Nonradiative decay mechanism of fluoren-9-ylidene malononitrile ambipolar derivatives.

We report recent results on the nonradiative decay (NRD) of fluoren-9-ylidene malononitrile (FM) ambipolar derivatives (FMDs). 2,7- and 3,6-disubstituted FMDs present distinctive photophysics. Charge separation was found dominant for excited state relaxation. The radiative decay (RD) is sensitive to changes in temperature and solvent medium only for the case of 3,6-FMDs. Excited state deactivation of carbazole-containing 3,6-FMD (CPAFM36) was exclusively nonradiative in polar solvents with excited state lifetimes shorter than 10 ps. The charge separation/recombination mechanism of the corresponding FMDs is suggested to fall in the inverted Marcus region of electron transfer. Given the electron-withdrawing properties of the FM unit, its ambipolar derivatives are suggested as potential candidates for air-stable organic thin-film transistors and molecular organic photovoltaics.

Carbazole-based cyano-stilbene highly fluorescent microcrystals.

Aggregation-induced enhanced emission (AIEE) is reported for 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE). The weak luminescence of dilute CN-CPE solutions is enhanced upon aggregate formation into 2-3 µm sized crystals. In contrast to general observations, crystal formation of CN-CPE causes a blue-shift in emission and enhances the intensity. X-ray crystallographic analysis revealed that key factors causing high luminescence efficiency in the crystal are a lack of strong cofacial π-π alignment and the existence of the strong supramolecular interactions due to the intermolecular H-bonding. These factors seem to be responsible for the AIEE phenomenon as molecules of CN-CPE are held in a rigid twisted conformation, thereby increasing the fluorescence intensity in the solid or aggregated states. Accordingly, conformational twisting in the crystal packing process may be responsible for the unusual emission blue-shift in the aggregate.

[Effects of erlong zuoci pills and its effective disassembled prescriptions on gentamycin induced hair cell apoptosis].

OBJECTIVE: To investigate the effects of erlong zuoci (ELZC) pills and its disassembled prescriptions (Shudi-huang-Zexie group and Zexie group) on the enzymatic activity and protein expression changes of the key apoptosis molecules in the gentamycin injured hair cells. METHOD: The model of gentamycin induced ototoxicity in mice cochlear primary cultures was copied. Cochlear organotypic cultures of postnatal day 3-5 (P3-P5) mice were treated with gentamycin alone or in combination with ELZC pills, Shudihuang-Zexie group or Zexie group respectively. The enzymatic activity of Caspase-9 and Caspase-3 was determined by means of fluorescence staining in situ. The protein expression of Bcl-2 and Bax in the hair cell area was examined by immunofluorescence in normal and treated specimens. RESULT: Average optical density analysis indicated that, compared to the normal group, 0.03 mmol x L(-1) gentamycin could significantly activate Caspase-9 and Caspase-3, downregulate the ratio of Bcl-2 and Bax protein expression. Compared to the gentamycin model group, ELZC pills significantly inhibited the enzymatic activity of Caspase-9 and upregulated the ratio of Bcl-2 and Bax protein expression, showing inhibition trend toward the enzymatic activity of Caspase-3. Both Shudihuang-Zexie group and Zexie group could effectively inhibit the enzymatic activity of Caspase-9 and Caspase-3, upregulate the ratio of Bcl-2 and Bax protein expression. CONCLUSION: ELZC pills, Shudihuang-Zexie group and Zexie group can effectively protect hair cells from gentamycin by correcting the abnormal changes of the mitochondrion-dependent signal transduction pathway.

Photopolymerization of conductive polymeric metal nanoparticles.

5-Mercapto-2,2'-bithiophene functionalized metal nanoparticles BTSMs [M: copper (Cu), silver (Ag), and gold (Au)] of different diameters (2-8 nm) were synthesized. Conductive polymeric metal nanoparticles were formed from BTSM by UV irradiation. The photopolymerization mechanism was investigated using transient absorption measurements. Intramolecular electron transfer from the ligand to the metal nano-core was confirmed. Nanoparticle size, as well as plasmon electronic interactions, are important factors. The smaller the nanoparticle and the stronger the electronic interactions, the faster the electron transfer is. The three-dimensional structure of the polymerized BTSM was identified using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The conductivity of polymerized BTSM measured in poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) film is higher than that of the nonpolymerized BTSM.

Photoinduced hole transfer forming polymer quantum dot nanocomposites.

The study of hole transfer from CdSe QD core to ligands under visible light irradiation and conductive polymer-QD composites formation without conventional photoinitiator.

Alpha-bond dissociation of p-phenylbenzoyl derivatives in the higher triplet excited state studied by two-color two-laser flash photolysis.

Photochemical properties in the lowest singlet and triplet excited states (S(1) and T(1)) and in the higher triplet excited states (T(n)) of p-phenylbenzoyl derivatives (PB-X) having C-O and C-S bonds were investigated in solution using a stepwise two-color two-laser flash photolysis technique. PB-Xs (X = OPh and SPh as a leaving group) undergo alpha-bond dissociation in the S(1) state, while the C-O bonds in PB-OH and -OMe were stable upon the 266-nm laser flash photolysis. The T(1)(pi,pi*) states of PB-X were efficiently produced during the 355-nm laser flash photolysis of PB-X in the presence of benzophenone as a triplet sensitizer. The T(1)(pi,pi*) states of PB-Xs deactivate to the ground-state without producing any intermediates. However, when PB-Xs (X = OPh and SPh) in the T(1) states (PB-X(T(1))) were excited upon the 430-nm laser flash photolysis, their disappearance was observed. These observations indicate that PB-X(T(1)) (X = OPh and SPh) is excited to PB-Xs in the T(n) states which decompose through the C-X bond cleavage. From the transient absorption measurements, quantum yields (Phi(dec)) of the disappearance of PB-X(T(1)) were determined while bond dissociation energies (BDE) of the C-X bonds of PB-X were calculated by computations. On the basis of the Phi(dec) and the BDE values, it was shown that the rates of the decomposition process of PB-X in the T(n) states were expressed in an energy gap low form as a function of BDE. Features of the C-X bond cleavage of PB-X in the T(n) states were discussed.

Isomer restriction on a nanoparticle surface and enhanced blue emission.

5-Mercapto-2,2'-bithiophene (BTSH) functionalized copper (Cu), silver (Ag), and gold (Au) nanoparticles (NPs) of various diameters have been synthesized. Emission with a peak around 455 nm is detected on irradiation with UV light. This emission, however, disappears upon photopolymerization of the NPs. The photophysical study of BTSH at room and low temperatures indicates that the trans-isomer should be dominant when attached onto the surface of an NP. The electronic interaction among the ligands and NP core likely restricts the isomerization of BTSH and forms an emissive excited state. Therefore, we suggest that the trans isomer assembles on the NP surface giving the enhanced blue emission. As a control, no such isomer restriction in the case of 5-(5-mercaptopentyl)-2,2'-bithiophene (BTC(5)SH) is observed when attached on the NP surface.

Photochemical synthesis of 2,3,9,10-tetrabromopentacene: its unusual photodimerization.

The photogeneration of 2,3,9,10-tetrabromopentacene from its precursor dione by photodecarbonylation was investigated. An unusual photodimer, 3,3',9,9',10,10'-hexabromo-2,2'-bipentacene, is produced.

Gold nanoparticle photosensitized radical photopolymerization.

We report the photopolymerization of an acrylic monomer using thiol-stabilized gold nanoparticles (AuNPs) and [4-[(octyloxy)phenyl] phenyl] iodonium hexafluoroantimonate (OPPI) as photoinitiator and coinitiator, respectively. Polymerization occurred only when the AuNPs, in the presence of the iodonium salt, were irradiated at the particle plasmonic absorption region (lambda>450 nm). The AuNPs activate the coinitiator by intermolecular electron transfer since OPPI has no absorption in the visible region. Fourier transform infrared spectroscopy was used to monitor polymerization. UV-Vis spectroscopy and transmission electron microscopy measurements were used to characterize the NPs.

Photodecomposition profiles of beta-bond cleavage of phenylphenacyl derivatives in the higher triplet excited states during stepwise two-color two-laser flash photolysis.

Photochemical properties of p-phenylphenacyl derivatives (PP-X) having C-halide, C-S, and C-O bonds in the lowest (T 1) and higher (T n ) triplet excited states were investigated in solution by using single-color and stepwise two-color two-laser flash photolysis techniques. PP-Xs (X = Br, SH, and SPh) undergo beta-bond dissociation in the lowest singlet excited states (S 1) while the C-X bonds of other PP-Xs are stable upon 266-nm laser photolysis. The T 1(pi,pi*) states of PP-X were efficiently produced during 355-nm laser photolysis of benzophenone as a triplet sensitizer. Triplet PP-Xs deactivate to the ground state without photochemical reactions. Upon 430-nm laser photolysis of the T 1 states of PP-X (X = Br, Cl, SH, SPh, OH, OMe, and OPh), decomposition of PP-X in the T n states was found. On the basis of the changes in the transient absorption, quantum yields (Phi dec) of the decomposition of PP-X in the T n states were determined, while bond dissociation energies (BDE) of the C-X bonds were calculated by computations. According to the relationship between the Phi dec and BDE values, it was shown that the decomposition of PP-X in the T n state is due to beta-cleavage of the corresponding C-X bond, and that the state energy of the reactive T n for the C-O bond cleavage differs from that for the C-halide and C-S bond cleavage. The reaction profiles of the C-X bond cleavage of PP-X in the T n states were discussed.

Photopolymerization of metal nanoparticles on multiwall carbon nanotubes.

A facile method for the functionalization of multiwall carbon nanotubes (MWCNT) by photopolymerization of 5-mercapto-2,2'-bithiophene modified metal (Au or Ag) nanoparticles on the surface of the MWCNT is developed.

Carbazole-based donor-acceptor compounds: highly fluorescent organic nanoparticles.

Carbazole-based donor-acceptor compounds 1,2-dicyano-trans-1,2-bis-4-(carbazolyl)phenylethylene (1) and 1,2-dicyano-trans-1,2-bis-4-(3,6-di-tert-butylcarbazolyl)phenylethylene (2) were synthesized. 1 and 2 show negative solvatochromic absorption behavior, but show both positive and negative solvatochromic behavior in the fluorescence spectra. In a water/THF mixture, 1 as well as 2 aggregate into 50-150 nm nanoparticles. The emission of nanoparticles of the new types of fluorescent organic nanoparticles (FONs) 1 and 2 is much higher than that of either 1 or 2 in solution.

Bimolecular hole transfer from the trimethoxybenzene radical cation in the excited state.

Bimolecular hole transfer quenching of the 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) by hole quenchers (Q) such as biphenyl (Bp), naphthalene (Np), anisole (An), and benzene (Bz) with higher oxidation potentials than that of TMB was directly observed during the two-color two-laser flash photolysis at room temperature. From the linear relationships between the inverse of the transient absorption changes of TMB*+ during the second 532-nm laser excitation versus the inverse of the concentration of Q, the rate constant of the hole transfer from TMB*+* to Q was estimated to be (8.5 +/- 0.4) x 10(10), (1.4 +/- 0.7) x 10(11), (1.3 +/- 0.6) x 10(11), and (6.4 +/- 0.3) x 10(10) M(-1)s(-1) for Bp, An, Np, and Bz, respectively, in acetonitrile based on the lifetime of TMB*+*. The estimated rate constants are larger than the diffusion-controlled rate constant in acetonitrile. Short lifetime, high energy, and high oxidation potential of TMB*+* cause the lifetime-dependent quenching process or static quenching process as the major process during the quenching of TMB*+* by Q as indicated by the Ware's theoretical model. The subsequent hole transfer from Q*+ to TMB, giving TMB*+, was found to occur at the diffusion-controlled rate for Bp and An as Q. For Q such as Np and Bz, the dimerization of Q*+ with Q to give dimer radical cation (Q2*+) occurred competitively with the hole transfer from Q*+ to TMB.

One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis.

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.

C-O-bond cleavage of esters with a naphthyl group in the higher triplet excited state during two-color two-laser flash photolysis.

A C-O-bond cleavage of esters having a naphthyl group, NpCO-OR and RCO-ONp (Np=alpha- and beta-naphthyl ((alpha)Np and (beta)Np, respectively), R=Ph and Me), was found during the two-color two-laser flash photolysis in acetonitrile. The C-O-bond cleavage occurred when NpCO-OR and RCO-ONp were excited to the singlet excited states (S1). On the other hand, no reaction occurred from the lowest triplet excited states (T1). When NpCO-OR(T1) and RCO-ONp(T1) were excited to the higher triplet excited states (Tn) using the second laser during the two-color two-laser flash photolysis, the C-O-bond cleavage occurred. The C-O-bond cleavage quantum yield (Phi) was estimated from the plots of the T1-state esters disappeared within a laser flash versus the second laser intensities. The C-O-bond cleavage in (beta)NpCO-OPh(Tn) occurred more efficiently than in (alpha)NpCO-OPh(Tn) and that in PhCO-O(beta)Np(Tn) occurred more efficiently than in PhCO-O(alpha)Np(Tn). The Phi value for ester with Ph and beta-Np groups was larger than that for ester with Ph and alpha-Np groups. The Phi value for MeCO-O(alpha)Np(Tn) was similar to those for PhCO-ONp(Tn), while that for MeCO-O(beta)Np(Tn) was much smaller than those for PhCO-ONp(Tn) and MeCO-O(alpha)Np(Tn). On the other hand, no C-O-bond cleavage was observed in NpCO-OMe(Tn). The Phi value depended on the characters of the groups (Np, Ph, and Me) on the ester. Whether R is Ph or Me with or without pi electron, respectively, is important for the C-O-bond cleavage. In other words, electronic delocalization of the T(n) state including Np and ester groups is necessary for the occurrence of the C-O-bond cleavage in NpCO-OR(Tn) and RCO-ONp(Tn).

Intermolecular electron transfer from excited benzophenone ketyl radical.

The electron transfer from the benzophenone ketyl radical in the excited state (BPH(.-)(D(1))) to several quenchers (Qs) was investigated using nanosecond/picosecond two-color two-laser flash photolysis and nanosecond/nanosecond two-color two-laser flash photolysis. The electron transfer from BPH(.-)(D(1)) to Qs was confirmed by the transient absorption and fluorescence quenching measurements. The intermolecular electron-transfer rate constants were determined using the Stern-Volmer analysis. The driving force dependence of the electron-transfer rate was revealed.

Solvent effect on the deactivation processes of benzophenone ketyl radicals in the excited state.

The solvent effects on ketyl radicals of benzophenone derivatives (BPD) in the excited state (BPDH*(D1)) were investigated. Absorption and fluorescence spectra of BPDH*(D1) in various solvents were measured using nanosecond-picosecond two-color two-laser flash photolysis. The fluorescence peaks from BPDH*(D1) showed a shift due to the dipole-dipole interaction with the solvent molecules. The dipole moments (mu(e)) of BPDH*(D1) were estimated to be 7-10 D, indicating that BPDH*(D1) are highly polarized. It was revealed that the fluorescence lifetime (tau(f)) depends on mu(e) in acetonitrile because the stabilization by solvent molecules affects the tau(f) value in polar solvents, predominantly. On the contrary, the conformation of BPDH*(D1) plays an important role in cyclohexane because the efficiency of the unimolecular reaction from BPDH*(D1) depends on the conformation. The substituent effect on the electron transfer from BPDH*(D1) to their parent molecules was also discussed.

Properties and reactivity of xanthyl radical in the excited state.

The properties and reactivity of the 9-xanthyl radical (X(*)) in the doublet excited state (X(*)(D(1))) were investigated using nanosecond-picosecond two-color two-laser flash photolysis. The absorption and fluorescence spectra of X(*)(D(1)) were observed for the first time. The reactivity of X(*)(D(1)) toward a series of halogen donors and electron acceptors in acetonitrile and 1,2-dichloroethane (DCE) was investigated. It is confirmed that X(*)(D(1)) has a halogen abstraction ability from a series of halogen donors. On the basis of the solvent effect on the quenching rate constants of X(*)(D(1)), an electron transfer from X(*)(D(1)) to CCl(4) was indicated.

Photodissociation of naphthalene dimer radical cation during the two-color two-laser flash photolysis and pulse radiolysis-laser flash photolysis.

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.