Solid-State Photochromism by Molecular Assembly of Bis-o-carboranyl Siloles.

A new type of solid-state photochromism was observed in an AB2 -type molecular assembly comprising a central silole and two peripheral o-carborane units, and in this assembly, depending on the assembling positions of those units at the adjoining benzene ring, two different regioisomers were formed: Si-m-Cb and Si-p-Cb. Each isomer showed different solid-state photochromism depending on its solid-state molecular conformation and was either in the crystalline or amorphous state. The crystals of each meta- or para-isomer, CSi-m-Cb or CSi-p-Cb , showed yellow or blue emission, and mechanically grinding those crystals into amorphous powders of ASi-m-Cb and ASi-p-Cb , switched their emissions to blue and yellow, respectively. Photophysical studies revealed that the electronic interaction between silole and o-carborane units determined the emission color. The crystal and DFT-optimized structures each account for the crystalline and amorphous structures, respectively, and are correlated well with the electronic interactions in the molecular assembly in the solid state, thus enabling the prediction of the solid-state molecular conformational change.

Photoinduced electron and hole transfers in carbazole dendrimers with heteroleptic Ir-complex cores.

We synthesised carbazole (Cz) dendrimers with heteroleptic Ir-complex cores. Upon excitation of the carbazole (Cz) dendrons, the phosphorescence of the core Ir(iii) complex was quenched due to the photoinduced electron transfer (PET) process. The PET dynamics of the excited Cz-dendrons were investigated using the femtosecond time-resolved transient absorption technique. A broad transient absorption (TA) band attributed to the S1-Sn transition of the 1Cz*-dendron was observed at around 630 nm in the first generation Cz-dendrimer (G1). This TA band in the second-generation dendrimer (G2) decayed with a longer lifetime of 55.5 ps compared to that of G1 (9.8 ps), because G2 has a larger distance between the Cz-dendron and Ir-complex core than that of G1. The decay time of the free 1Cz*-dendron was 6.3 ns, and thus, the reduced decay time in Gn corresponds to the PET dynamics. As a result of the PET process, the Cz cationic radical species (Cz˙+) was observed at around 780 nm. Interestingly, when the core Ir-complex in the dendrimer was excited with a 400 nm pulse selectively, the TA band of Cz˙+ was also detected at around 780 nm. This may be due to the photoinduced hole transfer (PHT) from the highest occupied molecular orbital (HOMO) energy state of Cz to the lowest singly occupied molecular orbital (LSOMO) energy state of the excited Ir-complex. The oxidation potential of Cz is lower than that of the Ir-complex, indicating that the HOMO of the Cz-dendron is located at a higher energy state than that of the Ir-complex. To investigate the relative order of the energy states and their orbital shapes, we performed theoretical calculations using density functional theory. The TA spectra were globally deconvoluted to generate the decay-associated spectra (DAS), from which the species-associated spectra (SAS) were calculated. The SAS can distinguish the individual intermediate species participating in the PET and PHT processes. The analysed rate constants of SAS were consistent with the results determined by the TA decays.

Excitation spectroscopic and synchronous fluorescence spectroscopic analysis of the origin of aggregation-induced emission in N,N-diphenyl-1-naphthylamine-o-carborane derivatives.

We have synthesised mono-(NpCb) and bis-[(N,N-phenyl-1-naphthylamino)benzo]-o-caboranes (NpCbNp), which show anomalously intense aggregation-induced emission (AIE) at long wavelengths and monomer emission at short wavelengths. The actual concentration of the aggregator in intense AIE is very low, so absorption spectroscopy is unsuitable for detecting small changes in the absorbance. Hence, to understand the aggregation pattern, we employ excitation spectroscopy, since this method has excellent sensitivity in compliance with the emission intensity. Moreover, we carried out synchronous fluorescence spectroscopic measurements to confirm that the aggregator is different from the monomeric species. The excitation spectrum shows distinguishable differences between the AIE and the normal emission. For the triad NpCbNp, the excitation spectrum for the AIE is located at a shorter wavelength than that for the monomeric emission spectrum, which means that the AIE is attributed to the H-type aggregator. On the other hand, for the dyad NpCb, the excitation spectrum for the AIE is observed at an identical wavelength as that for the monomeric species, which indicates that the aggregator is of the oblique type.

Photoinduced Electron Transfer in a BODIPY- ortho-Carborane Dyad Investigated by Time-Resolved Transient Absorption Spectroscopy.

We report the results of photoinduced electron transfer (PET) in a novel dyad, in which a boron dipyrromethene (BODIPY) dye is covalently linked to o-carborane ( o-Cb). In this dyad, BODIPY and o-Cb act as electron donor and acceptor, respectively. PET dynamics were investigated using a femtosecond time-resolved transient absorption spectroscopic method. The free energy dependence of PET in the S1 and S2 states was examined on the basis of Marcus theory. PET in the S1 state occurs in the Marcus normal region. Rates are strongly influenced by the driving force (-Δ G), which is controlled by solvent polarity; thus, PET in the S1 state is faster in polar solvents than in nonpolar ones. However, PET does not occur from the higher energy S2 state despite large endothermic Δ G values, because deactivation via internal conversion is much faster than PET.

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)-Platinum(II) and Iridium(III)-Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine.

We investigated the electrochemical and excited-state properties of 2,3-bis(2-pyridyl)pyrazine (dpp)-bridged bimetallic complexes, (L)2Ir-dpp-PtCl [1, L = 2-(4',6'-difluorophenyl)pyridinato-N,C2 (dfppy); 2, L = 2-phenylpyridinato-N,C2 (ppy)] and [(L)2Ir]2(dpp) [3, L = dfppy; 4, L = ppy] compared to monometallic complexes, (L)2Ir-dpp (5, L = dfppy; 6, L = ppy) and dpp-PtCl (dpp-PtIICl2; 7). The single-crystal X-ray crystallographic structures of 1, 3, 5, and 6 showed that 1 and 3 have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in 5 and 6 is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir-Pt and Ir-Ir bimetallic complexes (1-4) could be explained by assuming the involvement of crossing to platinum- and iridium-based d-d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d-d states from the emissive triplet metal-to-ligand charge-transfer (3MLCT) state, followed by cleavage of the dpp-Pt and (L)2Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based 3MLCT to the Ir d-d or Pt d-d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d-d or Pt d-d) with a significant dependency on the relative 3MLCT levels of the (L)2Ir-dpp component.

Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO2 by Re(I)-Complex-Anchored TiO2 Hybrid Catalyst.

A series of cationic Ir(III) complexes ([Ir(btp)2(bpy-X2)]+ (Ir-X+: btp = (2-pyridyl)benzo[b]thiophen-3-yl; bpy-X2 = 4,4'-X2-2,2'-bipyridine (X = OMe, tBu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO2 reduction to CO using a hybrid catalyst (TiO2/ReP) prepared by anchoring of Re(4,4'-Y2-bpy)(CO)3Cl (ReP; Y = CH2PO(OH)2) on TiO2 particles. Irradiation of a solution containing Ir-X+, TiO2/ReP particles, and an electron donor (1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction of CO2 to CO with efficiencies in the order X = OMe > tBu ≈ Me > H; Ir-CN+ has no photosensitization effect. A notable observation is that Ir-tBu+ and Ir-Me+ are less efficient than Ir-OMe+ at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the Ir-X+ sensitizers are commonly superior compared to Ru(bpy)32+, a widely used transition-metal photosensitizer, and (2) the system comprising Ir-OMe+ and TiO2/ReP is much more efficient than a homogeneous-solution system using Ir-OMe+ and Re(4,4'-Y'2-bpy)(CO)3Cl (Y' = CH2PO(OEt)2). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail.

The effect of interligand energy transfer on the emission spectra of heteroleptic Ir complexes.

In order to understand the causes of the emission shape and colour changes of heteroleptic Ir3+ complexes containing 2-(2,4-difluorophenyl)pyridine (dfppy) as the main ligands, we introduced two types of ancillary ligands: (1) non-luminescent ancillary ligands, namely tetrakis(pyrazolyl)borate (bor) and picolinate (pic), which were employed for the preparation of Ir(dfppy)2(bor) and Ir(dfppy)2(pic), respectively, and (2) luminescent ancillary ligands, namely 1,10-phenanthroline (phen), bipyridine (bpy), and 2,3-dipyridylpyrazine (dpp), which were employed for the preparation of Ir(dfppy)2(phen), Ir(dfppy)2(bpy), and Ir(dfppy)2(dpp), respectively. In a glassy matrix at 77 K, the Ir complexes showed well-structured emission spectra, except Ir(dfppy)2(dpp). The vibronic structures in the emission spectra of Ir(dfppy)2(bor) and Ir(dfppy)2(pic) were maintained even at 300 K. However, Ir(dfppy)2(phen), Ir(dfppy)2(bpy), and Ir(dfppy)2(dpp) showed markedly red-shifted and broad emission spectra. The anomalous rigidochromism was attributed to an interligand energy transfer (ILET), and showed a strong temperature dependence. The excited states of dfppy are higher than those of phen, bpy, and dpp; thus, ILET occurs from dfppy to the other ligands lying in lower energy states. The ILET dynamics were probed directly using femtosecond transient absorption (TA) spectroscopy after the excitation of dfppy. As the time delay increased, the intensity of the TA band of dfppy decreased, while those of the bands related to the phen, bpy, and dpp ancillary ligands increased. On the other hand, no changes in the TA spectra were observed for Ir(dfppy)2(bor) and Ir(dfppy)2(pic). The TA spectral behaviours can be explained in terms of the relative ordering of the emissive states for cyclometalating and ancillary ligands.

Time-resolved spectroscopic analysis of the light-energy harvesting mechanism in carbazole-dendrimers with a blue-phosphorescent Ir-complex core.

In order to investigate the light-energy harvesting mechanism, a series of dendrimers with a heteroleptic iridium(iii) complex core, [Ir(dmb)2(pic-Czn)] (Gn: n = 1, 2, and 3), with 2,6-difluoro-3-(4-methylpyridin-2-yl)benzonitrile (dmb) as the cyclometallating ligand and 3-hydroxypicolinate (pic) as the ancillary ligand, connected to carbazole-based dendrons (Czn: n = 2, 4, and 8) was synthesized. The Ir centred complex [Ir(dmb)2(pic-OCH3), G0] shows a blue emission at <500 nm, which is assigned to metal-to-ligand charge-transfer (3MLCT) phosphorescence. This phosphorescence was enhanced with increasing generations due to the increase in the total absorbance of the Cz-dendron. The light-harvesting efficiencies determined by various methods were approximately 160 (G1), 220 (G2), and 330% (G3). The energy transfer efficiencies for G1-G3 from the peripheral Cz-dendron to the Ir-core complex were above 97%, determined using the reduction in the lifetime of the excited 1Cz*-dendrons. G1-G3 showed a transient absorption (TA) band at 600 nm, which was attributed to the Sn ← S1 transition of the Cz-dendrons. The fast decay of these TA bands was consistent with the fast emission decay times. The time-resolved TA band correlated with the core Ir-complex was observed at 500 nm, though it overlapped and interfered with the intense TA band of the Cz-dendrons. Therefore, we attempted a global analysis by singular value decomposition (SVD). The determination of the absorption spectra of the individual species participating in the energy transfer process by SVD analysis can distinguish between different mechanistic models. The analysed rate constants were consistent with the results determined by the emission decays.

A spectroscopic study on the satellite vibronic band in phosphorescent Pt-complexes with high colour purity.

To understand the relationship between the narrowing of an emission band and structural changes, we synthesised tetradentate Pt-complexes. Pt-1 has two directly connected carbazole (Cz) moieties, Pt-2 has two additional methyl groups to Pt-1, and Pt-3 has one Cz moiety. The absorption and emission spectra of Pt-2 were identical to those of Pt-1. Pt-3's emission was observed at a shorter wavelength compared to the others. We achieved phosphorescence with high colour purity by introducing a tetradentate ligand. All the Pt-complexes showed a vibronic structure in the emission spectra measured at 77 and 300 K. The 0-0 vibronic band of the Pt-complexes is quite intense compared to the 0-1 vibronic band, which may be due to less structural change of the fused tetradentate ligand in the excited state relative to the ground state. The spacing of the 0-0 and 1-0 vibronic bands is 1487 and 1323 cm-1, respectively. To understand the origin of the satellite vibronic bands, we carried out vibrational spectroscopic (IR and Raman) measurements and theoretical calculations to analyse the infrared spectrum. In addition, we carried out a transient Raman experiment to obtain the vibronic information of an excited Pt-1. The vibronic spacing in the emission was caused by the displacement of the potential energy curve in the excited state. The highest occupied molecular orbital is populated with a Cz moiety and the lowest unoccupied molecular orbital is localized at the terminal pyridine moiety. For the triplet state, however, the highest singly occupied molecular orbital is delocalized on the pyrazole or imidazole moiety, as well as the pyridine moiety. These groups are located at the terminal site of the ligand, and are less rigidified and more flexible. Therefore, the major origin of the satellite vibration band in emission spectra is the stretching of the terminal groups.

Steric effect on excimer formation in planar Pt(ii) complexes.

In order to understand the steric influence on excimer formation in square planar metal complexes, three different Pt(ii) complexes were prepared by modifying the substituents in the main ligand: Pt(ii)(dfppy)(acac) (Pt-1, where dfppy is difluorophenylpyridine, acac is acetylacetonate); the bulky triphenyl silyl (Ph3Si-) group was substituted at the pyridine moiety (Pt-2) and at the phenyl moiety (Pt-3) of the main ligand of Pt-1. The Pt-complexes showed sky-blue emission at ∼460 nm. In addition, Pt-1 and Pt-3 showed excimer emission at ∼600 nm in the concentrated solution and the solid sample. The emission lifetimes and intensities for monomeric Pt-1 and Pt-3 showed strong concentration dependence. Indeed, the lifetime of the monomer was reduced in highly concentrated solutions due to excimer formation. The intrinsic emission lifetimes were determined as 364 ns (Pt-1) and 300 ns (Pt-3) by Stern-Volmer analysis, considering the self-quenched lifetime of monomer emission. Pt-2 did not show any excimer emission in the concentrated solution or solid sample. The crystal structures of Pt-1 and Pt-3 were analysed by X-ray crystallographic measurements. The results revealed that the LUMO moiety closely overlapped with that of another Pt-complex. In this study, based on the influence of steric hindrance of the bulky Ph3Si group, we concluded that the LUMO-LUMO interaction between the pyridine moieties of the main ligand is the main factor responsible for excimer formation.

Direct observation of the photoinduced electron transfer processes of bis(4-arylphenylamino benzo)-ortho-carborane using transient absorption spectroscopic measurements.

The intramolecular photoinduced electron transfer (PET) processes of three bis(4-arylphenylamino benzo)-ortho-carboranes (ArCbAr, Ar = phenyl (Ph), naphthyl (Np) and pyrenyl (Py)) triads were investigated in CH2Cl2 and n-hexane using the femtosecond time-resolved transient absorption (TA) spectroscopic technique. In CH2Cl2, the transient S1-Sn absorption band of 1ArCbAr* was observed at short delay times. Concomitant with the decay of 1ArCbAr*, the TA bands for cation radical species were detected at around 700 nm for Ph˙+, 650 nm for Np˙+, and 580 nm Py˙+. At the same time, the TA band of the carborane anion radical (Cb˙-) was observed at around 430 nm. This implies that the intramolecular PET process occurs from the 1ArCbAr* state. The TA bands of the cationic and anionic radical species can be assigned by comparison to the absorption spectra of an electrochemically generated radical species. The PET process also occurs in n-hexane, as demonstrated by the monitoring of radical species in the TA spectra. In n-hexane, the TA band for Ar˙+ interfered with the aggregation induced emission (AIE) at early delay times. The long lived Ar˙+ species can be observed in the TA spectra at long delay times after fading of the AIE. The PET is exergonic in both solvents, as shown by the negative Gibbs energies (ΔG) for the PET processes.

Stable Blue Phosphorescence Iridium(III) Cyclometalated Complexes Prompted by Intramolecular Hydrogen Bond in Ancillary Ligand.

Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir(III) complexes is designed and prepared: Ir(dfppy)2(pic-OH) (1a), Ir(dfppy)2(pic-OMe) (1b), Ir(ppy)2(pic-OH) (2a), and Ir(ppy)2(pic-OMe) (2b). The emission lifetime of Ir(dfppy)2(pic-OH) (1a) (τem = 3.19 µs) in dichloromethane solution was found to be significantly longer than that of Ir(dfppy)2(pic-OMe) (1b) (τem = 0.94 µs), because of a substantial difference in the nonradiative decay rate (knr = 0.28 × 10(5) s(-1) for (1a) vs 2.99 × 10(5) s(-1) for (1b)). These results were attributed to the intramolecular OH···O═C hydrogen bond of the 3-hydroxy-picolinato ligand. Finally, device lifetime was significantly improved when 1a was used as the dopant compared to FIrpic, a well-known blue dopant. Device III (1a as dopant) achieved an operational lifetime of 34.3 h for an initial luminance of 400 nits compared to that of device IV (FIrpic as dopant), a value of 20.1 h, indicating that the intramolecular hydrogen bond in ancillary ligand is playing an important role in device stability.

The influence of π-conjugation on competitive pathways: charge transfer or electron transfer in new D-π-A and D-π-Si-π-A dyads.

In order to elucidate the influence of π-conjugation on photoinduced electron transfer (PET) and intramolecular charge transfer processes, donor-π-acceptor dyads (D-π-A (1) and D-π-Si-π-A (2)) were newly synthesized. In these dyads, carbazole and triazine moieties acted as the electron donor and acceptor, respectively. The fluorescence of dyad 1 red-shifted as the solvent polarity was increased. The electron charge distribution of the excited state of dyad 1 was delocalized in the acceptor moiety, forming the charge transfer D(δ+)-π-A(δ-) dyad in the excited state. In the excited state of dyad 1, the π-conjugation acted as the linker for charge transfer between the donor and acceptor moieties. A large dipole moment change (Δµ = 45.6 D) between the ground and excited states was determined using the Lippert-Mataga plot. Furthermore, the fluorescence of dyad 1 was observed upon two-photon excitation. In contrast, dyad 2, in which the π-conjugation is disconnected by a Si-atom in the linker, displayed weak dual-emission: a short-wavelength emission at around 350 nm arising from the monomeric species and a long-wavelength one assigned to the emission from an intramolecular exciplex between the donor and acceptor moieties. The weak emission of dyad 2 indicates that the D(+)˙-π-Si-π-A(-)˙ species was generated through a PET process in the excited state. The cationic radical species of the carbazole and the anionic radical species of the triazine, which show transient absorption (TA) bands at around 780 and 530 nm, respectively, were characterized using the femtosecond TA method.

Influence of π-conjugation structural changes on intramolecular charge transfer and photoinduced electron transfer in donor-π-acceptor dyads.

The influence of π-conjugation structural changes on photoinduced electron transfer (PET) and intramolecular charge transfer (ICT) processes in π-conjugated donor (D)-acceptor (A) dyads (D-π-A) was investigated. Three types of D-π-A dyads were prepared through the modification of the structure of their π-conjugated linker, including D-π-A (1) and D-πtw-A (2) having a twisted π-conjugation, and D-π-Si-π-A (3) with a π-conjugation severed by a Si-atom. In these dyads, carbazole (Cz) and oxadiazole (Oz) moieties act as an electron donor and acceptor, respectively. The emission maxima of dyads 1 and 3 red-shifted with the increase in polarity, which could be attributed to the ICT process. The fluorescence lifetimes of dyads 1 and 3 were 2.64 and 4.29 ns in CH2Cl2, respectively. In contrast, dyad 2 showed dual emission at 350 and 470 nm in CH2Cl2. The emission of dyad 2 at 380 nm corresponded to the monomer fluorescence in the locally excited state. Moreover, the emission at 470 nm increased simultaneously with the diminishing of the fluorescence at 380 nm. This emission band can be assigned as the intramolecular exciplex emission, and showed a strong solvatochromic shift. The low emission quantum yield (<3%) of dyad 2 is due to the PET process. In dyad 2, the cationic and anionic radical species generated by the PET process were confirmed by femtosecond transient absorption (fs-TA) spectroscopy. Upon photoexcitation at 290 or 340 nm, the A or D moieties can be selectively excited. Upon excitation at 290 nm, the acceptor moiety can be excited to the 1A* state, thus the photoinduced hole transfer (PHT) takes place from 1A* to D through the HOMO levels within a few picoseconds. On the other hand, when the donor moiety is excited at 340 nm, the PET process occurs from 1D* to A. Based on the fs-TA studies, it was found that the dynamics and mechanisms for the electron (or charge) transfer were strongly affected by the variation of the π-conjugation of the linker. Herein, we can conclude that the PET and ICT processes are strongly influenced by the π-conjugation properties and their mechanisms are also affected by whether selective excitation of the donor or acceptor moiety occurs. Moreover, unit electron transfers (PET or PHT) were observed dominantly in the dyads having severed/twisted linkers in π-conjugation. However, dyad 1 possessing a well-conjugated linker showed a partial charge transfer character.

Ligand-to-ligand charge transfer in heteroleptic Ir-complexes: comprehensive investigations of its fast dynamics and mechanism.

To gain new insights into ligand-to-ligand charge-transfer (LLCT) dynamics, we synthesised two heteroleptic Ir(3+) complexes: (Ir(dfppy)2(tpphz)) and (Ir(dfppy)2(dpq)), where dfppy, tpphz, and dpq are 2-(4,6-difluorophenyl)pyridine, tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, and 2,3-bis-(2-pyridyl)-quinoxaline, respectively. The tpphz and dpq ligands have longer π-conjugation than dfppy. Therefore, the excited ligand-centred (LC) state and the metal-to-ligand charge transfer (MLCT) state of dfppy are higher than those of tpphz and dpq. The LLCT dynamics from dfppy to tpphz (or dpq) was probed using femtoscond transient absorption (TA) spectroscopy after the selective excitation of dfppy. The TA band for the LC/MLCT state of dfppy is observed at 480 nm. Because of the LLCT process, the TA bands related to the MLCT states of tpphz and dpq ligands increased, whereas those of dfppy decreased. The time constants for the LLCT process were 17 ps for Ir(dfppy)2(tpphz) and 5 ps for Ir(dfppy)2(dpq). The MLCT emission of Ir(dfppy)2(tpphz) showed strong temperature dependence, indicating that the LLCT process has a significant energy barrier. In comparison, the temperature weakly influenced the emission of Ir(dfppy)2(dpq), and thus, its LLCT process may have a smaller barrier. The anomalous rigidochromism and photodynamic behaviours can be explained in terms of the barrier between cyclometalating and ancillary ligands.

Aggregation-induced emission of diarylamino-π-carborane triads: effects of charge transfer and π-conjugation.

Carborane-based donor-π-acceptor triads (D-π-A-π-D) bearing triarylamine moieties were synthesised. All the monomeric triads showed a blue-green emission in a dilute solution, which was assigned as an intramolecular charge-transfer (CT) emission. The intramolecular CT emission showed large Stokes shifts at a higher solvent polarity. The intramolecular CT emission further shifted to a longer wavelength with the increase in π-conjugation. Interestingly, a strong red emission was observed in highly concentrated solutions or in the solid state, which was assigned as an aggregation-induced emission (AIE). Moreover, the AIE strongly depended on solvent polarity. A large Stokes shift in AIE was attributed to the strong CT character. The changes in the dipole moment for the AIE state and monomer emission were evaluated using the Lippert-Mataga relationship. The density functional theory calculations showed that the change in electron distribution between the aryl amino group (highest occupied molecular orbital, HOMO) and the carborane moiety (lowest unoccupied molecular orbital, LUMO) indicates the intramolecular CT character, and the emission colour changes were attributed to the HOMO-LUMO energy gap controlled by the π-extension of the phenylene linker. The electrochemical properties such as oxidation and reduction potentials were consistent with theoretical calculation results. The emission properties were affected by two main factors: solvent polarity and solubility.

Electronic Alteration on Oligothiophenes by o-Carborane: Electron Acceptor Character of o-Carborane in Oligothiophene Frameworks with Dicyano-Vinyl End-On Group.

We studied electronic change in oligothiophenes by employing o-carborane into a molecular array in which one or both end(s) were substituted by electron-withdrawing dicyano-vinyl group(s). Depending on mono- or bis-substitution at the o-carborane, a series of linear A1-D-A2 (1a-1c) or V-shaped A1-D-A2-D-A1 (2a-2c) oligothiophene chain structures of variable length were prepared; A1, D, and A2, represent dicyano-vinyl, oligothiophenyl, and o-carboranyl groups, respectively. Among this series, 2a shows strong electron-acceptor capability of o-carborane comparable to that of the dicyano-vinyl substituent, which can be elaborated by a conformational effect driven by cage σ*-π* interaction. As a result, electronic communications between o-carborane and dicyano-vinyl groups are successfully achieved in 2a.

Intriguing emission properties of triphenylamine-carborane systems.

Electron donor-acceptor (D-A) systems with a triphenylamino moiety (D) and ortho-carborane (A) show three kinds of intriguing emissions that can be attributed to the local excited state, the intramolecular charge-transfer state, and the aggregation-induced emission state. The emission behaviors depend on which positions of the carborane are substituted.

Carborane dyads for photoinduced electron transfer: photophysical studies on carbazole and phenyl-o-carborane molecular assemblies.

o-Carborane-based donor-acceptor dyads comprising an o-carboranyl phenyl unit combined with N-carbazole (1) or 4-phenyl-N-carbazole (2) were prepared, and their dyad characters were confirmed by steady-state photochemistry and photodynamic experiments as well as electrochemical studies. The absorption and electrochemical properties of the dyads were essentially the sum of those of the carbazole and o-carboranyl phenyl units; this indicates negligible interaction between the carbazole and o-carborane units in the ground state. However, the emission spectra of 1 and 2 indicated that carbazole fluorescence was effectively quenched and a new charge-transfer (CT) emission was observed in solvents, varying from hexane to acetonitrile, which exhibited large Stoke shifts. The CT emission properties of o-carborane-based dyads were further analyzed by using Lippert-Mataga plots to show that unit charge separation occurred to form a charge-separated species in the excited state, namely, 1⋅2. This excited-state species was confirmed by nanosecond transient absorption spectra and spectroelectrochemical measurements; the photoexcitation of carbazole generated the CT state in which a radical cation and anion were formed at the carbazole and o-carborane units, respectively, within a few nanoseconds. DFT calculations corroborated the presence of this CT species and showed localized populations of the highest singly occupied molecular orbital on 2 in the reduced anionic state. As a result, molecular assemblies formed by linking the carbazole group with the o-carborane cage through a phenylene or multi-phenylene spacer revealed that the photoinduced electron-transfer process occurred intramolecularly.

Efficient light harvesting and energy transfer in a red phosphorescent iridium dendrimer.

A series of red phosphorescent iridium dendrimers of the type [Ir(btp)2(pic-PCn)] (Ir-Gn; n = 0, 1, 2, and 3) with two 2-(benzo[b]thiophen-2-yl)pyridines (btp) and 3-hydroxypicolinate (pic) as the cyclometalating and ancillary ligands were prepared in good yields. Dendritic generation was grown at the 3 position of the pic ligand with 4-(9H-carbazolyl)phenyl dendrons connected to 3,5-bis(methyleneoxy)benzyloxy branches (PCn; n = 0, 2, 4, and 8). The harvesting photons on the PCn dendrons followed by efficient energy transfer to the iridium center resulted in high red emissions at ∼600 nm by metal-to-ligand charge transfer. The intensity of the phosphorescence gradually increased with increasing dendrimer generation. Steady-state and time-resolved spectroscopy were used to investigate the energy-transfer mechanism. On the basis of the fluorescence quenching rate constants of the PCn dendrons, the energy-transfer efficiencies for Ir-G1, Ir-G2, and Ir-G3 were 99, 98, and 96%, respectively. The energy-transfer efficiency for higher-generation dendrimers decreased slightly because of the longer distance between the PC dendrons and the core iridium(III) complex, indicating that energy transfer in Ir-Gn is a Förster-type energy transfer. Finally, the light-harvesting efficiencies for Ir-G1, Ir-G2, and Ir-G3 were determined to be 162, 223, and 334%, respectively.