DCM Molecules Research Articles

Effect of polar dopant on energetic and positional disorders in tris(8-hydroxyquinolinato) aluminum (Alq3)

The influence of polar dopant on the charge carrier transport in amorphous tris (8-hydroxyquinolinato) aluminum (Alq3) was studied by time-of-flight measurement. The 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was doped into Alq3 with various concentration from 0.5 to 24 wt. %. The electron mobility was reduced by about one order by DCM doping in Alq3. The electric-field dependence electron mobility in Alq3:DCM films separated into two discrete regions of critical fields Ec1/2. The value of Ec1/2ranged from 360 to 405 (V/cm)1/2 depending on the DCM concentration in Alq3 films. The energetic disorder in Alq3:DCM films increased from 0.01 eV to 0.09 eV with DCM doping concentration. The positional disorder in Alq3:DCM films also increased from 0.3 to 6.5 with DCM doping concentration up to 24 wt. %. These results indicated the strong Coulombic and dipole–dipole interactions between DCM and Alq3 molecules. The interactions between randomly located DCM molecules, Alq3 dipoles and oriented dipoles are the major caused of positional disorder.

Tunable Emission from Doped 1,3,5‐Triphenyl‐2‐pyrazoline Organic Nanoparticles

Tunable fluorescence emission from blue to red (see Figure) is exhibited by nanoparticles of 1,3,5-triphenyl-2-pyrazoline (TPP) doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM); the emission is found to be dependent on the amount of DCM dopant used. Efficient energy transfer is also shown to occur between TPP and DCM molecules.

Time-resolved transient electroluminescence measurements of emission from DCM-doped Alq 3 layers

Organic light-emitting devices (OLEDs) using NPB (α-naphthylphenylbiphenyl diamine) as the hole-transport layer and DCM [4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4H-pyran] doped Alq 3[tris-(8-hydroxyquinoline)] as the emission layer were studied using a time-resolved transient electroluminescence method. Upon application of a pseudo-rectangular voltage pulse, the luminance increased and overshot to maxima. Using suitable spectrum filters the overshoot luminance peaks were identified to originate solely from the DCM emission. When two consecutive voltage pulses are applied, the overshoot vanished during the second pulse when time gap is shorter than 1 ms. It was considered to be related to carrier traps in the DCM molecules.

Surface Properties of Surfactant-Free Oil Droplets Dispersed in Water Studied by Confocal Fluorescence Microscopy

The fluorescence spectrum of dye molecules, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyron (DCM), dissolved in surfactant-free n-decane droplets (average diameters of approximately 300 and approximately 2000 nm) dispersed in water was measured by a confocal microscope. The fluorescence spectra for 300- and 2000-nm droplets are found to exhibit a peak at 640 and 625 nm, respectively, and the peak red shifts with a decrease in the droplet diameter (solvatochromic shift of DCM molecules). It is concluded that (1) DCM molecules are located in a polar surface region of n-decane droplets and (2) the polarity increases with decreasing the droplet diameter.

Electron blocking and hole injection: The role of N,N′-Bis(naphthalen-1-y)-N,N′-bis(phenyl)benzidine in organic light-emitting devices

The current density-luminance-voltage characteristics of organic light-emitting devices (OLEDs) with N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB) of various thicknesses as the hole transport layer have been investigated. It is found that for conventional structures of indium–tin–oxide/NPB/tris(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/LiF (0.5 nm)/Al the optimal hole injection and luminescence efficiencies appear at NPB thicknesses of 5 and 20 nm, respectively. The large difference between the two optimal thicknesses suggests that the effective block of the NPB layer against electrons from across the Alq3/NPB interface is essential for high-efficiency operation of the OLEDs. The electron blocking effect of NPB is further confirmed by the electroluminescence (EL) behavior of devices with the structure of ITO/NPB(5 nm)/Alq3:4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) (30 nm)/NPB/Alq3(60 nm)/LiF(0.5 nm)/Al. The proportion of DCM EL to the whole EL decreases with increasing NPB thickness. This suggests that the NPB layer blocks electron transport to the Alq3:DCM layer. The Förster energy transfer from the 60 nm Alq3 layer to the DCM molecules is ruled out by the EL behavior observed after quenching excitons in the Alq3 layer. The origin of the difference in the optimal N,N′-Bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) thicknesses reported by other two different groups is also discussed.

Effect of electric fields on photoluminescence of 4-(dicyanomethylene)-2-methyl- 6-(p-dimethylaminostyryl)-4H-pyran

Electric-field-induced photoluminescent variation has been observed from the red-emitting dopant, 4-(di cyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). The layered structure of Al/tris-(8-hydroxyquinoline) aluminum (Alq)/DCM-doped Alq (Alq:DCM)/indium tin oxide studied, exhibits relatively enhanced red emission when a voltage is applied such that the electric field runs directly from the Alq:DCM layer to the Alq layer, and relatively enhanced green emission in the opposite direction of the field . This phenomenon is attributed to field-enhanced aggregation and permeation of DCM molecules due to the polarity of the molecule.

Spectroscopy and dynamics of DCM encapsulated in MCM-41 and Y zeolite mesoporous materials

The laser dye 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4H-pyran (referred to as DCM) was encapsulated in mesoporous MCM-41 and Y-zeolite. The resultant composites were studied by X-ray diffraction, UV-Vis absorption and emission spectroscopies, and fluorescence lifetime spectrophometry at room temperature. The interactions between occluded DCM and the internal surfaces of the porous materials are found to modify the optical spectra and dynamics of the confined DCM molecules. In particular, two fluorescence bands are observed for DCM occluded within MCM-41, while one fluorescence band is observed for DCM occluded within Y-zeolite or dissolved in DMSO. The dual bands are attributed to emissions from the lowest excited (LE) state and a state created as a result of intramolecular charge transfer, specifically, the twisted intramolecular charge transfer (TICT) state. The decay lifetimes for the LE and TICT states are found to be 0.6±0.1 and 1.9±0.1 ns, respectively, with 480 nm excitation; the decay lifetimes for 514 excitation are essentially the same. The single emissions from the DCM/Y-zeolite composite and the homogeneous DMSO solution show single exponential decays with lifetimes of 1.3±0.1 and 2.0±0.1 ns, respectively, both assigned to the TICT state.

Aggregation and permeation of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran molecules in Alq

The morphologic and luminescent behaviors of various 4-(dicyanomethylene)-2methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) containing films have been investigated. This involves pure DCM layers deposited on top of a tris-(8-hydroxyquinoline) aluminum (Alq) layer or positioned between two Alq layer, DCM-doped Alq films, and periodically arranged Alq/DCM multilayer structures. The occurrence over a period of time of aggregation and permeation of DCM molecules at room temperature and at temperatures as low as ∼260 K is found in all the cases studied. Such a phenomenon will result in degradation of related organic light-emitting devices and is closely related to the electric polarity of the DCM molecule.

In situ photoluminescence investigation of doped Alq

We report the photoluminescence (PL) properties measured in situ from vacuum-deposited organic films of tris-(8-hydroxyquinoline) aluminum (Alq) doped with 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran (DCM), where the red emission from the guest molecules is due to Förster energy transfer of excited state energy from host to guest. Both bare DCM-doped Alq (Alq:DCM) and bilayer Alq/Alq:DCM films have been studied, with the thickness of the Alq overlayer continuously varied in the latter case. The PL spectra from the bilayer structure contain no Alq contribution when its thickness is below 2.4 nm. Taking the value as the maximum distance for which the Alq exciton can travel in the film and still transfer its energy to a DCM molecule, the minimum DCM concentration in Alq:DCM necessary to produce red emission only can be estimated at 0.31 wt %. The most efficient red emission appears at the DCM concentration of about 1.7 wt %, at which more than 90% Alq-originated excitons are involved in the Förster energy transfer process. There exists a gradual red shift of the PL spectra with increasing DCM concentration, which can be attributed to the polarization and accompanying aggregation tendency of DCM molecules in Alq during the formation of the doped films.

Picosecond amplified spontaneous emission bursts from a molecularly doped organic semiconductor

We report the dynamics of amplified spontaneous emission (ASE) in thin organic films of tris-(8-hydroxyquinoline)-aluminum (Alq3) doped with small amounts of the laser dye 4-dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). The energy transfer from the initially photoexcited Alq3 to the DCM molecules affects the high excitation density dynamics significantly. The time delay between pulsed photoexcitation and the ASE burst depends on the pumping level. For higher pumping levels the competition between depopulation by ASE and refilling of the DCM states via energy transfer leads to an oscillatory emission with a fundamental frequency of up to 0.2 THz.

CONTROL OF THE ENERGY TRANSFER WITH THE OPTICAL MICROCAVITY

We present the first experimental observation that dipole-dipole interaction can be strongly enhanced by placing the system in a microcavity. We have studied the excitation energy transfer in poly(phenyl-p-phenylene vinylene) (PPPV) doped with DCM molecules, placed within a Fabry-Perot resonator. As the spectral position of the cavity resonant mode in tuned across the DCM absorption profile, the transfer efficiency from PPPV to DCM changes dramatically as revealed by photoluminescence (PL) spectra. This behavior is clear evidence for the increase of the dipole-dipole interaction strength at the cavity resonances mediated by propagating modes emitted from the excited dipoles.

Transient absorption experimental set-up with femtosecond time resolution. Femto- and picosecond study of DCM molecule in cyclohexane and methanol solution

The experimental set-up for time dependent transient absorption/gain measurements with femtosecond time resolution is presented. Using pump-probe spectroscopy technique with femtosecond pulses from Ti:Sapphire laser system (Spectra Physics), the optical density of transient absorption and transient gain in the spectral range of 330–700nm, time resolution of 120fs, can be measured with the precision of up to 0.005. Knowing the real time resolution and “zero time” dispersion of the system from two-photon absorption measurements, we performed femto- and picosecond study of electronically excited DCM molecule in c-C6H12 and MeOH solution. The emission decay of the DCM molecule in the locally excited singlet state, S1-LE, was measured with picosecond time resolution using TCSPC set-up (Time Correlated Single Photon Counting). The steady-state and time resolved measurements permitted the determination of the properties of this locally excited singlet S1-LE state and the pathways of its deactivation.

Solvation dynamics of 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4H-pyran (DCM) in a microemulsion

The photophysical process of the laser dye 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4H-pyran (DCM) is studied in aerosol-OT (AOT) microemulsions in n-heptane using picosecond spectroscopy. When AOT and water are added to a solution of DCM in n-heptane, some of the DCM molecules migrate from bulk n-heptane to the water pool. The absorption and emission peaks of the DCM molecules in the polar water pool are markedly red shifted from those in the bulk n-heptane and the emission intensity in the water pool is nearly 40 times higher. Dual emission is not observed in the microemulsions. DCM exhibits slow solvation dynamics in the water pool with an average solvation time of 1.23 ns.

Application of energy transfer model to partially DCM-doped Alq3 light-emitting diode

Theelectroluminescent (EL) properties of partially-DCM doped Alq3 (tris(8-quinoline) aluminum) light-emitting diodes (LEDs) are discussed on the basis of a simple energy transfer model due to exciton diffusion. The EL process is described as the product of the distribution of exciton density of Alq3 and the radiative probability of excited molecule. Using the separated EL intensities from Alq3 and DCM, the various ratios of EL intensities calculated from the model were compared with the experimental results. The diffusion length of the Alq3 exciton was estimated to be 20 nm. The radiative probability of DCM-doped Alq3 was found to be 4.4 times higher than that of Alq3. The ratio of the former to the latter almost agreed with the ratio of the quantum photoluminescent yield. The doped DCM molecules are speculatd to suppress the generation of the Alq3 exciton. The model is very simple, but explains the practical EL characteristics in organic dye-doped Alq3 LEDs. © 1997 John Wiley & Sons, Ltd.