Yi-Hung Liu

Vacuum-Deposited Small-Molecule Organic Solar Cells with High Power Conversion Efficiencies by Judicious Molecular Design and Device Optimization

Three new tailor-made molecules (DPDCTB, DPDCPB, and DTDCPB) were strategically designed and convergently synthesized as donor materials for small-molecule organic solar cells. These compounds possess a donor-acceptor-acceptor molecular architecture, in which various electron-donating moieties are connected to an electron-withdrawing dicyanovinylene moiety through another electron-accepting 2,1,3-benzothiadiazole block. The molecular structures and crystal packings of DTDCPB and the previously reported DTDCTB were characterized by single-crystal X-ray crystallography. Photophysical and electrochemical properties as well as energy levels of this series of donor molecules were thoroughly investigated, affording clear structure-property relationships. By delicate manipulation of the trade-off between the photovoltage and the photocurrent via molecular structure engineering together with device optimizations, which included fine-tuning the layer thicknesses and the donor:acceptor blended ratio in the bulk heterojunction layer, vacuum-deposited hybrid planar-mixed heterojunction devices utilizing DTDCPB as the donor and C(70) as the acceptor showed the best performance with a power conversion efficiency (PCE) of 6.6 ± 0.2% (the highest PCE of 6.8%), along with an open-circuit voltage (V(oc)) of 0.93 ± 0.02 V, a short-circuit current density (J(sc)) of 13.48 ± 0.27 mA/cm(2), and a fill factor (FF) of 0.53 ± 0.02, under 1 sun (100 mW/cm(2)) AM 1.5G simulated solar illumination.

A Low-Energy-Gap Organic Dye for High-Performance Small-Molecule Organic Solar Cells

A novel donor-acceptor-acceptor (D-A-A) donor molecule, DTDCTB, in which an electron-donating ditolylaminothienyl moiety and an electron-withdrawing dicyanovinylene moiety are bridged by another electron-accepting 2,1,3-benzothiadiazole block, has been synthesized and characterized. A vacuum-deposited organic solar cell employing DTDCTB combined with the electron acceptor C(70) achieved a record-high power conversion efficiency (PCE) of 5.81%. The respectable PCE is attributed to the solar spectral response extending to the near-IR region and the ultracompact absorption dipole stacking of the DTDCTB thin film.

Cooperative Recognition of a Copper Cation and Anion by a Calix[4]arene Substituted at the Lower Rim by a β‐Amino‐α,β‐Unsaturated Ketone

We report herein a new ditopic calix[4]arene receptor 25,27-bis-{[4-amino-4-(1-naphthyl)-2-oxo-3-butenyl]oxy}-26,28-dihydroxycalix[4]arene (2) for the simultaneous complexation of anionic and cationic species. The host molecule 25,27-bis{[3-(1-naphthyl)-5-isoxazolyl]methoxy}-26,28-dihydroxycalix[4]arene (1) was synthesised first and was followed by a [Mo(CO)6]-mediated ring-opening reaction to give the target receptor 2. The binding properties of ligands 1 and 2 towards metal ions in CH3CN were investigated by UV/Vis and fluorescence spectroscopies. The results showed that both ligands 1 and 2 were highly selective for Cu(II) ions. Upon titration with Cu(II), the fluorescence of 1 was severely quenched, whereas 2 showed strong fluorescence enhancement because the metal ions help to lock the conformation of the fluorophores. During the complexation of 2 with Cu(II), the Cu(II) was reduced to Cu(I) by the free phenolic OH of 2, whereas the phenol was oxidised by Cu(II), after which it assisted in the trapping of Cu(I). Ditopic behaviour was observed for the complex 2.Cu(I), which showed further enhancement of its fluorescence intensity upon complexation with anions such as acetate or fluoride.

Acid/Base‐ and Anion‐Controllable Organogels Formed From a Urea‐Based Molecular Switch

Low-molecular-weight organogels have applications in several fields, including molecular sensing, nanostructure assembly, and drug delivery. Ideally, these materials would switch reversibly between their solution and gel states through the addition or removal of heat, electrons, or ions. Although these modes of operation are similar to those employed for switches based on interlocked molecules, organogels formed from pseudorotaxaneor rotaxane-type gelators are rare. Indeed, we are aware of only a few previously reported examples, all of which feature long alkyl chains or cholesterol units incorporated into the molecular structures to assist the gelation process. Predicting the molecular structures of potential gelators and their preferred solvents remains difficult, and developing new rotaxane-based gelators that do not feature commonly used types of gelation units (e.g., long alkyl chains, steroids) in their structures is particularly challenging. Herein we report the serendipitous discovery of a urea-based [2]rotaxane that behaves as both a molecular switch and an organogelator; both functions are mediated by acid/base and anion control. The reaction of the macrocycle 1, the amino-terminated salt [2-H][PF6], [7] and the isocyanate 3 in CH3NO2 gave the dumbbell-shaped salt [4-H][PF6] and the [2]rotaxane [5-H][PF6] in 49 and 46% yield, respectively (Scheme 1). The binding constant for the assembly formed from the macrocycle 1 and dibenzylammonium hexafluorophosphate ((DBA)PF6) in CD3NO2 is (300 30)m , and 1 interacts only negligibly with diphenylurea derivatives in this solvent. 8] Therefore we suspected that the interlocked macrocycle in the [2]rotaxane [5-H][PF6] would prefer to encircle the DBA station, rather than the diphenylurea station, when dissolved in CD3NO2. Indeed, the 2D NOESY spectrum of the [2]rotaxane [5-H][PF6] in CD3NO2 shows cross-signals between the ethylene glycol protons of the macrocyclic unit and the aromatic protons of the 3,5-di-tert-butylphenyl stopper adjacent to the DBA center, however, no crosssignals are seen between the macrocyle and the stopper unit adjacent to the urea station. As expected, addition of potassium tert-butoxide (1 equivalent) to a solution of the [2]rotaxane [5-H][PF6] (CD3NO2, 13.6 mm) resulted in significant shifts in the locations of many of the signals in the H NMR spectrum (Figure 1). The significant downfield shift of the signal for the macrocyle NH protons, and the appearance of signals for the formerly severely broadened urea protons suggested the formation of hydrogen bonds to the carbonyl group of the urea station (Figure 1b). The addition of perchloric acid (70% in H2O, 1 equivalent) to this solution afforded a spectrum similar to that of the original [2]rotaxane. These observations suggest that the [2]rotaxane [5-H][X] is an acid/base-controllable molecular switch; the interlocked macrocyclic unit can be Scheme 1. Synthesis and switching of the [2]rotaxane [5-H][PF6].

A Molecular Cage-Based [2]Rotaxane That Behaves as a Molecular Muscle

We report a molecular cage-based [2]rotaxane that functions as an artificial molecular muscle through the control of the addition and removal of fluoride anions. The percentage change in molecular length of the [2]rotaxane is about 36% between the stretched and contracted states, which is larger than the percentage change (approximately 27%) in human muscle.

Carbazole–benzimidazole hybrid bipolar host materials for highly efficient green and blue phosphorescent OLEDs

In this study, we synthesized a series of bipolar hosts (CbzCBI, mCPCBI, CbzNBI, and mCPNBI) containing hole-transporting carbazole and electron-transporting benzimidazole moieties and then examined the morphological, thermal, and photophysical properties and carrier mobilities of these bipolar host materials. Altering the linking topology (C- or N-connectivity of the benzimidazole) changed the effective conjugation length and led to different excited-state solvent relaxation behavior. The N-connected compounds (CbzNBI, mCPNBI) possessed higher triplet energies (ET) than those of their C-connected analogues (CbzCBI, mCPCBI) by 0.23 eV. The higher values of ET of CbzNBI and mCPNBI endowed them with the ability to confine triplet excitons on the blue-emitting guest. A blue PhOLED device incorporating mCPNBI achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 16.3%, 35.7 cd A−1, and 23.3 lm W−1, respectively; confirming the suitability of using N-connected bipolar hosts for the blue phosphor. The donor/acceptor interactions of the C-connected analogue resulted in a lower triplet energy, making it a suitable bipolar host for green phosphors. A green-phosphorescent device incorporating CbzCBI as the host doped with (PBi)2Ir(acac) achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 20.1%, 70.4 cd A−1, and 63.2 lm W−1, respectively.

Solvent‐Free Synthesis of the Smallest Rotaxane Prepared to Date

[2]Rotaxanes—supermolecules comprising interlocked macrocyclic and dumbbell-shaped components—are fascinating materials for the construction of molecular devices because of the machinelike movement of their constituent parts. The development of efficient, convenient, and environmentally friendly methods for the synthesis of these functional interlocked molecules has progressed tremendously in the past decade. We became interested, however, in answering the following fundamental question: What is the smallest [2]rotaxane that can be synthesized, either in terms of molecular weight or the number of constituent atoms? We identified the crown ether/secondary dialkylammonium ion pair, which can be simplified into a few repeating CH2CH2O units that encircle a threadlike component as small as a dimethylammonium (CH3NH2 CH3) ion, as the simplest and smallest recognition system for preparing [2]rotaxanes. Herein, we report a new and efficient solvent-free reaction which involves ball-milling of the [2]pseudorotaxane formed from dipropargylammonium tetrafluoroborate and the crown ether [21]crown-7 (21C7) on SiO2 with 1,2,4,5-tetrazine. This led to the isolation in high yield (81 %) of the smallest [2]rotaxane reported to date (Scheme 1). Although it has been postulated for some time that macrocycles possessing 21 or more atoms in their ring will be able to accommodate an alkyl chain, it was only recently reported that a secondary dialkylammonium ion could be threaded through a 21-membered ring macrocycle, namely benzo[21]crown-7 (B21C7). In addition, a phenyl group can act as the stopper that prevents the unthreading of the interlocked ring-shaped and linear components when this small macrocycle is used. We proposed that Diels–Alder reactions of 1,2,4,5-tetrazine with the terminal alkyne units of a 21C7-based [2]pseudorotaxane would produce pyridazine end groups, which are slightly less bulky than phenyl groups, and might also function as stoppers in a 21C7-containing [2]rotaxane. We chose the dipropargylammonium ion (1-H) as the alkyne-terminated linear component in the small [2]pseudorotaxane precursor, expecting its small CH2NH2 CH2 unit to reside within the cavity of the crown ether 21C7, stabilized through N H···O and C H···O hydrogen bonds. The alkyne termini are available for functionalization (Scheme 1) through Diels–Alder reactions with 1,2,4,5-tetrazine to generate small, but nevertheless sufficiently bulky, pyridazine rings for stoppering the pseudorotaxanes under solvent-free conditions. The H NMR spectrum (Figure 1b) of an equimolar (5 mm) mixture of 21C7 and 1-H·BF4 in CD3CN at room temperature shows the chemical shifts of the protons of the complex are significantly different from those of its free components. The appearance of broad signals for both the free and complexed thread 1-H·BF4 in the H NMR spectrum (Figure 1c) of a 1:2 molar ratio mixture of 21C7 and 1-H·BF4 in CD3CN suggested that the rates of exchange during the complexation and decomplexation processes were slow on the H NMR spectroscopic timescale at 400 MHz under these conditions, but not sufficiently slow to provide the sharp signals required to obtain an accurate value for the association constant through the single-point method. Instead, we used isothermal titration calorimetry (ITC) to determine an association constant of (14 00

Effect of Ground-State Twisting on the trans → cis Photoisomerization and TICT State Formation of Aminostilbenes§

The synthesis, X-ray crystal structures, and photochemical behavior of a series of methyl- and ethylene-bridge-substituted trans-4-(N-(4-cyanophenyl)amino)stilbenes (3-8) are reported and compared to those of the parent compound 1CN. Aminostilbene 1CN displays dual fluorescence in polar solvents due to planar and twisted intramolecular charge-transfer (PICT and TICT) states. Alkyl substitution on the stilbene group of 1CN significantly perturbs its photochemistry, including fluorescence, trans --> cis photoisomerization, and TICT state formation. The alkyl substituent effect can be dissected into electronic and steric influences, and both are position dependent, which is vinyl alpha-carbon > vinyl beta-carbon > phenyl o-carbon. The main outcome of the alkyl substituent effect is to lower the barrier for the singlet-state photoisomerization. As a result, the quantum yield for photoisomerization is increased, and that for fluorescence is reduced. The corresponding quantum yield for TICT state formation in polar solvents is reduced only when significant ground-state twisting (a steric influence) is present. The alkyl substitution exerts little or no effect on the rate of intersystem crossing.

Biscarbene palladium(II) complexes. reactivity of saturated versus unsaturated N-heterocyclic carbenes.

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by (1)H and (13)C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd-C((saturated NHC)) is distinct from that of Pd-C((unsaturated NHC)). The Pd-C((saturated NHC)) bonds are fairly stable toward reagents such as CF(3)COOH, AgBF(4) and I(2), whereas Pd-C((unsaturated NHC)) bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki-Miyaura coupling follows the order: (sat-NHC)(2)PdCl(2) > (sat-NHC)(unsat-NHC)PdCl(2 )> (unsat-NHC)(2)PdCl(2).

Bis(diphenylamino)-9,9 '-spirobifluorene functionalized Ir(III) complex: a conceptual design en route to a three-in-one system possessing emitting core and electron and hole transport peripherals

Conceptual design of a three-in-one (luminescence chromophore with electron and hole transports) system was demonstrated by a functionalized Ir(III) complex 3, in which 4,5-diazafluorene and bis(diphenylamino) serve as electron and hole transporting sites, respectively. The poor emission quantum yield of 3 was systematically examined via a series of photophysical studies in combination with theoretical approaches. The far lifting of the π-electron from -NPh2 renders virtually no 3MLCT contribution to the lowest transition in the triplet manifold as compared with that of the parent model 2 without amino substituents. With an empirical approach, we conclude that an energy gap law may account for the major deactivation process. A light-emitting electrochemical cell (LEC) device based on 3 shows peak EQE, peak current efficiency and peak power efficiency at 2.4 V of 0.020%, 0.013 cd A−1 and 0.017 lm/W, respectively. The low device efficiencies are in accordance with the low PL quantum yield, stemming from the ligand-centered radiationless deactivation. The conceptual design presented here should provide valuable information for future progress en route to an ideal three-in-one system suited for OLEDs.