Synthesis and spectroscopic studies of 4-Formyl-4′-N,N-dimethylamino-1,1′-biphenyl: The unusual red edge effect and efficient laser generation

The synthesis and photophysics of 4-formyl-4-N,N-dimethylamino-1,1-biphenyl are reported. The emission spectrum in various solvent polarities demonstrates solvatochromism, indicating that the fluorescence originates from an electronically excited species with a strong charge transfer character. The change in [ _max(absorption) – _max(emission)] varies from 1500 cm^–1 inn-heptane to as much as 7500 cm^–1 in acetonitrile. In protic solvents, the unusual excitation energy-dependent steady-state emission (red edge effect), resulting from solvent dielectric relaxation, was observed in media with a low viscosity. The large Stokes-shifted and high-yield fluorescence led to the observation of the efficient lasing action. The frequency tunability of the laser output is strongly solvent dependent, generating a new charge transfer laser dye in the blue-green region.     

Studies of the triplet state of the proton-transfer tautomer in salicylaldehydes

The spectroscopy and dynamics of the low-lying triplet state of the proton-transfer tautomer in salicylaldehydes have been studied via internal heavy-atom effects coupled with a sensitive near-IR detecting system. For 3,5-diiodosalicylaldehyde a weak proton-transfer keto-tautomer phosphorescence was resolved with a maximum at 710 nm (τ_p1.8 μs, Φ_obs5.23×10⁻⁴) in a 77 K methylcyclohexane glass. The results, in combination with the time-resolved thermal lensing experiment, further deduced the triplet-state population yield and radiative decay rate to be 0.20 (298 K) and 3.12×10² s⁻¹, respectively. Consequently, the energetics and dynamics of the triplet states during a proton transfer cycle are discussed in detail.     

A remarkable ligand orientational effect in osmium-atom-induced blue phosphorescence

A new series of Os(II)-based carbonyl complexes cis(CO),trans(Npy,Npy),cis(Ntz,Ntz)-[Os(CO)2(bptz)2] (1), cis(CO),cis(Npy,Npy),trans(Ntz,Ntz)-[Os(bptz)2(CO)2] (2), and cis(CO),trans(Npy,Npy),cis(Ntz,Ntz)-[Os(CO)2(fptz)2] (3), where bptz and fptz denote 3-tert-butyl-5-(2-pyridyl)- and 3-trifluoromethyl-5-(2-pyridyl)-1,2,4-triazolate, respectively, have been designed and synthesized in an effort to achieve high efficiency, room-temperature blue phosphorescence. Although 1 and 2 are geometric isomers, remarkably different excited-state relaxation pathways were observed. Complex 1 exhibits strong phosphorescence in CH3CN (Phi(p) approximately 0.47) and as a single crystal at room temperature, whereas complex 2 is nearly nonemissive under similar conditions. The associated relaxation dynamics have been comprehensively investigated by spectroscopic and relaxation dynamics as well as by theoretical approaches. Our results lead us to the conclusion that for complex 2, the "loose bolt" effect of metal-ligand bonding interactions plays a crucial role in the fast radiationless deactivation of this type of geometrical isomer. Fine adjustment can also be achieved by functionalizing the ligands so that the electron-withdrawing nature of the CF3 group in 3 stabilizes the HOMO of the triazolate moiety, thus moving the emission further into the pure "blue" region; this results in highly efficient phosphorescence and renders 3 particularly attractive for application in blue OLED devices.     

REINVESTIGATION OF THE PHOTOPHYSICS OF 2-(2'-HYDROXY-4'-DIETHYLAMINOPHENYL)BENZOTHIAZOLE

Abstract— The photophysical properties of 2-(2'-hydroxy-4'-diethylaminophenyl) benzothiazole (HABT) have been investigated by steady-state and time-resolved spectroscopies. In n-heptane HABT exhibits both normal and tautomer emissions with ∼equal fluorescence intensity at room temperature, in contrast to a previous report in which negligible tautomer emission was observed. The normal/tautomer (400/500 nm) ratio of emission intensity increases as the temperature decreases. Two possible excited-state intramolecular proton transfer (ESIPT) mechanisms are proposed, which cannot be resolved at the present stage. One proposed mechanism incorporates state mixing between -OH and -N(C₂H₅)₂ charge transfer states, resulting in a significant energy barrier for ESIPT. An alternative mechanism is also proposed in which fast proton tunneling may take place between enol and keto forms, which are in equilibrium in the excited singlet state.     

Excited-State THz Vibrations in Aggregates of PtII Complexes Contribute to the Enhancement of Near-Infrared Emission Efficiencies**

The exploration of deactivation mechanisms for near-infrared(NIR)-emissive organic molecules has been a key issue in chemistry, materials science and molecular biology. In this study, based on transient absorption spectroscopy and transient grating photoluminescence spectroscopy, we demonstrate that the aggregated Pt^II complex 4H (efficient NIR emitter) exhibits collective out-of-plane motions with a frequency of 32 cm⁻¹ (0.96 THz) in the excited states. Importantly, similar THz characteristics were also observed in analogous Pt^II complexes with prominent NIR emission efficiency. The conservation of THz motions enables excited-state deactivation to proceed along low-frequency vibrational coordinates, contributing to the suppression of nonradiative decay and remarkable NIR emission. These novel results highlight the significance of excited-state vibrations in nonradiative processes, which serve as a benchmark for improving device performance.     

Simple organic molecules bearing a 3,4-ethylenedioxythiophene linker for efficient dye-sensitized solar cells

3,4-Ethylenedioxythiophene and bis[2-(2-methoxyethoxy)ethoxy]thiophene bridged donor-acceptor molecules for dye-sensitized solar cells have been synthesized, one of which achieved a solar-to-energy conversion efficiency of 7.3%, compared to 7.7% optimized for N719 dye.     

Modulate photoinduced electron transfer efficiency of bipolar dendritic systems

Covalent linkage of dendritic carbazole-based donors and 1,3,4-oxdiazole-based acceptors renders novel bipolar dendrimers that can efficiently facilitate the photoinduced electron transfer (PET) process. Photodynamic studies indicated that the PET rate of bipolar dendrimers DA1 and DA2 can be modulated by the number of acceptors presented in the molecule.     

Preparations and photophysical properties of fused and nonfused thienyl bridged MM (M = Mo or W) quadruply bonded complexes

A series of metal-metal quadruply bonded compounds [(tBuCO2)3M2]2(mu-TT) where TT = thienothiophenedicarboxylate and M = Mo, 1A, and M = W, 1B and [(tBuCO2)3M2]2(mu-DTT) where DTT = dithienothiophenedicarboxylate and M = Mo, 2A, and M = W, 2B, has been prepared and characterized by elemental analysis, ESI- and MALDI-TOF mass spectrometry and 1H NMR spectroscopy. Their photophysical properties have also been investigated by steady-state absorption as well as transient absorption and emission spectroscopy. The optimized structures and the predicted low energy electronic transitions were obtained by DFT and time-dependent DFT calculations, respectively, on model compounds. These results, in combination with the respective properties of the compounds [(tBuCO2)3M2]2(mu-BTh) (BTh = 2,5'-bithienyldicarboxylate, M = Mo, 3A, and M = W, 3B), allow us to make a comprehensive comparison of the fused (compounds 1A, 1B, 2A, and 2B) and the nonfused thienyl (compounds 3A and 3B) dicarboxylate bridged compounds of molybdenum and tungsten. The electrochemical studies show singly oxidized radical cations that are valence trapped on the EPR time-scale and are classified as Class 1 (M = Mo) or Class 2 (M = W) on the Robin and Day scale for mixed valence compounds. The new compounds exhibit intense metal to bridge ligand charge transfer absorption bands in the far visible and near IR (NIR) region. Both molybdenum and tungsten complexes show dual emission, but for molybdenum, the phosphorescence is dominant while for tungsten the emission is primarily fluorescence. Femtosecond transient absorption spectroscopy shows that the relaxation dynamics of the S1 states which have lifetimes of approximately 10 ps is dominated by intersystem crossing (ISC), leading to T1 states that in turn possess long lifetimes, approximately 70 micros (M = Mo) or 3 micros (M = W). These properties are contrasted with the photophysical properties of conjugated organic systems incorporating metal ions of the later transition elements.     

Spectroscopy and femtosecond dynamics of excited-state proton transfer induced charge transfer reaction

Fluorescence spectroscopy and femtosecond relaxation dynamics of 2-{[2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl]methylene}malononitrile (diCN-HBO) and 2-{[2-(2-hydroxyphenyl)benzo[d]thiazol-6-yl]methylene}malononitrile (diCN-HBT) are studied to probe the excited-state proton transfer (ESPT) coupled charge transfer (ESCT) reaction. Unlike most of the ESPT/ESCT systems previously designed, in which ESCT takes place prior to ESPT, both diCN-HBO and diCN-HBT undergo ESPT, concomitantly accompanied with the charge transfer process, such that the ESPT reaction dynamics are directly coupled with solvent polarization effects. The long-range solvent polarization interactions result in a solvent-induced barrier that affects the overall proton transfer reaction rate. In cyclohexane, the rate constant of ESPT of diCN-HBO is measured to be 1.1 ps (9.1 x 10(11) s(-1)), which is apparently slower than that of 150 fs for the parent molecule 2-(2'-hydroxyphenyl)benzoxazole (HBO). Upon increasing solvent polarity to, for example, CH 3CN, the rate of ESPT is increased to 300 fs (3.3 x 10(12) s(-1)). The results are rationalized by the stabilization of proton transfer tautomer, which possesses a large degree of charge transfer character via an increase of the solvent polarity, such that the corresponding solvent-induced barrier is reduced. We thus demonstrate a prototypical system in which the photon-induced nuclear motion (proton transfer) is directly coupled with solvent polarization and the corresponding mechanism is reminiscent of that applied in an electron transfer process.