Cofacial Dimers Research Articles

Distinct vibrational motions promote disparate excited-state decay pathways in cofacial perylenediimide dimers.

A complex interplay of structural, electronic, and vibrational degrees of freedom underpins the fate of molecular excited states. Organic assemblies exhibit a myriad of excited-state decay processes, such as symmetry-breaking charge separation (SB-CS), excimer (EX) formation, singlet fission, and energy transfer. Recent studies of cofacial and slip-stacked perylene-3,4:9,10-bis(dicarboximide) (PDI) multimers demonstrate that slight variations in core substituents and H- or J-type aggregation can determine whether the system follows an SB-CS pathway or an EX one. However, questions regarding the relative importance of structural properties and molecular vibrations in driving the excited-state dynamics remain. Here, we use a combination of two-dimensional electronic spectroscopy, femtosecond stimulated Raman spectroscopy, and quantum chemistry computations to compare the photophysics of two PDI dimers. The dimer with 1,7-bis(pyrrolidin-1'-yl) substituents (5PDI2) undergoes ultrafast SB-CS from a photoexcited mixed state, while the dimer with bis-1,7-(3',5'-di-t-butylphenoxy) substituents (PPDI2) rapidly forms an EX state. Examination of their quantum beating features reveals that SB-CS in 5PDI2 is driven by the collective vibronic coupling of two or more excited-state vibrations. In contrast, we observe signatures of low-frequency vibrational coherence transfer during EX formation by PPDI2, which aligns with several previous studies. We conclude that key electronic and structural differences between 5PDI2 and PPDI2 determine their markedly different photophysics.

A Cofacial Porphyrin Dimer Generated by Cooperative Zinc Ion Binding

Noncovalent cofacial porphyrin dimers have attracted interest due to their potential use as host materials or catalysts. In this study, we introduce a novel coordination‐directed cofacial porphyrin dimer from a meso‐tetraphenylporphyrin derivative modified with 2,2'bipyridine moieties at the meta positions of the phenyl groups. Spectroscopic investigation has shed light on the binding processes and the structure of the dimer in solution in detail. Initial Zn2+ ion binding to the porphyrin hinders the subsequent Zn2+ binding and the binding of the second porphyrin. However, once the dimer is formed, additional Zn2+ binding is notably enhanced, resulting in a pronounced S‐shaped binding curve. The detailed structure of the dimer in solution was established based on UV‐vis spectroscopy, various 1H NMR spectroscopy techniques and mass spectrometry. Simulations of ringcurrent‐induced chemical shift changes provided evidence for the face‐to‐face porphyrin structure with tetrahedral coordination of the bipyridine ligands. The X‐ray single crystal structure provided conclusive evidence for the structure of the face‐to‐face dimer, which remarkably resembles the model structure from the 1H NMR simulation. The stability of the dimer structure makes it promising for further exploration of this motif in developing new supramolecular structures and for their potential applications as host compounds and catalysts.

Tetracene Diacid Aggregates for Directing Energy Flow toward Triplet Pairs.

A comprehensive investigation of the solution-phase photophysics of tetracene bis-carboxylic acid [5,12-tetracenepropiolic acid (Tc-DA)] and its related methyl ester [5,12-tetracenepropynoate (Tc-DE)], a non-hydrogen-bonding counterpart, reveals the role of the carboxylic acid moiety in driving molecular aggregation and concomitant excited-state behavior. Low-concentration solutions of Tc-DA exhibit similar properties to the popular 5,12-bis((triisopropylsilyl)ethynl)tetracene, but as the concentration increases, evidence for aggregates that form excimers and a new mixed-state species with charge-transfer (CT) and correlated triplet pair (TT) character is revealed by transient absorption and fluorescence experiments. Aggregates of Tc-DA evolve further with concentration toward an additional phase that is dominated by the mixed CT/TT state which is the only state present in Tc-DE aggregates and can be modulated with the solvent polarity. Computational modeling finds that cofacial arrangement of Tc-DA and Tc-DE subunits is the most stable aggregate structure and this agrees with results from 1H NMR spectroscopy. The calculated spectra of these cofacial dimers replicate the observed broadening in ground-state absorption as well as accurately predict the formation of a near-UV transition associated with a CT between molecular subunits that is unique to the specific aggregate structure. Taken together, the results suggest that the hydrogen bonding between Tc-DA molecules and the associated disruption of hydrogen bonding with solvent produce a regime of dimer-like behavior, absent in Tc-DE, that favors excimers rather than CT/TT mixed states. The control of aggregate size and structure using distinct functional groups, solute concentration, and solvent in tetracene promises new avenues for its use in light-harvesting schemes.

Facilitating intrinsic delayed fluorescence of conjugated emitters by inter-chromophore interaction.

Delayed fluorescence (DF) is a unique emitting phenomenon of great interest for important applications in organic optoelectronics. In general, DF requires well-separated frontier orbitals, inherently corresponding to charge transfer (CT)-type emitters. However, facilitating intrinsic DF for local excited (LE)-type conjugated emitters remains very challenging. Aiming to overcome this obstacle, we demonstrate a new molecular design strategy with a DF-inactive B,N-multiple resonance (MR) emitter as a model system. Without the necessity of doping with heavy atoms, we synthesized a co-facial dimer in which an excimer-like state (Sexc) was expected to facilitate efficient reverse intersystem crossing (RISC, T1 → Sexc) and intrinsic DF. Benefiting from greatly enhanced SOC and reduced ΔE ST, the proof-of-concept emitter Np-2CzB exhibited k RISC up to 6.5 × 105 s-1 and intrinsic DF with >35% contribution (Φ DF/Φ F) in dilute solution. Further investigation indicated that Sexc state formation relies on an optimized co-facial distance (d = ∼4.7 Å), strong inter-chromophore interaction (J coul > 450 cm-1) and a rigid structure (Γ S1→S0 < 350 cm-1). Although our strategy was demonstrated with a B,N-MR emitter, it can be applicable to many LE-type conjugated emitters without intrinsic DF. By triggering potential DF emission, many classic emitters might play a more important role in optoelectronics.

Crystal structure, host-guest interactions and supramolecular features of a novel μ-hydroxy bridged eclipsed dimer based on indium(III) tetrapyrazinoporphyrazine

The crystal structure of the indium(III) derivative of a chloro-substituted tetrapyrazinoporphyrazine (TPyzPz), which is dimerized through hydroxy bridges is reported. The In(III) ion resides well outside the mean plane of the distorted pyrazinoporphyrazine core and thus each TPyzPz fragment has a domed shape. Two axial hydroxy groups are coordinated to the indium ions of two adjacent In(III) tetrapyrazinoporphyrazine fragments to form a co-facial dimer (InTPyzPz) which are eclipsed in geometry. Each indium(III) is six coordinated and the whole molecule is neutral in charge. Four pyridine molecules are encapsulated between TPyzPz fragments in the dimer, thereby forming a stable host-guest system. The host-guest interactions between the TPyzPz and pyridines are achieved by efficient O-H...N and [Formula: see text]...[Formula: see text] type nonbonding interactions. The packing pattern of the crystal shows that the InTPyzPz system formed a herringbone-type assembly in its network. A combination of multiple Cl...Cl, C-H...O, and C-H...[Formula: see text] and interactions between adjacent TPyzPz have been observed which in turn favored and stabilized this supramolecular network. The dimeric InTPyzPz system is further studied by the Hirshfeld Surfaces analysis which also indicated significant nonbonding interactions in the crystal and the related 2D fingerprint plots provided a quantitative idea of the inter-atomic interactions among the adjacent dimers.

π-Stacks of radical-anionic naphthalenediimides in a metal-organic framework.

Radical-ionic metal-organic frameworks (MOFs) have unique optical, magnetic, and electronic properties. These radical ions, forcibly formed by external stimulus-induced redox processes, are structurally unstable and have short radical lifetimes. Here, we report two naphthalenediimide-based (NDI-based) Ca-MOFs: DGIST-6 and DGIST-7. Neutral DGIST-6, which is generated first during solvothermal synthesis, decomposes and is converted into radical-anionic DGIST-7. Cofacial (NDI)2•- and (NDI)22- dimers are effectively stabilized in DGIST-7 by electron delocalization and spin-pairing as well as dimethylammonium counter cations in their pores. Single-crystal x-ray diffractometry was used to visualize redox-associated structural transformations, such as changes in centroid-to-centroid distance. Moreover, the unusual rapid reduction of oxidized DGIST-7 into the radical anion upon infrared irradiation results in effective and reproducible photothermal conversion. This study successfully illustrated the strategic use of in situ prepared cofacial ligand dimers in MOFs that facilitate the stabilization of radical ions.

Charge transfer excitons in π-stacked thiophene oligomers and P3[Alkyl]T crystals: CIS calculations and electroabsorption spectroscopy.

Poly(3-alkylthiophenes) (P3[Alkyl]T) exhibit high mobility and efficiency of formation of polaronic charge carriers generated by light absorption, thus finding applications in field effect devices. Excited states of π-stacked dimers of tetra-thiophene oligomers (T4), infinite isolated polythiophene (PT) chains, and P3[Alkyl]T crystals are modeled using configuration interaction singles (CIS) calculations. Excited states in cofacial T4 dimers are mostly localized Frenkel states except for two low energy charge transfer (CT) exciton states, which become the ionization potential and electron affinity levels of T4 molecules at large dimer separation. The lowest excited states in infinite, isolated PT chains and P3[Alkyl]T crystals are intra-chain excitons where the electron and hole are localized on the same chain. The next lowest excited states are interchain, CT excitons in which the electron and hole reside on neighboring chains. The former capture almost all optical oscillator strength and the latter may be a route to efficient formation of polaronic charge carriers in P3[Alkyl]T systems. Changes in optical absorption energies of T4 dimers as a function of molecular separation are explained using CIS calculations with four frontier orbitals in the active space. Shifts in optical absorption energy observed on going from isolated chains to P3[Alkyl]T lamellar structures are already present in single-particle transition energies induced by direct π-π interactions at short range. The electroabsorption spectrum of T4 dimers is calculated as a function of dimer separation and states that are responsible for parallel and perpendicular components of the spectrum are identified.

Complete Set of Diketopyrrolopyrrole Centrosymmetrical Cofacial Stacked Pairs.

Stacked centrosymmetrical dimers and simultaneously H-bonded and stacked hexamers of thiophene-substituted diketopyrrolopyrrole (ThDPP) were studied using DFT as models for crystals with slipped-stacked molecules in 1D columns. Eight stacked dimer arrangements were found, six of which are driven by the minimisation of electron repulsion and realised by placing the partially negatively charged atoms of the diketopyrrolopyrrole rings below the centre of an adjacent thiophene ring. Four of these stacks are related to N,N'-dialkylated derivatives of ThDPP found in the literature, while a further one is related to an N,N'-diacylated derivative. An analogous set of eight stacks was discovered computationally for phenyl-substituted DPP (PhDPP), four of which are known among H-bonded DPP pigments, and one more among N,N'-dialkylated PhDPP derivatives. The results shed more light on the mechanisms that drive the formation of stacks between nonaromatic (DPP) and aromatic (Th, Ph) rings. The excitation energies of the lowest four singlet states computed by TD DFT enabled excitonic coupling and energy separation between Frenkel-resonance-type and charge-transfer states to be established, depending on the equilibrium stack geometry.

Synthetic Control of Exciton Dynamics in Bioinspired Cofacial Porphyrin Dimers.

Understanding how the complex interplay among excitonic interactions, vibronic couplings, and reorganization energy determines coherence-enabled transport mechanisms is a grand challenge with both foundational implications and potential payoffs for energy science. We use a combined experimental and theoretical approach to show how a modest change in structure may be used to modify the exciton delocalization, tune electronic and vibrational coherences, and alter the mechanism of exciton transfer in covalently linked cofacial Zn-porphyrin dimers (meso-beta linked ABm-β and meso–meso linked AAm-m). While both ABm-β and AAm-m feature zinc porphyrins linked by a 1,2-phenylene bridge, differences in the interporphyrin connectivity set the lateral shift between macrocycles, reducing electronic coupling in ABm-β and resulting in a localized exciton. Pump–probe experiments show that the exciton dynamics is faster by almost an order of magnitude in the strongly coupled AAm-m dimer, and two-dimensional electronic spectroscopy (2DES) identifies a vibronic coherence that is absent in ABm-β. Theoretical studies indicate how the interchromophore interactions in these structures, and their system-bath couplings, influence excitonic delocalization and vibronic coherence-enabled rapid exciton transport dynamics. Real-time path integral calculations reproduce the exciton transfer kinetics observed experimentally and find that the linking-modulated exciton delocalization strongly enhances the contribution of vibronic coherences to the exciton transfer mechanism, and that this coherence accelerates the exciton transfer dynamics. These benchmark molecular design, 2DES, and theoretical studies provide a foundation for directed explorations of nonclassical effects on exciton dynamics in multiporphyrin assemblies.

HAB79: A new molecular dataset for benchmarking DFT and DFTB electronic couplings against high-level ab initio calculations

A new molecular dataset called HAB79 is introduced to provide ab initio reference values for electronic couplings (transfer integrals) and to benchmark density functional theory (DFT) and density functional tight-binding (DFTB) calculations. The HAB79 dataset is composed of 79 planar heterocyclic polyaromatic hydrocarbon molecules frequently encountered in organic (opto)electronics, arranged to 921 structurally diverse dimer configurations. We show that CASSCF/NEVPT2 with a minimal active space provides a robust reference method that can be applied to the relatively large molecules of the dataset. Electronic couplings are largest for cofacial dimers, in particular, sulfur-containing polyaromatic hydrocarbons, with values in excess of 0.5eV, followed by parallel displaced cofacial dimers. V-shaped dimer motifs, often encountered in the herringbone layers of organic crystals, exhibit medium-sized couplings, whereas T-shaped dimers have the lowest couplings. DFT values obtained from the projector operator-based diabatization (POD) method are initially benchmarked against the smaller databases HAB11 (HAB7-) and found to systematically improve when climbing Jacob's ladder, giving mean relative unsigned errors (MRUEs) of 27.7% (26.3%) for the generalized gradient approximation (GGA) functional BLYP, 20.7% (15.8%) for hybrid functional B3LYP, and 5.2% (7.5%) for the long-range corrected hybrid functional omega-B97X. Cost-effective POD in combination with a GGA functional and very efficient DFTB calculations on the dimers of the HAB79 database give a good linear correlation with the CASSCF/NEVPT2 reference data, which, after scaling with a multiplicative constant, gives reasonably small MRUEs of 17.9% and 40.1%, respectively, bearing in mind that couplings in HAB79 vary over 4 orders of magnitude. The ab initio reference data reported here are expected to be useful for benchmarking other DFT or semi-empirical approaches for electronic coupling calculations.

Outstanding Enhancement in the Axial Coordination Ability of the Highly Rigid Cofacial Cyclic Metalloporphyrin Dimer

AbstractCopper‐ and nickel−porphyrin complexes show extremely weak axial coordination ability without any electron‐withdrawing groups. Herein, we report axial ligation on CuII‐ and NiII−porphyrins in a highly rigid cofacial porphyrin dimer with a bidentate ligand, 1,4‐diazabicyclo[2.2.2]octane (DABCO). To the best of our knowledge, this is the first report on the use of CuII‐ and NiII−porphyrins for coordination‐induced guest binding of porphyrin‐based host molecules without the help of other metal ions. The high rigidity of the dimer induces guest binding through the cooperative effect of weak axial ligation. The results showed that CuII‐ and ZnII‐complexes bind one DABCO molecule inside their cavities, whereas the NiII‐complex binds two additional DABCO molecules outside to form a stable 6‐coordinate paramagnetic NiII‐complexes. The binding constants were determined by the UV/vis titration experiments.

A flexible cofacial Fe porphyrin dimer as an extremely efficient and selective electrocatalyst for the CO2 to CO conversion in non-aqueous and aqueous media

A cofacial iron porphyrin dimer with a flexible structure (f-Fe2PD) showed extremely efficient CO2-to-CO conversion in aqueous and nonaqueous media compared to the one with a rigid structure (r-Fe2PD) and reported catalysts.

Modulation of the optical properties of chiral porphyrin dimers by introducing bridged chiral amide-bonds

The d/l-enantiomers of a series of three Zn(II)tetraarylporphyrin dimers were synthesized and isolated by incorporating a bridging amide-bonded xanthene moiety at the para-position of one of the meso-aryl rings. The electronic structures and optical properties were modulated by incorporating chiral amino acid moieties into the amide-bonding moieties of the xanthene bridge that contain methyl, tolyl and 2-methylindole substituents. A cofacial dimer was formed in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) resulting in a significant red shift of the B band, due to a relative destabilization of the HOMO, which has large MO coefficients on the pyrrole nitrogens. The sign sequences observed in the B band region of the CD spectra due to the presence of the chiral amino acid moieties were modified due to this change in geometry. Significant CD intensity is also observed in the B band region of the CD spectra of anion radical species during in situ spectroelectrochemical measurements.

Control over Kinetic and Thermodynamically Driven Pathways of Crystallization to Yield Cofacial and Slipped-Stack Dimers in Single Crystals.

Control over the molecular packing in the solid state is of utmost importance in regulating the bulk optical properties of organic semiconductors. The electronic coupling between the molecules makes it possible to improve the properties of the bulk materials. This work reports an example of control over the selective formation of polymorphic single crystals of donor-acceptor-type small-molecule compound 25TR by 1) kinetic or 2) thermodynamic course of crystallisation to yield slipped stack (S) and cofacial (C) dimers in the single crystals. The distinct optical characteristics of the C-dimer and S-dimer are summarised. Both forms show significant excitonic interactions in the solid state, and the S-dimeric form has strong yellowish orange fluorescence, whereas the C-dimeric form is non-fluorescent in the crystalline state. DFT calculations and differential scanning calorimetric experiments revealed that the C-dimer polymorph is the thermodynamically stable form with a free energy offset of 0.43 eV in comparison with the S-dimer. Interestingly, the thermodynamically driven non-fluorescent single crystal was found to be convertible to its fluorescent form irreversibly by thermal trigger. The charge-carrier-transport characteristics of these two polymorphs were computed by using the Marcus-Hush formalism. The computations of the charge-carrier-transport behaviour revealed that the S-dimer (25TR(R) ) is ambipolar, whereas the C-dimer (25TR(Y) ) is predominantly n-type.

Synthesis of Double-Bridged Cofacial Nickel Porphyrin Dimers with 2,2'-Bipyridyl Pillars and Their Restricted Coordination Space.

Double-bridged cofacial Ni porphyrin dimers 2 with 2,2'-bipyridyl pillars were effectively prepared by a one-step reductive homocoupling reaction of bis(chloropyridyl)-substituted Ni porphyrin derivatives followed by a specific separation of a cyanopropyl-modified silica gel column using pyridine eluent systems. The structural analyses of 2 and its Pd complex were carried out in their solid and solution states by means of X-ray single crystal analysis and NMR, respectively. The complexation of η3-allylpalladium chloride (Pd) with 2 on the spatially restricted 2,2-bipyridine moieties on 2 gave a 2:1 (Pd:2) complex, in which the 2,2'-bipyridine ligands only provided one of the N atoms on a 2,2'-bipyridine ligand to a Pd. Therefore, the 2,2-bipyridine moieties acted as a monodentate ligand.

5,8-Disubstituted crown-naphthalonitriles as a platform for highly soluble naphthalocyanines

The approach toward synthesis of 5,8-disubstituted naphthalonitriles as the universal platform for the range of novel functionalized highly soluble and nonaggregating naphthalocyanines was developed. The selective bromination of 15-crown-5-naphthalonitrile was described, that can pave way to wide application of transition-metal catalyzed cross-coupling reactions for the synthesis of functionalized naphthalonitriles and naphthalocyanines based on 6,7-dialkoxy-substituted precursors. Copper(II) complexes with heptynyl- and heptyl-substituted tetra-15-crown-5-naphthalocyaninates (2Cu and 3Cu) were synthesized from corresponding 5,8-disubstituted naphthalonitriles. It was shown that introduction of substituents into these positions efficiently suppresses aggregation of naphthalocyanines – the studied complexes were highly soluble in organic solvents and stayed in monomeric form in solutions up to 10−3 M according to UV–Vis and ESR studies. In contrast to previously described tetra-15-crown-5-naphthalocyanines 1M (M = Zn(II), Mg(II), 2H), complex 2Cu form cation-induced cofacial supramolecular dimer only in the presence of excess of potassium cations, while complex 3Cu didn't form supramolecular cofacial dimer. In addition the studied complexes demonstrate high photostability in comparison with Mg(II) and Zn(II) derivatives. Notably, introduction of heptynyl-substituents into 5,8-positions of naphthalocyanines did not lead to significant shift of the Q-band in UV–Vis spectrum, which was explained by quantum-chemical calculations using simplified TD-DFT approach, firstly applied herein to naphthalocyanine systems.

Photoinduced electron transfer from zinc meso-tetraphenylporphyrin to a one-dimensional perylenediimide aggregate: Probing anion delocalization effects

Organic photovoltaics incorporating non-fullerene acceptors based on perylenediimide (PDI) now rival fullerene acceptor-based devices in performance, although the mechanisms of charge generation in PDI-based devices are not yet fully understood. Fullerene-based systems are proposed to undergo electron transfer directly from the photoexcited donor into a band of delocalized acceptor states, thus increasing charge generation efficiency. Similarly, anion delocalization has been shown to enhance the rate of electron transfer from a photoexcited donor to two electronically coupled PDI acceptors. Here we investigate how additional electron acceptors may further increase the rate of electron transfer from the donor zinc meso-tetraphenylporphyrin (ZnTPP) to an aggregate of PDI acceptors (PDI[Formula: see text]. Femtosecond transient visible and mid-infrared absorption spectroscopies show that the rate of electron transfer from 1*ZnTPP to the PDI assembly ZnTPP2-PDI3 is statistically identical to that of the previously examined ZnTPP-PDI2. A Marcus theory analysis indicates that the parameters governing electron transfer are nearly identical for the two molecules, suggesting that the maximum electron transfer rate enhancement has been achieved in a cofacial PDI dimer because the ZnTPP directly couples to the first two PDI acceptors whereas the coupling to the third PDI is too weak.

Coordination Chemistry of Anticrowns. Synthesis and Structures of Complexes of the Perfluorinated Three‐Mercury Anticrown (o‐C6F4Hg)3 with Monohalogenobenzenes

The paper reports the reactions of the perfluorinated three‐mercury anticrown (o‐C6F4Hg)3 (1) with monohalogenobenzenes. It has been found that the interaction of 1 with fluorobenzene gives two 1:1 complexes 2 and 3, differing in their structures. Complex 2 forms in the crystal extended binary stacks wherein the molecules of 1 alternate with the halogenobenzene species, while complex 3 consists of the cofacial dimers formed from the corresponding monomeric units {[(o‐C6F4Hg)3](C6H5F)}. Each fluorobenzene molecule in both adducts is bonded to 1 by its phenyl ring through secondary π‐Hg–C interactions, whereas the fluorine atom is coordinated with one of the Hg sites of 1. Analogous types of bonding are realized in the chlorobenzene and bromobenzene complexes, {[(o‐C6F4Hg)3]2(C6H5X)4}, (X = Cl, Br), containing four halogenobenzene molecules per two molecules of 1. The complexes include two monomeric {[(o‐C6F4Hg)3](C6H5X)2} fragments connected with one another by two bridging η1 coordinated halogen atoms of the halogenobenzene ligands. The halogen atoms of two other halogenobenzene species are η3 coordinated each with the Hg centers of the neighboring molecule of 1. The reaction of iodobenzene with 1 affords a 3:1 complex, {[(o‐C6F4Hg)3](C6H5I)3}, which is unstable and readily transformed into the corresponding 1:1 adduct. Both compounds form in the crystal extended stacks wherein each sandwiched iodobenzene molecule is coordinated via the iodine atom with the adjacent molecule(s) of 1.

Through‐Space Spin Coupling in a Silver(II) Porphyrin Dimer upon Stepwise Oxidations: AgII⋅⋅⋅AgII, AgII⋅⋅⋅AgIII, and AgIII⋅⋅⋅AgIII Metallophilic Interactions

AbstractInvited for the cover of this issue are Sankar Prasad Rath and co‐workers at the Indian Institute of Technology (IIT Kanpur) and the University of Sassari. The image depicts through space interactions in a cofacial AgII porphyrin dimer upon stepwise oxidations, which allow the estimation of the structure and energy characteristics of metallophilic interactions between AgII/AgII (d9/d9), AgIII/AgIII (d8/d8) and also mixed‐valent AgII/AgIII (d9/d8) complexes. Read the full text of the article at 10.1002/chem.201901731.

Phosphorescent Molecules That Resist Concentration Quenching in the Solution State: Concentration-Driven Emission Enhancement of Vaulted trans-Bis[2-(iminomethyl)imidazolato]platinum(II) Complexes.

In this paper, we describe the first phosphorescent molecules that do not exhibit the concentration quenching in the homogeneous solution state throughout the entire range of concentrations. A series of newly designed polymethylene-vaulted trans-bis[2-(iminomethyl)imidazolato]platinum(II) complexes (1a, n = 10; 1b, n = 12; 1c, n = 14) was prepared by treating [PtCl2(CH3CN)2] with the corresponding N,N'-bis[(1H-imidazol-2-yl)methylene]-1,ω-alkanediamines. The trans coordination and vaulted structures of 1 have been unequivocally established from X-ray diffraction studies. When the concentration of a clear homogeneous solution of 1a-c in organic solvents increases from the diluted to the saturated state, the emission intensity and quantum efficiency increase continuously without concentration quenching at ambient temperature. This is in contrast to the emission profiles of other analogues 2-4 and typical AIEgens, which show ordinary concentration quenching under the same measurement conditions. The present concentration-driven emission enhancement is observed more intensely in a solution of a racemic mixture of 1 in comparison to that of the optically pure solution. Kinetic studies, 1H NMR, XRD analyses, and DFT calculations revealed that this specifically intense emission enhancement of 1 is attributed to an increase in the contribution of a 3MMLCT to 1GS transition, which is caused by the specific ability for the formation of a cofacial association dimer of 1.