V. Jankus
The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N-'-dicarbazolyl-1,1(')-biphenyl
Jankus, V.; Winscom, C.; Monkman, A.P.
Abstract
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.
Citation
Jankus, V., Winscom, C., & Monkman, A. (2009). The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N-'-dicarbazolyl-1,1(')-biphenyl. The Journal of Chemical Physics, 130(7), Article 074501. https://doi.org/10.1063/1.3077163
Journal Article Type | Article |
---|---|
Publication Date | Feb 1, 2009 |
Deposit Date | Feb 16, 2012 |
Publicly Available Date | Oct 24, 2012 |
Journal | Journal of Chemical Physics |
Print ISSN | 0021-9606 |
Electronic ISSN | 1089-7690 |
Publisher | American Institute of Physics |
Peer Reviewed | Peer Reviewed |
Volume | 130 |
Issue | 7 |
Article Number | 074501 |
DOI | https://doi.org/10.1063/1.3077163 |
Keywords | Excited states, Fluorescence, Organic compounds, Phosphorescence, Spectral line intensity, Time resolved spectra |
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Copyright Statement
© 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Jankus, V. and Winscom, C. and Monkman, A.P. (2009) 'The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N-'-dicarbazolyl-1,1(')-biphenyl.', Journal of chemical physics., 130 (7). 074501 and may be found at http://dx.doi.org/10.1063/1.3077163
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