Spirally Configured ( cis-Stilbene) Trimers: Steady-State and Time-Resolved Photophysical Studies and Organic Light-Emitting Diode Applications.

This article reports for the time-resolved photophysical studies of spirally configured ( cis-stilbene) trimers and their spin-coated organic light-emitting diode (OLED) device performances. Transient absorption profiles of spirally configured, ter-( cis-stilbene) were studied by pulse radiolysis. The emission profiles after charge recombination of their incipient radical ions in benzene provides insights into the emission mechanism and efficiency in OLED devices.

Emission mechanism of doubly ortho-linked quinoxaline/diphenylfluorene or cis-stilbene/fluorene hybrid compounds based on the transient absorption and emission measurements during pulse radiolysis.

A series of bipolar OLED materials were subjected to pulsed radiolysis experiments to determine their transient absorption and lifetime profiles of the independently in situ generated radical cations and anions in solutions. Moreover, their emission behaviors from the charge recombination of their radical ions were also determined by the pulse radiolysis method. It was found the absorption bands in doubly ortho-linked quinoxaline/diphenylfluorene hybrids 1a-e are red-shifted progressively with increasing electron-donating nature at the C5 and C8 positions of the quinoxaline template.

Emission from regioisomeric bis(phenylethynyl)benzenes during pulse radiolysis.

Emission from charge recombination between radical cations and anions of a series of regioisomeric 1,4-, 1,3-, and 1,2-bis(phenylethynyl)benzenes (bPEBs) substituted by various electron donor and/or acceptor groups was measured during pulse radiolysis in benzene (Bz). The formation of bPEB in the excited singlet state ((1)bPEB*) can be attributed to the charge recombination between bPEB(*+) and bPEB(*-), which are initially generated from the radiolytic reaction. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (-DeltaH(o)) for the charge recombination of bPEB(*+) and bPEB(*-) and excitation energy of (1)bPEB*.

Properties of excited radical cations of substituted oligothiophenes.

Excited-state properties of radical cations of substituted oligothiophenes ( nT (*+), n denotes the number of thiophene rings, n = 3, 4, 5) in solution were investigated by using various laser flash photolysis techniques including two-color two-laser flash photolysis. nT (*+) generated by photoinduced electron transfer to p-chloranil or resonant two-photon ionization (RTPI) by using the first 355-nm ns laser irradiation was selectively excited with the second picosecond laser (532 nm). Bleaching of the absorption of nT (*+) together with growth of a new absorption was observed during the second laser irradiation, indicating the generation of nT (*+) in the excited state ( nT (*+)*).

Fine-tuning of radiolysis induced emission by variable substitution of donor-/acceptor-substituted tetrakis(arylethynyl)benzenes.

Emission from charge recombination between radical cations and anions of various tetrakis(arylethynyl)benzenes (TAEBs) was measured during pulse radiolysis in benzene (Bz). The formation of TAEB in the excited singlet state ((1)TAEB*) can be attributed to the charge recombination between TAEB (*+) and TAEB (*-), which is initially generated from the radiolytic reaction. It was found that the charge recombination between TAEB (*+) and TAEB (*-) gave (1)TAEB* as the emissive species but not excimers because of the large repulsion between substituents caused by the rotation around C-C single bonds.

Donor-acceptor-substituted tetrakis(phenylethynyl)benzenes as emissive molecules during pulse radiolysis in benzene.

Emission from charge recombination between radical cations and anions of various tetrakis(phenylethynyl)benzenes (TPEBs) was measured during pulse radiolysis in benzene (Bz). The formation of TPEB in the singlet excited state (1TPEB*) can be attributed to the charge recombination between TPEB*+ and TPEB*-, which are initially generated from the radiolytic reaction in Bz. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (-DeltaH degrees) for the charge recombination of TPEB*+ and TPEB*- and excitation energy of 1TPEB*.

Emission from charge recombination during the pulse radiolysis of 9-cyano-10-phenylethynylanthracenes with donor and acceptor substituents.

Emission from 9-cyano-10-phenylanthracene and 9-cyano-10-phenylethynylanthracenes having donor and acceptor substituents (RA = PA, PEA, OEA, NEA, and DEA) was studied with the time-resolved fluorescence measurement during the pulse radiolysis of RAs in benzene (Bz). PA and DEA showed only monomer emission, while other RAs (PEA, OEA, and NEA) showed both monomer and excimer emissions with much lower intensities. On the basis of the steady-state and transient absorption and emission measurements, the formation of RA in the singlet excited state ((1)RA*) can be attributed to the charge recombination between RA radical cation and anion (RA*+ and RA*-, respectively) which are initially generated from the radiolytic reaction in Bz.

Inhibition of the formation and decay of stilbene core radical cations by the dendron during the photoinduced electron transfer.

Formation and decay processes of stilbene core radical cation (ST*+) during the photoinduced electron transfer have been studied for a series of stilbene bearing benzyl ether-type dendrons (D). ST*+ and the radical cation of peripheral dendron (D*+) were generated by intermolecular hole transfer from biphenyl radical cation, which was generated from photoinduced electron transfer from biphenyl to the singlet-excited 9,10-dicyanoanthracene in a mixture of acetonitrile and 1,2-dichloroethane (3:1). An intramolecular dimer radical cation of benzyl groups at the terminal of stilbene dendrimer was indicated as a hole trapping site.

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent substituted quinolines with donor-acceptor character.

Efficient emission from various donor-acceptor quinolines with an ethynyl linkage (PnQ), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PnQ in the singlet excited state can be interpreted by charge recombination between the PnQ radical cation and the PnQ radical anion which are generated initially from the radiolytic reaction in benzene. The strong electronic coupling between the donor and acceptor through conjugation is responsible for the efficient emission during the pulse radiolysis of PnQ in benzene.

Formation of highly stabilized intramolecular dimer radical cation and pi-complex of [3n]cyclophanes (n = 3, 5, 6) during pulse radiolysis.

Formation of radical cation and charge-transfer complex of [3n]cyclophanes (n = 3, 5, 6) was investigated by transient absorption spectroscopy during pulse radiolysis. Radical cations of [3n]cyclophanes showed the charge resonance band around 700 nm which exhibited a blue-shift as the number of trimethylene bridges increased, indicating formation of highly stabilized intramolecular dimer radical cation of [3n]cyclophanes. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accord with the oxidation potential of [3n]cyclophanes.

Importance of properties of the lowest and higher singlet excited states on the resonant two-photon ionization of stilbene and substituted stilbenes using two-color two-lasers.

Radical cations of trans-stilbene and substituted trans-stilbenes (stilbenes and the radical cations denote Sand S(*+), respectively) were generated from the resonant two-photon ionization (TPI) in acetonitrile with irradiation of one-laser (266- or 355-nm laser) and with simultaneous irradiation of two-color two-lasers (266- and 532-nm or 355- and 532-nm lasers) with the pulse width of 5 ns each. The formation yields of S(*+), the TPI efficiency, depended on the properties of S in the lowest and higher singlet excited state (S(S(1)) and S(S(n))), generated from one-photon excitation with 266- or 355-nm laser and from two-photon excitation with simultaneous irradiation of 266- and 532-nm or 355- and 532-nm lasers, respectively. The TPI efficiency using two-color two-lasers increased compared with that using one-laser.

Emission from charge recombination during the pulse radiolysis of arylethynylpyrenes.

Emission from several 1-(arylethynyl)pyrenes with a substituent on the aryl group (REPy, R = phenyl (PEPy), 4-dimethylaminophenyl (NPEPy), 4-isopropoxyphenyl (OPEPy), 2-quinonyl (QEPy), and 9-(10-cyanoanthracenyl) (AEPy)) was studied with time-resolved fluorescence measurements during pulse radiolysis in benzene. NPEPy and AEPy showed only monomer emission, while PEPy, OPEPy, and QEPy showed both monomer and excimer emissions during pulse radiolysis. In addition, REPy's also showed long-lived emissions with very weak intensities in the absence of oxygen, which were assigned to the "P-type" delayed fluorescence derived from the triplet-triplet annihilation.

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent donor-acceptor molecules with an ethynyl linkage.

Efficient monomer and excimer emission from various donor-acceptor substituted phenylethynes (PE), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PE in the singlet excited state (1PE*) and the excimer (1PE2*) can be interpreted by the charge recombination between the PE radical cation (PE.+) and the PE radical anion (PE.

Effect of oxygen on the formation and decay of stilbene radical cation during the resonant two-photon ionization.

Formation and decay of radical cations of trans-stilbene and p-substituted trans-stilbenes (S.+) during the resonant two-photon ionization (TPI) of S in acetonitrile in the presence and absence of O(2) have been studied with laser flash photolysis using a XeCl excimer laser (308 nm, fwhm 25 ns). The transient absorption spectra of S.

Dihydrophenanthrene-type intermediates during photoreaction of trans-4'-benzyl-5-styrylfuran.

[structure: see text] Photoreaction of trans-4'-benzyl-5-styrylfuran (trans-BSF) has been studied by the 355-nm laser flash photolysis (LFP) in CH2Cl2 using a Nd3+:YAG laser (30 ps, 5 mJ pulse(-1) or 5 ns, 30 mJ pulse(-1)). Transient fluorescence and absorption spectra assigned to the singlet excited trans-BSF were observed during the 30-ps LFP, whereas a transient absorption spectrum with two peaks at 400 and 510 nm, assigned to the trans-fused dihydrophenanthrene (DHP)-type intermediate (DP1), was observed during the 5-ns LFP. It is clearly suggested that a two-photon absorption process is involved in the formation of DP1.

Effects of benzyl ether type dendrons as hole-harvesting antennas, and shielding for the neutralization of stilbene core radical cations with chloride ion during two-photon ionization of stilbene dendrimers having stilbene core and benzyl ether type dendrons.

The two-photon ionization (TPI) process (308 and 266 nm) of stilbene dendrimers having a stilbene core and benzyl ether type dendrons has been investigated in an acetonitrile and 1,2-dichloroethane mixture (3:1) in order to elucidate the dendrimer effects. The quantum yield of the formation of stilbene core radical cation during the 308-nm TPI was independent of the dendron generation of the dendrimers, whereas a generation dependence of the quantum yield of the radical cation was observed during the 266-nm TPI, where both the stilbene core and benzyl ether type dendron were ionized, suggesting that the subsequent hole transfer occurs from the dendron to the stilbene core, and that the dendron acts as a hole-harvesting antenna. The neutralization rate of the stilbene core radical cation with the chloride ion, generated from the dissociative electron capture by 1,2-dichloroethane, decreased with the increase in the dendrimer generation, suggesting that the dendron is an effective shield of the stilbene core radical cation against the chloride ion.