Dynamics of triplet migration in films of N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine.

We study triplet migration properties in NPB (N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine) films using time resolved gated spectroscopy and dispersive migration theory as our main tools of analysis. We show that in NPB, a well-known hole transporter in organic light emitting diodes, at high excitation densities triplet migration follows two regimes--a dispersive non-equilibrium regime (distinguished by exciton energetical relaxation within the distribution of hopping sites and as a consequence the hopping frequency being time dependent) that evolves into a second, non-dispersive equilibrium regime. Further, we observe a third region, which we term acceleration. From the turning over time between dispersive and non-dispersive dynamics, we deduce the width of the triplet density of states (DOS). We observe how the DOS variance changes when one decreases the thickness of the NPB film and note how surface effects are becoming important. Furthermore, the DOS variance of NPB changes when another organic layer is evaporated on top, namely Ir(piq)3 (tris(1-phenylisoquinoline)iridium(III)). We believe that these changes are due to the different polarizable media in contact with the NPB film, either vacuum or Ir(piq)3. We also show in this paper that the triplet level when time approaches zero is much higher in energy than the relaxed triplet levels, as quoted in most published papers; these values are thus incorrect for NPB. Lastly, it is possible that even at room temperature, the dispersive regime might be important for triplet migration at high initial triplet concentrations and might affect the diffusion length of triplets to a certain extent. However, more experimentation needs to be performed in order to address this question. Overall, we have characterized the triplet migration dynamics of NPB fully and shown that it agrees with previously published observations for other organic semiconductors and theoretical considerations.

The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N(')-dicarbazolyl-1,1(')-biphenyl.

In this paper we report the results of optical characterization of 4,4-N,N(')-dicarbazolyl-1,1(')-biphenyl (CBP), known as a host material for phosphorescent light emitting devices. Using absorption, steady state, and time-resolved spectroscopy, we explore the singlet and triplet states in solid and solution samples of CBP. In solutions we observe two distinct short-lived states with well-resolved emission originating from individual molecule singlet states (at 365 and 380 nm) and "quenching" low energy (LE) states (at 404 and 424 nm). The latter are seen only in saturated solutions and solid samples. Both of those species have different lifetimes. After UV exposure of very concentrated degassed solution the intensities of the LE bands starts to decrease. The longer the solution is exposed to UV, the less emission is seen at 404 and 424 nm, until it is totally gone. The spectrum of the highly concentrated solution is then the same as the spectrum of dilute solution, i.e., only emission at 365 and 380 nm is present. An increase in intensities of the singlet emission peaks correlates with an increase in UV exposure time. Similar behavior is observed in evaporated CBP film. We propose that this behavior is due to chemical instability of the weak N-C bonding of carbazolyl moiety-this creates new degradational species over time which dissociate after exposure to UV. We believe this to be the reason for variation in CBP fluorescence and delayed fluorescence spectra recorded by various research groups. Further, we detected two types of very long-lived states. One of these states (higher energy) is ascribed to molecular phosphorescence emission, the other to emission from low energy triplet trap states which we relate to degradational species. We propose that triplets are more easily caught by these latter sites when their hopping rate increases, and they emit inefficiently from these lower energy sites.

Triplet energies and the singlet oxygen quenching mechanism for 7H-pyrazolo[5,1-c]-1,2,4-triazole azomethine dyes.

Analysis of triplet energy transfer rate constants gives the triplet energies of six 7H-pyrazolo[5,1-c]-1,2,4-triazole azomethine dyes, with lambda max values in the range 546-633 nm in ethanol, to lie in the range 115-88 kJ mol-1. Energy transfer rates from porphyrin and phthalocyanine sensitisers can be well approximated using the Balzani equation with a zero or small reorganization energy, and a transmission coefficient ca. 1/1000 that of the fully adiabatic value. A comparison of data on triplet energies of azomethine dyes suggests a relationship between the dye absorption energies and triplet energies of the form: ET = 0.69(+/- 0.04)(E lambda max)-33(+/- 9) kJ mol-1. A detailed study of the quenching of 1O2* by one of the dyes shows that this reaction is accompanied by isomerisation of the dye. This is interpreted as strong indirect evidence for an energy transfer mechanism for the process, a conclusion which is supported in a general way by the value of the 1O2* quenching rate constant.