Chin-Hung Lai

Neutral, panchromatic Ru(II) terpyridine sensitizers bearing pyridine pyrazolate chelates with superior DSSC performance

A new series of neutral, panchromatic Ru(II) terpyridine sensitizers (PRT1-PRT4) exhibit much higher molar extinction coefficients at 400-550 nm and superior DSSC performance in terms of conversion efficiency (eta = 10.05 for PRT4) and stability.

Excited-state intramolecular proton transfer molecules bearing o-hydroxy analogues of green fluorescent protein chromophore.

o-Hydroxy analogues, 1a-g, of the green fluorescent protein chromophore have been synthesized. Their structures and electronic properties were investigated by X-ray single-crystal analyses, electrochemistry, and luminescence properties. In solid and nonpolar solvents 1a-g exist mainly as Z conformers that possess a seven-membered-ring hydrogen bond and undergo excited-state intramolecular proton transfer (ESIPT) reactions, resulting in a proton-transfer tautomer emission. Fluorescence upconversion dynamics have revealed a coherent type of ESIPT, followed by a fast vibrational/solvent relaxation (<1 ps) to a twisted (regarding exo-C(5)-C(4)-C(3) bonds) conformation, from which a fast population decay of a few to several tens of picoseconds was resolved in cyclohexane. Accordingly, the proton-transfer tautomer emission intensity is moderate (0.08 in 1e) to weak (∼10(-4) in 1a) in cyclohexane. The stronger intramolecular hydrogen bonding in 1g suppresses the rotation of the aryl-alkene bond, resulting in a high yield of tautomer emission (Φ(f) ≈ 0.2). In the solid state, due to the inhibition of exo-C(5)-C(4)-C(3) rotation, intense tautomer emission with a quantum yield of 0.1-0.9 was obtained for 1a-g. Depending on the electronic donor or acceptor strength of the substituent in either the HOMO or LUMO site, a broad tuning range of the emission from 560 (1g) to 670 nm (1a) has been achieved.

Harvesting Highly Electronically Excited Energy to Triplet Manifolds: State-Dependent Intersystem Crossing Rate in Os(II) and Ag(I) Complexes

A series of newly synthesized Os(II) and Ag(I) complexes exhibit remarkable ratiometric changes of intensity for phosphorescence versus fluorescence that are excitation wavelength dependent. This phenomenon is in stark contrast to what is commonly observed in condensed phase photophysics. While the singlet to triplet intersystem crossing (ISC) for the titled complexes is anomalously slow, approaching several hundred picoseconds in the lowest electronic excited state (S(1) → T(1)), higher electronic excitation leads to a much accelerated rate of ISC (10(11)-10(12) s(-1)), which is competitive with internal conversion and/or vibrational relaxation, as commonly observed in heavy transition metal complexes. The mechanism is rationalized by negligible metal d orbital contribution in the S(1) state for the titled complexes. Conversely, significant ligand-to-metal charge transfer character in higher-lying excited states greatly enhances spin-orbit coupling and hence the ISC rate. The net result is to harvest high electronically excited energy toward triplet states, enhancing the phosphorescence.

Mesomorphism and Luminescence Properties of Platinum(II) Complexes with Tris(alkoxy)phenyl‐Functionalized Pyridyl Pyrazolate Chelates

A series of new mesomorphic platinum(II) complexes 1-4 bearing pyridyl pyrazolate chelates are reported herein. In this approach, pyridyl azolate ligands have been strategically functionalized with tris(alkoxy)phenyl groups with various alkyl chain lengths. As a result, they are ascribed to a class of luminescent metallomesogens that possess distinctive morphological properties, such as their intermolecular packing arrangement and their associated photophysical behavior. In CH(2) Cl(2), independent of the applied concentration in the range 10(-6)-10(-3)  M, all Pt(II) complexes exhibit bright phosphorescence centered at around 520 nm, which is characteristic for monomeric Pt(II) complexes. In stark contrast, the single-crystal X-ray structure determination of [Pt(C4pz)(2)] (1) shows the formation of a dimeric aggregate with a notable Pt⋅⋅⋅Pt contact of 3.258 Å. Upon heating, all Pt(II) complexes 1-4 melted to form columnar suprastructures, for which similar intracolumnar Pt⋅⋅⋅Pt distances of approx. 3.4-3.5 Å are observed within an exceptionally wide temperature range (>250 °C), according to the powder XRD data. Upon casting into a neat thin film at RT, the luminescence of 1-4 is dominated by a red emission that spans 630-660 nm, which originates from the one-dimensional, chainlike structure with Pt-Pt interaction in the ground state. Taking complex 4 as a representative, the emission intensity and wavelength were significantly decreased and blueshifted, respectively, on heating from RT to 250 °C. Further heating to liquefy the sample alters the red emission back to the green phosphorescence of the monomer. The results highlight the pivotal role of tris(alkoxy)phenyl groups in the structural versus luminescence behavior of these Pt(II) complexes.

Dual Excited-State Intramolecular Proton Transfer Reaction in 3-Hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one

The synthesis, characterization and fundamental of the dual excited-state proton-transfer properties of 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one (1a) are reported. In the electronic ground state, there exist two competitive hydrogen bonding (HB) isomers for 1a. Conformer 1a(O) reveals a five-membered ring HB structure between O-H and carbonyl oxygen, while conformer 1a(N) possesses a six-membered ring HB formation between O-H and pyridyl nitrogen. In a single crystal, the X-ray crystallography unveils an exclusive formation of conformer 1a(N). In solution such as CH(2)Cl(2), 1a(O) and 1a(N) are in equilibrium, and their respective absorption chromophores are significantly different due to different degrees of hydrogen-bond induced pi electron delocalization. Upon excitation, both conformers 1a(O) and 1a(N) undergo excited-state intramolecular proton transfer (ESIPT) reaction. Following ESIPT, 1a(O) gives rise to a tautomer emission maximized at 534 nm in CH(2)Cl(2). Conversely, due to dominant radiationless quenching processes the tautomer emission for 1a(N) cannot be obtained with a steady-state manner but can be resolved from time-resolved fluorescence. Time resolved fluorescence estimates an equilibrium constant of 27 +/- 5 in favor of 1a(N) in CH(2)Cl(2). Ultrafast ESIPT also takes place for the unique 1a(N) form in the crystal. Due to the prohibition of quenching processes in the solid state, bright tautomer emission maximized at 540 nm is resolved for 1a(N) (Phi(f) approximately 0.3). The interplay between two HB conformers with on(1a(O))/off(1a(N)) character in tautomer emission may find future applications such as the recognition of organic Lewis acid/base in organic solvents.

Extensive spectral tuning of the proton transfer emission from 550 to 675 nm via a rational derivatization of 10-hydroxybenzo[h]quinoline

Via a systematic derivatization of the excited-state intramolecular proton-transfer system, 10-hydroxybenzo[h] quinoline, the proton-transfer emission can be extensively tuned from 550 nm (1) to 675 nm (6), in which amplified spontaneous emission was readily observed for , generating a new family of proton transfer laser dyes.

Syntheses, photophysics, and application of iridium(III) phosphorescent emitters for highly efficient, long-life organic light-emitting diodes.

Rational design and synthesis of Ir(III) complexes (1-3) bearing two cyclometalated ligands (C--N) and one 2-(diphenylphosphino)phenolate chelate (P--O) as well as the corresponding Ir(III) derivatives (4-6) with only one (C--N) ligand and two P--O chelates are reported, where (C--NH)=phenylpyridine (ppyH), 1-phenylisoquinoline (piqH), and 4-phenylquinazoline (nazoH). Single crystal X-ray diffraction studies of 3 reveal a distorted octahedral coordination geometry, in which two nazo ligands adopt an eclipsed configuration, with the third P--O ligand located trans to the phenyl group of both nazo ligands, confirming the general skeletal pattern for 1-3. In sharp contrast, complex 4 reveals a trans-disposition for the PPh2 groups, along with the phenolate groups residing opposite the unique cyclometalated ppy ligand, which is the representative structure for 4-6. These Ir(III) complexes exhibit green-to-red photoluminescence with moderate to high quantum efficiencies in the degassed fluid state and bright emission in the solid state. For 1-6, the resolved emission spectroscopy and relaxation dynamics are well rationalized by the computational approach. OLEDs fabricated using 12 wt. % of 3 doped in CBP and with BCP as hole blocking material, give bright electroluminescence with lambda(max)=628 nm and CIE(xy) coordinates (0.65, 0.34). The turn-on voltage is 3.2 V, while the current efficiency and the power efficiency reach 11.2 cd A(-1) and 4.5 lm W(-1) at 20 mA cm(-2). The maximum efficiency reaches 14.7 cd A(-1)and 6.8 lm W(-1) upon switching to TPBI as hole blocking material. For evaluating device lifespan, the tested device incorporating CuPc as a passivation layer, 3 doped in CTP as an emitting layer, and BAlq as hole blocking material, shows a remarkably long lifetime up to 36,000 h at an initial luminance of 500 cd m(-2).

Excited‐State Double Proton Transfer in Model Base Pairs: The Stepwise Reaction on the Heterodimer of 7‐Azaindole Analogues

A four fused-ring system 11-propyl-6H-indolo[2,3-b]quinoline (6 HIQ) is strategically designed and synthesized; it possesses a central moiety of 7-azaindole (7AI) and undergoes excited-state double proton transfer (ESDPT). Despite a barrierless type of ESDPT in the 6 HIQ dimer, femtosecond dynamics and a kinetic isotope effect provide indications for a stepwise ESDPT process in the 6 HIQ/7AI heterodimer, in which 6 HIQ (deuterated 6 HIQ) delivers the pyrrolyl proton (deuteron) to 7AI (deuterated 7AI) in less than 150 fs, forming an intermediate with a charge-transfer-like ion pair, followed by the transfer of a pyrrolyl proton (deuteron) from cation-like 7AI (deuterated 7AI) to the pyridinyl nitrogen of the anion-like 6 HIQ (deuterated 6 HIQ) in approximately 1.5+/-0.3 ps (3.5+/-0.3 ps). The barrier of second proton transfer is estimated to be 2.86 kcal mol(-1) for the 6 HIQ/7AI heterodimer.

Surface plasmon enhanced energy transfer between type I CdSe/ZnS and type II CdSe/ZnTe quantum dots

Fluorescence resonant energy transfer (FRET) has been investigated between donor-acceptor pairs of type I CdSe/ZnS and type II CdSe/ZnTe quantum dots (QDs). An Au nanoparticles assisted FRET enhancement was clearly demonstrated. It is found that the efficiency of the energy transfer depends on the excitation wavelength and is largest when in resonance with the Au surface plasmon mode. With the large tunability of the emission intensity in near infrared region, our finding paves an excellent route for creating highly efficient optoelectronic devices and bioimaging labels derived from type II QDs.

Can an OH radical form a strong hydrogen bond? A theoretical comparison with H2O

In this study, we apply UCCSD/6‐31++G** to investigate the ability of an OH radical acting as a hydrogen bond acceptor with HF, HCl, and H2O (HO…HX; XF, Cl, OH) or as a hydrogen bond donor with H2O and H2S (OH…XH2; XO and S). We also replace OH with H2O and make a fair comparison between them. Additionally, the counterpoise method (CP) has been used to examine the effect of basis set superposition error (BSSE). Our results reveal that OH is a stronger hydrogen bond donor but a weaker hydrogen bond acceptor than H2O. This conclusion is independent of the correction for BSSE and can be rationalized by the NBO analysis, the results of which indicate that OH radical has a lower nO and σ*O–H in energy than that of H2O.