Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission
Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission
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DOI：
10.1063/1.3495764
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发表时间：
2010-10-14
影响因子：
4.4
通讯作者：
Bardeen, Christopher J.
中科院分区：
文献类型：
--
作者：
Burdett, Jonathan J.;Mueller, Astrid M.;Bardeen, Christopher J.
利用皮秒荧光和飞秒瞬态吸收研究并四苯多晶薄膜中的激发态动力学。将固态结果与稀溶液中单体并四苯的结果进行比较。室温下固态荧光衰减与考虑激子 - 激子湮灭和激子裂变的早期模型一致，但延迟荧光寿命缩短，为20 - 100纳秒，而单晶中为2微秒或更长。对溶液中单体的飞秒瞬态吸收测量揭示了几个与基态漂白和受激发射信号重叠的激发态吸收特征。在较长时间尺度上，由于系间窜越，最初激发的单重态完全衰减，并且观察到叠加在漂白信号上的三重态吸收，这与早期的闪光光解实验一致。在固态中，瞬态吸收动力学主要由一个负的受激发射信号主导，其以9.2皮秒的时间常数衰减。固态中增强的漂白和受激发射信号归因于一个超辐射的、离域的S - 1态，该态迅速裂变成三重态，并且还能产生第二个超辐射态，很可能是一个晶体缺陷，它主导皮秒发光信号。S - 0→S - 1跃迁吸收强度的增强以及我们多晶薄膜的部分取向性质，掩盖了较弱的T - 1→T - N吸收特征。为了确认三重态是最初激发态弛豫产生的主要物种，对延迟荧光和基态漂白恢复进行了比较。它们相同的衰减与三重态在陷阱或缺陷位点的扩散和复合一致。结果表明，为了全面描述并四苯薄膜的光物理行为，必须考虑诸如激子离域、发光缺陷位点的存在以及微晶取向等复杂因素。实验结果与并四苯的光动力学由激子裂变和三重态复合主导的传统观点一致，但表明裂变在10皮秒内发生，比以前认为的要快得多。（C）2010美国物理学会。[doi:10.1063/1.3495764]
The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20-100 ns as opposed to 2 mu s or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are dominated by a negative stimulated emission signal, decaying with a 9.2 ps time constant. The enhanced bleach and stimulated emission signals in the solid are attributed to a superradiant, delocalized S-1 state that rapidly fissions into triplets and can also generate a second superradiant state, most likely a crystal defect, that dominates the picosecond luminescence signal. The enhanced absorption strength of the S-0 -> S-1 transition, along with the partially oriented nature of our polycrystalline films, obscures the weaker T-1 -> T-N absorption features. To confirm that triplets are the major species produced by relaxation of the initially excited state, the delayed fluorescence and ground state bleach recovery are compared. Their identical decays are consistent with triplet diffusion and recombination at trapping or defect sites. The results show that complications like exciton delocalization, the presence of luminescent defect sites, and crystallite orientation must be taken into account to fully describe the photophysical behavior of tetracene thin films. The experimental results are consistent with the traditional picture that tetracene's photodynamics are dominated by exciton fission and triplet recombination, but suggest that fission occurs within 10 ps, much more rapidly than previously believed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3495764]