Assembly Induced Super-Large Red-Shifted Absorption: The Burgeoning Field of Organic Near-Infrared Materials

Open AccessCCS ChemistryMINI REVIEW1 May 2021Assembly Induced Super-Large Red-Shifted Absorption: The Burgeoning Field of Organic Near-Infrared Materials Luyang Zhao, Xiaokang Ren and Xuehai Yan Luyang Zhao State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, Xiaokang Ren State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049 and Xuehai Yan *Corresponding author: E-mail Address: email protected State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049 Center for Mesoscience, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190 https: //doi. org/10. 31635/ccschem. 021. 202100771 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Supramolecular assembly of organic dye compounds with J-aggregation leads to a red-shifted absorption spectrum that greatly facilitates the construction of near-infrared (NIR) materials. A considerable improvement of the material functions requires that the absorption red-shift be larger than 100 nm, but such a super-large red-shift is challenging, and the rules leading to the super-large red-shifted absorption is still not explicit. In this review, we focused on those J-aggregated organic dye materials with super-large red-shifted absorption. The nature of the super-large red-shift is originated from the intermolecular charge transfer between neighboring chromophores. The super-large red-shift can be obtained by tuning either the molecular structure or kinetic assembly process in a delicate manner. Materials with super-large red-shifted absorption have been successfully applied to biological imaging, phototherapy, electronic devices, and solar cells, and show great potential in many other fields. The elaboration of assembly induced super-large red-shifted absorption is promising for design of supramolecular NIR materials with tuned structures, enhanced functionalities, and a wide array of applications. Download figure Download PowerPoint Introduction Organic dye materials with near-infrared (NIR) absorption (i. e. , NIR materials or more specifically, NIR absorption materials) are highly attractive in many fields due to their unique advantages of low cost, narrow band gaps, tunable energy levels, and high biocompatibility. 1–5 Supramolecular assembly of organic dye molecules may lead to J-aggregate states with red-shifted absorption, which has the potential to reach the NIR region in a much more convenient and flexible way than conventional molecular design and synthesis. Ideally, for a dye compound with absorption in the far-red region, a larger than 100 nm of absorption red-shift, which we defined as super-large red-shift, 6 is required to achieve the NIR region. However, conventional J-aggregates merely gave rise to less than 30 nm of absorption red-shift, 7–9 resulting in quite limited improvement in material properties. How to enlarge the assembly induced absorption red-shift to more than 100 nm becomes the key task for developing supramolecular NIR materials. To date, there have been few reports of developing super-large red-shifted absorption materials. 10, 11 These materials exhibited larger than 100 nm or even 200 nm of red-shift in comparison with their assembly units, enabling the absorption of the final materials strongly expanding to the NIR region. Consequently, these materials show the feasibility to fabricate NIR materials via simple supramolecular assembly. Despite these successes, assembly methods for these super-large red-shifted absorptions differ from case to case, and assembly units have special molecular structures where the relationship between the molecular structure and the super-large red-shift is still not explicit. In other words, a general rule that governs the fabrication of super-large red-shifted absorption material is still deficient. It is extremely important to reveal the inherent mechanisms of super-large red-shifted absorption for further development of supramolecular NIR materials. In this review, we focus on the assembly induced super-large red-shifted absorption of organic dye compounds and their various applications. We first elaborate the intermolecular aggregation theory beyond the conventional Kasha model, in which the absorption red/blue shifts are controlled by the intermolecular charge transfer (CT). Next, we summarize the kinetic assembly methods that may lead to the aggregation state with super-large red-shifted absorption. Finally, we highlight the applications of the materials with super-large red-shifted absorption. We believe that this timely review will prompt the development of supramolecular chemistry as well as their prospective applications. Theory Conventional J-aggregates with small red-shifted absorption have been well explained by Kasha's theory. 12 It assumes that the intermolecular interactions between neighboring chromophores only include the long-range Coulombic coupling, which arises from the interaction between the local excited transition dipole moments (μ) of each chromophore (Figure 1a). The electron configuration under Coulombic coupling is depicted in Figure 1b. On the one hand, the Coulombic coupling determines the excitation energy by the equation E F (k) = E S 1 + 2 J Coul cos k (1) where E S 1 is the excitation energy of the monomeric chromophores, and k a phase parameter. On the other hand, the Coulombic coupling is dependent on the geometry of the chromophore aggregates with the relationship J Coul = μ 2 (1 − 3 cos 2 θ) 4 π R 3 (2) where R is the interchromophore distance, and θ is the angle between μ and R. As a total consequence, the relative energy between the chromophore aggregates and monomers, that is, EF (k) − E S 1, crucially relies on the chromophore geometries. When JCoul is negative, the excitation energy of the aggregates is smaller than the monomers, resulting in a red-shifted absorption; the aggregates are slipped "head-to-tail" and are defined as J-aggregates. Comparably, when JCoul is positive, the excitation energy of the aggregates is larger than the monomers, resulting in a blue-shifted absorption, and the corresponding aggregate geometries are defined as H-aggregates. Since Coulomb coupling is a weak intermolecular interaction "through space", its perturbation over the energy level of S1 is slight, and the absorption red- or blue-shift is thereby relatively small. The Kasha's theory based on the Frenkel exciton model has shown great success in describing those systems in which chromophores stay far from each other (usually <5 Å) or chromophores are disorderly arranged. Figure 1 | (a) Schematic illustration of long-range interaction (JCoul) and short-range interaction (JCT). (b) Energy-level diagram depicting the electronic configuration and JCT. Adapted with permission from ref 13. Copyright 2017 American Chemical Society. (c) The stacking model of a phthalocyanine dimer with intermolecular distance d. (d) CT-related integrals as the functions of d. (e) Calculated absorption spectra of the phthalocyanine dimer with different d values. Adapted with permission from ref 6. Copyright 2019 Chinese Chemical Society. Download figure Download PowerPoint However, Kasha's theory fails when chromophores get close to each other (usually <5 Å, and especially at ∼3. 5 Å, the distance of π–π stacking), as widely seen in crystallized one-dimensional fibrils, because such a geometry allows wave function overlap between neighboring chromophores. The wave function overlap is a short-range superexchange interaction that contrasts with the Coulombic interaction, enabling the dissociation of local excitations into CT states where electrons and holes reside on neighboring chromophores (Figure 1b). 13 The effect of the short-range exciton coupling is given by J CT = − 2 t e t h E CT − E S 1 | E CT − E S 1 | ≫ | J Coul |, | t e |, | t h | (3) where ECT is the energy of the CT state, and te, th are the electron/hole transfer integrals, respectively, with the definitions t e ≡ ⟨ ϕ L 1 | h ^ | ϕ L 2 ⟩ (4) t h ≡ ⟨ − ϕ H 1 | h ^ | ϕ H 2 ⟩ (5) in which h ^ is the single-electron Hamiltonian operator. Similar to the role of JCoul in the Frenkel exciton model, the sign of JCT in the CT model determines whether the absorption spectrum appears red- or blue-shift, and the detailed relationship is E − (k) = E S 1 − 2 t e 2 + t h 2 E CT − E S 1 + 2 J CT cos k (6) E + (k) = E CT + 2 t e 2 + t h 2 E CT − E S 1 − 2 J CT cos k (7) under the perturbation limit of JCoul = 0. 14 Although the interference between Coulombic and CT-mediated coupling makes the mathematics more complicated, attention should be paid to the qualitative inferences drawn from the CT-mediated exciton model. Due to the existence of te and th, the energy level of the excited state is drastically perturbed, leading to severe spectral variation from which the super-large red-shifted absorption stems. Clearly, the difference between super-large red-shift and conventional small red-shift is that the former has remarkable electron/hole transfer in addition to the larger shift scale. Moreover, it is easy to investigate the relationship between supramolecular structure and super-large red-shifted absorption from two independent dimensions, that is, the vertical and planar dimensions, as follows. The effect of vertical intermolecular distance As mentioned earlier, CT coupling is a short-range interaction, and the intermolecular distance has significant influences on the CT coupling strength. This is because both te and th decay exponentially with the intermolecular distance. Taking the distance along the direction vertical to the π-plane of the chromophore, the influence of intermolecular distance on the absorption red-shift will be clearer. Taking the phthalocyanine dimer as an example, we calculated the relationship between its vertical distance and the absorption spectrum (Figure 1c). 6 The geometry of the phthalocyanine dimer was deduced from the experimental results and optimized to an energy minimum. The te and th were evaluated by the frontier orbital energy splitting with the relationships te = 2ΔEL and th = −2ΔEH, where ΔEL was the energy difference of the lowest unoccupied molecular orbitals (LUMOs) and ΔEH was the energy difference of the highest occupied molecular orbitals (HOMOs). There were two tes, namely te1 and te2, because monomeric phthalocyanine has two degenerated LUMOs. Accordingly, both te and th decreased dramatically with increasing intermolecular distance (Figure 1d). When the distance was larger than 5. 0 Å, the CT coupling almost diminished, leaving the Coulombic coupling effect alone, and the corresponding spectrum merely showed a small blue-shift (Figure 1e). In contrast, when the distance was close to 3. 4 Å, the geometric energy-minimal distance, te and th were large, resulting in a significant absorption red-shift. The remarkable split of absorption in the Q-band region was caused by the two tes with opposite signs, which lead to a relatively large red-shift and a relatively small blue-shift. The effect of in-plane slip In addition to the vertical distance, intermolecular displacement (or the slip to the π-plane of the chromophore) also significantly affects the supramolecular absorption spectrum because the π-conjugated chromophores have nodal frontier orbitals, and the slide geometry directly influences the orbital overlapping. 15 Taking perylene as the example, the nodal patterns of HOMO and LUMO of the perylene molecule determine the sign and the magnitude of th and te, which further leads to the transverse displacement-dependent teth (∝JCT). 16 The teth is extremely sensitive to the intermolecular slide so that an absorption red-shift can be obtained only if the supramolecular geometry of the aggregates is located in the adequate region with large and negative teth. Comparably, the Coulombic coupling is less sensitive to the intermolecular displacement, because along with the long molecular axis, the JCoul suffers only one conversion from positive to negative, which corresponds to the H- and J-aggregate patterns, respectively. The effect of in-plane slide on the absorption was also manifested by a cyanine derivative (3, 3′-bis (4-sulfobutyl) -5, 5′-dichloro-11-diphe- nylamino-10, 12-ethylenethiatricarbocyanine, ammonium salt, Cy7-DPA) dye, as investigated by Caram and co-workers. 17 The Cy7-DPA monomers had intrinsic absorption at 807 nm. In comparison, their self-assembled two-dimensional aggregates showed super-large red-shifted absorption lying at ∼1050 nm. The slip between neighboring chromophores of this aggregates was estimated to be 7∼10 Å, which was correlated to 0. 36 times of the chromophore length (Figure 2a). Such a long slip was considered reasonable because the steric diphenylamino group on Cy7-DPA was too large to allow a slip less than 7 Å. Furthermore, the calculation of density of states (DOS) and bright state as the functions of intermolecular slip indicated that along with the slip increasing, the aggregates would alternately exhibit H- and J-type geometries with corresponding blue- and red-shifted absorption (Figure 2b). The experimentally obtained aggregates with super-large red-shifted absorption was just located on one of the slip regions of J-type geometries. Moreover, an I-type geometry that fell between H- and J-type aggregates was identified, which showed unique temperature-dependent absorption spectrum and was probably the "Null"-aggregate referred by the CT exciton theory with JCoul + JCT = 0. 18 Although this investigation was depicted using the language of Kasha's theory, it definitely represented the picture of CT coupling. Figure 2 | (a) Predicted molecular stacking pattern of the Cy7-DPA dye. (b) Bright states and the blue-/red-shift alternation as the function of slip, where b refers to the molecular length. Adapted with permission from ref 17. Copyright 2019 American Chemical Society. Download figure Download PowerPoint As a short summary of this section, a few principles that lead to the super-large absorption red-shift could be drawn from the complicated mathematics of CT exciton model. Namely, the assembled chromophores should (1) have very short intermolecular distances (in general, ∼3. 5 Å) with neighboring molecules, and (2) achieve an intermolecular transverse displacement with a strong negative CT coupling effect. Due to these requirements, an ordered stacking pattern of the building blocks is usually necessary to achieve the super-large red-shifted absorption, and a more precise control over both the molecular structure of the building block and the assembly process is also indispensable. Strategies for Obtaining Super-Large Red-Shift Based on the CT exciton model and quantum chemistry calculation, one can predict whether there is a supramolecular structure exhibiting the super-large red-shifted absorption starting from a certain molecular building block. However, these structures with super-large red-shifted absorption are not always thermodynamically or kinetically accessible. To obtain the super-large red-shifted absorption, extraordinary efforts need to be made on the molecular structure of the building blocks and the supramolecular assembly process. Molecular modification of substitute groups Organic dye compounds are generally constituted by a π-conjugated chromophore and peripheral substitutes. While it is difficult to form the aggregation pattern with super-large red-shifted absorption from a single chromophore, it is possible to tune the thermodynamics of the aggregates via substituent couplings so that the aggregate geometries can locate in the adequate region. 19 The effect of substituents on the absorption spectra variance was represented by Würthner and co-workers. 20 They used a series of dye compounds with an identical backbone of conjugated 2-aminothiophene and 2-4- (tert-butyl) thiazol-2 (3H) -ylidenemalononitrile and variable substituents on the nitrogen site, including several kinds of alkyl and phenyl groups. The monomeric solution of all these compounds exhibited similar absorption with maxima at 649∼664 nm. In contrast, the solid states of these compounds prepared by spin coating from chloroform solution exhibited either blue- or/and red-shifted absorptions with maxima at 479∼489 nm (H-band) and 735−751 nm (J-band), respectively (Figure 3). For substituents with low steric demand and/or rigidity, a tight card stack-like packing with no longitudinal displacement can be adopted in the solid state, yielding the largest dipole–dipole interaction and the strongest H-coupling of the dyes' transition dipole moments. In contrast, for larger, flexible, or bulky aromatic substituents, where such a cofacial arrangement is disfavored due to the increased steric demand, a zig-zag packing is induced in the solid state by large longitudinal shifts within the dimer synthon, thus giving rise to J-coupling. Figure 3 | Rigid and small peripheral substitutes lead to H-aggregates of the merocyanine chromophore with blue-shifted absorption, while flexible and large peripheral substitutes lead to J-aggregates with red-shifted absorption. Adapted with permission from ref 20. Copyright 2018 WILEY-VCH Verlag GmbH otherwise, the cis-conformation cannot remain stable. The aggregation structure and the absorption spectrum may also be synergistically controlled by multiple substituents on one chromophore backbone. For example, naturally derived light-harvesting chlorins exhibit fantastic assembly features with possibly as large as ∼90 nm of bathochromic shifted absorption from the far-red region to the NIR region. However, the large red-shift relies on three-substituted groups on the chlorin structure: (1) the central Zn ion coordination; (2) 31-methoxy (or hydroxyl) substitution; and (3) a steric hydrophobic ester on the 17-carboxylic group. The chlorin derivative could not form the aggregate with the large red-shifted absorption unless all of the three groups were contained. 22 This is because the tetra-coordinated Zn ion enabled extra axial coordination with the electronegative oxygen on the 31-site of the neighboring molecule, leading to a long-range ordered and slipped π-stacking arrangement. 23 In addition, the steric hydrophobic group probably further stabilized the large intermolecularly slipped geometry. Therefore, rational design of molecular structure is important for achieving the supramolecular aggregates with the desired red-shifted absorption spectra. Supramolecular assembly via intermolecular interactions Sometimes two or more thermodynamically accessible aggregates with local minimal energy in the energy landscape can be obtained from the same building blocks. The super-large red-shifted absorption may be hidden in one of these aggregates but is difficult to achieve via conventional assembly methods like rapid precipitation due to the pathway complexity. Achieving such aggregation states therefore requires kinetic control over the assembly process, or more precisely, control of noncovalent intermolecular interactions in the dimensions of both time and space via several parameters, including temperature, external electric or magnetic field, solvent pH, the order of addition, and so forth. 24–27 With suitable kinetic methodologies, different aggregate morphologies and molecular arrangements can be rationally selected from the same starting building blocks. 28–30 Kinetic control of self-assembly Recently, we showed a pair of pathway-dependent assembly structures with and without super-large red-shifted absorption, where a short peptide decorated phthalocyanine (phthalocyanine–diphenylalanine, PF) was employed as the model assembly unit (Figure 4a). 6 Through conventional flash precipitation, which was an entropy-dominated assembly process, the phthalocyanines assembled to nanoparticles (PF nanoparticles, PFI) with the absorption of maximal-wavelength at 675 nm, which was the same as the monomeric molecules and at ∼630 nm as a blue-shifted absorption probably due to H-aggregation. In contrast, slowly adding water many times over 3 days to elongate the PF assembly process afforded fibril nanoassemblies (PF nanofibrils, PFII) with a 105 nm red-shifted absorption, remarkably expanded to the NIR region (Figures 4b–4d). Further experiments revealed that the PFII aggregates were thermodynamically more stable than PFI, but there was a large enough energy gap between these two assembly states such that conventional flash precipitation could not lead to PFII. However, increasing the starting volume ratio of dimethyl sulfoxide (as the good solvent to solve the phthalocyanine) would reduce the energy gap, thereby increasing the accessibility of PFII (Figure 4e). Figure 4 | Kinetically controlled supramolecular assembly enabling super-large red-shifted absorption. (a) The molecular structure of the assembly unit PF. (b) Two assembly processes. (c) Assembly morphologies of PFI and PFII resulting from the different assembly process. (d) The absorption spectra of PFI and PFII. (e) The schematic free-energy landscape of PF assemblies. Adapted with permission from ref 6. Copyright 2019 Chinese Chemical Society. Download figure Download PowerPoint A similar supramolecular assembly system that was controlled by solubility and assembly time to exhibit super-large red-shifted absorption was also found in a BF2-chelated azadipyrromethene (aza-BODIPY) system. 31 Kinetic assembly of the aza-BODIPY monomers led to metastable nanoparticles with small red-shifted absorption, which then slowly transformed to thermodynamically more stable rod-like micelles with sharp and super-large red-shifted absorption. The nanoparticle and the micelle as two different aggregates constitute a moderate energy barrier, thereby enabling the spontaneous transformation. Herein, both the aza-BODIPY and the aforementioned phthalocyanine with super-large red-shifted absorption was thermodynamically more stable in the energy while their a special kinetic assembly process. the over the kinetic assembly and over the molecular structure may be for the super-large red-shifted absorption. Zhao and showed that an aromatic molecule with long alkyl substitutes exhibited absorption at nm as monomers and at nm as thermodynamically stable J-aggregates (Figures and The J-aggregates could be obtained by either the solution or increasing the solvent However, other relatively stable H-aggregates that as were to the J-aggregation assembly In the there is space on both of the molecules to these alkyl As the of H-aggregates larger and larger, the space for each molecule and steric when two of the alkyl were by groups with low steric as well as was and only the J-aggregates were shown by the at nm. Since both of these two molecules showed similar super-large red-shifted absorption as the of substitutes only the assembly process but not the aggregation Figure | (a) Molecular (b) spectra of the and J-aggregate of compound The is the schematic energy level diagram of different assembly states of Adapted with permission from ref Copyright 2018 American Chemical Society. Download figure Download PowerPoint self-assembly In addition to the kinetically controlled self-assembly of molecular external assembly could lead to the special assembly states with super-large red-shifted absorption. A cyanine dye, is in with absorption at nm and in adding the to water or to an solution gave rise to H-aggregates with a blue-shifted absorption Comparably, adding the to the solution of quantum a J-aggregation of the dye was a super-large red-shifted absorption to or nm. The J-aggregation of on the quantum was to be controlled by the interaction between the and the groups on the quantum Such J-aggregates with two red-shifted absorption maxima were further found by the pH, which was probably because the interaction other than the interaction a role in the supramolecular In many a compound could form J-aggregation with the of an which was also as showed intrinsic absorption at and nm. with like or the nanoparticles showed a significantly red-shifted at In a similar of an aza-BODIPY compound and under afforded nanoparticles two absorption at and nm, while the aza-BODIPY and its H-aggregates showed absorptions at and nm, of an and phthalocyanine lead to a J-aggregate with the absorption from to ca. With the of the process, of and phthalocyanine lead to a larger absorption red-shift from ca. to all of these the were for the super-large red-shifted absorption. Despite the of detailed assembly it is reasonable to that intermolecular interactions a role in the of J-aggregates. The external that the assembly could be covalently conjugated to the chromophores. For example, of a chlorin a to and a afforded