyrinquinone exhibits broad panchromatic absorption properties in solution owing to its low symmetry. Metal complexation markedly extends its absorption range into the near-infrared region. In contrast, the metal complexes of the triply linked diporphyrinquinones exhibit sharp and strong absorption bands in the visible to near-infrared region owing to their higher symmetry. The longest absorption wavelength of the triply linked diporphyrinquinones was approximately 1500 nm, which was significantly more red-shifted than that of the doubly linked ones.

Near-Infrared-Responsive Hydrocarbons Designed by π-Extension of Indeno[1,2,3,4-pgra]perylene at the 1,2,12-Positions.

The relationship between the overall electronic structure of π-conjugated molecules and the arrangement of their constituent elements is of fundamental importance. Establishing rational design guidelines for conjugated hydrocarbons with narrow HOMO-LUMO gaps is useful to develop near-infrared (NIR) responsive dyes and redox-active materials. This study describes the synthesis and properties of three conjugated hydrocarbons, i. e., an indenonaphthoperylene, an indenoterrylene, and a diindenoterrylene. These molecules exhibit NIR absorption despite the absence of significant antiaromaticity and diradical character. Notably, the indenonaphthoperylene exhibits red-to-NIR emission in the 620-850 nm region. The indenoterrylene and the diindenoterrylene exhibit NIR absorption tailing to 870 and 940 nm, respectively. Moreover, the effect of the π-extension of indenoperylene is disclosed in order to propose guidelines for achieving a narrow HOMO-LUMO gap with negligible antiaromaticity and diradical character.

A Quadruply Bridged Non-Offset Face-to-Face Porphyrin Dimer and Cross-Shaped Pentameric Porphyrin Tapes Based on 2,7,12,17-Tetrakis(pinacolatoboryl) NiII Porphyrin.

2,7,12,17-Tetrakis(pinacolatoboryl) NiII porphyrin 5 Ni was synthesized in 75 % yield by Ir-catalyzed borylation of porphine followed by NiII metalation and has been demonstrated to be a useful synthon, giving 2,7,12,17-tetraaryated NiII porphyrins 6 a-d, peripherally octaarylated NiII porphyrins 8 a-d, quadruply bridged face-to-face non-offset NiII -porphyrin dimer 12, and cross-shaped ß-meso singly linked porphyrin pentamers and nonamers. Oxidation of cross-shaped ß-meso singly linked porphyrin pentamers 14 Ni and 14 Zn gave fused pentameric tapes 15 Ni and 15 Zn. The structures of 12, 14 Zn, and 15 Ni have been revealed by X-ray diffraction analysis. Optical separation of 12 has been accomplished, showing a bisignate coupling pattern for exciton-coupled blue-shifted Soret band. Pentameric porphyrin tape 15 Zn exhibits a red-shifted absorption band at 1156 nm and seven reversible redox waves.

Radical Complexes of Nickel(II)/Copper(II) and Redox Non-innocent MB-DIPY Ligands: Unusual Stability and Strong Near-Infrared Absorption at λmax ∼1300†nm.

The preparation of radicals with intense and redox-switchable absorption beyond 1000 nm is a long-standing challenge in the chemistry of functional dyes. Here we report the preparation of a series of unprecedented stable neutral nickel(II) and copper(II) complexes of "Manitoba dipyrromethenes" (MB-DIPYs) in which the organic chromophore is present in the radical-anion state. The new stable radicals have an intense absorption at λmax ∼1300 nm and can be either oxidized to regular [MII (MB-DIPY)]+ (M=Cu or Ni) or reduced to [MII (MB-DIPY)]- compounds. The radical nature of the stable [MII (MB-DIPY)] complexes was confirmed by EPR spectroscopy with additional insight into their electronic structure obtained by UV-Vis spectroscopy, electro- and spectroelectrochemistry, magnetic measurements, and X-ray crystallography. The electronic structures and spectroscopic properties of the radical-based chromophores were also probed by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. These nickel(II) and copper(II) complexes represent the first stable radical compounds with a MB-DIPY ligand.

Expanded Azaporphyrins Consisting of Multiple BODIPY Units: Global Aromaticity and High Affinities Towards Alkali Metal Ions.

The one-pot self-coupling reaction of 3-amino-5-bromo-BODIPY 9 under Buchwald-Hartwig amination conditions gave nitrogen-bridged cyclic BODIPY tetramer 10, pentamer 11, and hexamer 12. Oxidation of 10, 11, and 12 in the presence of alkali metal ions provided 13, 15, and 16, respectively, as the first examples of expanded azaporphyrins possessing more than six pyrrole rings. Curiously, the oxidation even in the absence of alkali metal ions gave the same complexes as a result of spontaneous complexation. 10-12 showed large association constants (104 -106  M-1 ) with K+ and Na+ , but the spontaneous complexations were not observed, indicating the extremely high affinities of 13-16 toward alkali metal ions, which can be ascribed to the multiple and effective ion-dipole interaction of the alkali metal ion with the negatively polarized F atoms of the BF2 walls surrounding the cavity. Importantly, 13, 14, and 15 show diatropic ring currents as a result of global aromaticity.

Optical Properties of Carbon Dots in the Deep-Red to Near-Infrared Region Are Attractive for Biomedical Applications.

Carbon dots (CDs) represent a recently emerged class of luminescent materials with a great potential for biomedical theranostics, and there are a lot of efforts to shift their absorption and emission toward deep-red (DR) to near-infrared (NIR) region falling in the biological transparency window. This review offers comprehensive insights into the synthesis strategies aimed to achieve this goal, and the current approaches of modulating the optical properties of CDs over the DR to NIR region. The underlying mechanisms of their absorption, photoluminescence, and chemiluminescence, as well as the related photophysical processes of photothermal conversion and formation of reactive oxygen species are considered. The already available biomedical applications of CDs, such as in the photoacoustic imaging and photothermal therapy, photodynamic therapy, and their use as bioimaging agents and drug carriers are then shortly summarized.

High-Performance Piezo-Phototronic Devices Based on Intersubband Transition of Wurtzite Quantum Well.

III-nitride semiconductors play much more important roles in the areas of modern photoelectric applications, whereas strong polarization in their heterostructures is always a challenge to restrict the efficiency and performance of photoelectric devices. In this study, piezo-phototronic effect on near-infrared intersubband absorption is explored based on polar GaN/AlN quantum wells. The results show that externally applied pressure leads to the redshift of absorption wavelength by reducing polarization field of the quantum well. The sensitivity to estimate pressure-dependent intersubband absorption wavelength is almost two orders of magnitude higher than interband photoelectric devices. Additionally, such sensitivity is further enhanced by 2.6 times at 20 GPa as a result of piezo-phototronic effect. This study paves avenue for designing high-performance near-infrared piezo-phototronic devices based on intersubband transition.

Design Strategies of Photoacoustic Molecular Probes.

Photoacoustic (PA) probes have been developed very quickly and applied in broad areas in recent years. Most of them are constructed based on organic dyes with intrinsic near-infrared (NIR) absorption properties. To increase PA contrast and improve imaging resolution and the sensitivity of detection, various methods for the design of PA probes have been developed. This minireview mainly focuses on the development and design strategies of activatable small-molecule PA probes in four aspects: reaction-cleavage, metal ion chelation, photoswitch, and protonation-deprotonation. It highlights some key points of designing PA probes corresponding to their properties and applications. The challenges and perspectives for small-molecule PA probes are also discussed.

B←N-Incorporated Dibenzo-azaacene with Selective Near-Infrared Absorption and Visible Transparency.

Organic compounds with selective near-infrared absorption and visible transparency are very desirable for fabrication of transparent/semitransparent optoelectronic devices. Herein, we develop a molecule with selective near-infrared absorption property, QBNA-O, in which four B←N units are incorporated to the core and two benzodioxin groups are introduced at the termini of the dibenzo-azaacene skeleton. QBNA-O exhibits a small optical gap of 1.39 eV due to the strong electron-donating benzodioxin groups and the strong electron-withdrawing B←N units. In toluene solution, QBNA-O shows a strong absorption peak at 856 nm with the full width at half maximum (FWHM) of only 41 nm as well as very weak absorption in the visible range from 380 nm to 760 nm. Thin films of QBNA-O exhibit the average visible transparency (AVT) of 78 % at the thickness of 205 nm and 90 % at the thickness of 45 nm. Solution-processed organic field-effect transistors (OFETs) of QBNA-O display ambipolar transporting behavior with the electron mobility of 0.52 cm2 V-1 s-1 and the hole mobility of 0.013 cm2 V-1 s-1 together with excellent air-stability. The selective NIR absorbing property and excellent charge transporting property imply that QBNA-O can be used to fabricate transparent organic optoelectronic devices.

Quinoid-Aromatic Resonance for Very Small Optical Energy Gaps in Small-Molecule Organic Semiconductors: A Naphthodithiophenedione-oligothiophene Triad System.

Organic semiconductors with very small optical energy gaps have attracted a lot of attention for near-infrared-active optoelectronic applications. Herein, we present a series of donor-acceptor-donor (D-A-D) organic semiconductors consisting of a highly electron-deficient naphtho[1,2-b:5,6-b']dithiophene-2,7-dione quinoidal acceptor and oligothiophene donors that show very small optical energy gaps of down to 0.72 eV in the solid state. Investigation of the physicochemical properties of the D-A-D molecules as well as theoretical calculations of their electronic structures revealed an efficient intramolecular interaction between the quinoidal acceptor and the aromatic oligothiophene donors in the D-A-D molecules; this significantly enhances the backbone resonance and thus reduces the bond length alternation along the π-conjugated backbones. Despite the very small optical energy gaps, the D-A-D molecules have low-lying frontier orbital energy levels that give rise to air-stable ambipolar carrier transport properties with hole and electron mobilities of up to 0.026 and 0.043 cm2 V-1 s-1 , respectively, in field-effect transistors.

Near-Infrared Photoactive Aza-BODIPY: Thermally Robust and Photostable Photosensitizer and Efficient Electron Donor.

We report herein the synthesis of aza-BODIPY substituted with strongly electron-donating p-(diphenylamino)phenyl substituents (p-Ph2 N-) at 3,5-positions. The presence of p-Ph2 N- groups lowers the energy of the singlet excited state (Es ) to 1.48 eV and induces NIR absorption with λabs at 789 nm in THF. The compound studied is weakly emissive with the emission band (λf ) at 837 nm and with the singlet lifetime (τS ) equal to 100 ps. Nanosecond laser photolysis experiments of the aza-BODIPY in question revealed T1 →Tn absorption spanning from ca. 350-550 nm with the triplet lifetime (τT ) equal to 21 µs. By introducing a heavy atom (Br) into the structure of the aza-BODIPY, we managed to turn it into a NIR operating photosensitizer. The photosensitized oxygenation of the model compound-diphenylisobenzofuran (DPBF)-proceedes via Type I and/or Type III mechanism without formation of singlet oxygen (1 O2 ). As estimated by CV/DPV measurements, the p-Ph2 N- substituted aza-BODIPYs studied exhibits oxidation processes at relatively low oxidation potentials (Eox1 ), pointing to the very good electron-donating properties of these molecules. Extremely high photostability and thermal robustness up to approximately 300 °C are observed for the p-Ph2 N- substituted aza-BODIPYs.

Establishment of the Qy Absorption Spectrum of Chlorophyll a Extending to Near-Infrared.

A weak absorption tail related to the Qy singlet electronic transition of solvated chlorophyll a is discovered using sensitive anti-Stokes fluorescence excitation spectroscopy. The quasi-exponentially decreasing tail was, at ambient temperature, readily observable as far as -2400 cm-1 from the absorption peak and at relative intensity of 10-7. The tail also weakened rapidly upon cooling the sample, implying its basic thermally activated nature. The shape of the spectrum as well as its temperature dependence were qualitatively well reproduced by quantum chemical calculations involving the pigment intramolecular vibrational modes, their overtones, and pairwise combination modes, but no intermolecular/solvent modes. A similar tail was observed earlier in the case of bacteriochlorophyll a, suggesting generality of this phenomenon. Long vibronic red tails are, thus, expected to exist in all pigments of light-harvesting relevance at physiological temperatures.

Thermally Activated Upconversion Near-Infrared Photoluminescence from Carbon Dots Synthesized via Microwave Assisted Exfoliation.

Upconversion near-infrared (NIR) fluorescent carbon dots (CDs) are important for imaging applications. Herein, thermally activated upconversion photoluminescence (UCPL) in the NIR region, with an emission peak at 784 nm, which appears under 808 nm continuous-wave laser excitation, are realized in the NIR absorbing/emissive CDs (NIR-CDs). The NIR-CDs are synthesized by microwave-assisted exfoliation of red emissive CDs in dimethylformamide, and feature single or few-layered graphene-like cores. This structure provides an enhanced contact area of the graphene-like plates in the core with the electron-acceptor carbonyl groups in dimethylformamide, which contributes to the main NIR absorption band peaked at 724 nm and a tail band in 800-850 nm. Temperature-dependent photoluminescence spectra and transient absorption spectra confirm that the UCPL of NIR-CDs is due to the thermally activated electron transitions in the excited state, rather than the multiphoton absorption process. Temperature dependent upconversion NIR luminescence imaging is demonstrated for NIR-CDs embedded in a polyvinyl pyrrolidone film, and the NIR upconversion luminescence imaging in vivo using NIR-CDs in a mouse model is accomplished.

Synthesis, Structures, and Near-IR Absorption of Heterole-Fused Earring Porphyrins.

A reaction sequence of regioselective peripheral bromination, Suzuki-Miyaura coupling with 2-borylated thiophene or pyrrole, and oxidative ring-closure with FeCl3 allowed the synthesis of heterole-fused earring porphyrins 4Pd and 9Pd from the parent earring porphyrin 1. Differently pyrrole-fused porphyrins 5H and 6H and their PdII complexes 5Pd and 6Pd were also synthesized. The structures of 4Pd, 5H, 6Pd, and 8Pd have been revealed by X-ray analysis to be slightly twisted owing to constraints imposed by heterole-fused structures. 5Pd exhibits an intensified band at 1505 nm, while 4Pd and 9Pd display small but remarkably red-shifted absorption bands reaching around 2200 nm.

Wurster-Type Nanographenes as Stable Diradical Dications.

Three new and stable diradical dications based on the 4,10-dibromoanthanthrone have been synthesized and characterized. In their dicationic state, the amines show a biradical character in the ground state. Their magnetic properties have been investigated by NMR and EPR spectroscopies. The dicationic salts shows strong and broad optical absorption in the near-infrared region. The main driving force to obtain a ground state biradical is the high steric hindrance that prevents planarization of the diphenylamines with the anthanthrone aromatic core. The radical cations are isolated in two perpendicular π-systems as demonstrated by X-ray diffraction structures. This work provides a new bridging unit in the search for biradical bis(triarylamines) salts.

The Diradical-Dication Strategy for BODIPY- and Porphyrin-Based Dyes with Near-Infrared Absorption Maxima from 1070 to 2040†nm.

Four stable boron dipyrromethene (BODIPY)- and porphyrin-based bis-arylamine diradical dications were synthesized by two-electron oxidation of their neutral molecules. The two BODIPY-based dications have open-shell singlet ground states. UV/Vis absorption spectra of all four dications showed large redshifts in the NIR region compared to their neutral precursors with absorption maxima at 1274 and 1068 nm for the two BODIPY-based dications and 1746 and 2037 nm for the two porphyrin-based dications. Thus, two new types of NIR dyes with longer wavelengths are provided by the diradical-dication strategy, which can be applied for the generation of other NIR dyes with a range of different chromophores and auxochromes.

Thiazoloisoindigo: A Building Block that Merges the Merits of Thienoisoindigo and Diazaisoindigo for Conjugated Polymers.

Thiazoloisoindigo, a novel structural variation of isoindigo, is for the first time utilized to synthesize conjugated polymers. The polymer based on thiazoloisoindigo merges the advantages of the one based on thienoisoindigo and diazaisoindigo; it not only exhibits a greatly redshifted UV/Vis absorption to the near-infrared region owing to its strong tendency to form quinoidal structures, similar to that based on thienoisoindigo, but also shows excellent ambipolar mobility (hole: 3.93, electron: 1.07 cm2 V-1 s-1 ) in organic field-effect transistors (OFETs), superior to that based on diazaisoindigo, showing the strong electron-withdrawing capability of thiazoloisoindigo.

Wurster-Type Anthanthrene Polyradicaloid Cations.

4,10-dibromoanthanthrone, a highly robust building block, is used to synthesize a bis(triarylamine) polymer. The polymer can be oxidized twice to form a polycationic macromolecule showing magnetic properties by electron paramagnetic resonance spectroscopy. In its dicationic state, the presence of isolated radicals is possible because of the interrupted conjugation between the diphenylamine with the anthanthrone core. The high steric hindrance prevents the planarity of the adjacent groups resulting in a polyradical cationic polymer. The oxidized polymer has a strong absorption in the near-infrared region along with reversible redox stages.

An Expanded Porphycene with High NIR Absorptivity That Stabilizes Two Different Kinds of Metal Complexes.

A new expanded porphycene with 26 π-electrons has been prepared by the McMurry coupling of 1,4-bis(3,4-diethyl-2-pyrryl)benzene dialdehyde. Expansion of the porphycene framework provides a ligand capable of stabilizing a bis(rhodium) and a monoruthenium complex. These new porphycene derivatives absorb strongly in the NIR spectral region, with appreciable absorptivity up to 1300 nm. On the basis of their ground- and excited-state spectroscopic features and structural parameters, both the free-base system and the bis(rhodium) complex are considered to be Hückel-type aromatic systems. This conclusion is supported by DFT calculations.

A Stable N-Annulated Perylene-Bridged Bisphenoxyl Diradicaloid and the Corresponding Boron Trifluoride Complex.

Organic π-extended radicals display unique electronic structures, and could be used as promising functional materials. However, design and synthesis of stable radicals are challenging for chemists due to their high reactivity. In this work, we synthesized a stable N-annulated perylene-bridged bisphenoxyl diradicaloid, and its complex with Lewis acid boron trifluoride. Their ground-state geometric and electronic structures were systematically studied by various experimental methods, including X-ray crystallographic analysis, variable-temperature NMR spectroscopy, electron-spin resonance spectroscopy (ESR), and superconducting quantum interference device (SQUID) measurements, supported by density functional theory. Both were observed as open-shell singlet diradicaloids in the ground state. The bisphenoxyl diradicaloid demonstrated string features of strong near-infrared (NIR) absorption, closely packed π-dimer structure in crystals, amphoteric redox behavior with a small HOMO-LUMO energy band gap, and a rather small singlet-triplet gap, whereas the complex showed very different photophysical, electrochemical, and magnetic properties. Our studies provide an efficient method of making stable diradicaloids by Lewis acid/base complexation.