Methylene Blue Exciton States Steer Nonradiative Relaxation: Ultrafast Spectroscopy of Methylene Blue Dimer
Methylene Blue Exciton States Steer Nonradiative Relaxation: Ultrafast Spectroscopy of Methylene Blue Dimer
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DOI：
10.1021/acs.jpcb.5b11847
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发表时间：
2016-01-28
影响因子：
3.3
通讯作者：
Scholes, Gregory D.
中科院分区：
文献类型：
--
作者：
Dean, Jacob C.;Oblinsky, Daniel G.;Scholes, Gregory D.
亚甲基蓝(MB)的光化学和聚集特性使其在光动力疗法中得到了广泛的应用。然而，在浓水溶液中容易形成强耦合的“面对面”H-聚集体，显著改变了它的光谱。最简单的MB聚集体MB2在飞秒到纳秒的时间尺度上被研究，揭示了连续的内部转换事件，完全松弛了激发的14布居。为了进行直接比较，对MB单体的动力学进行了串联分析。第一，超快内转换特性和光物理。用宽带暂态吸收(BBTA)和二维电子能谱(2DES)研究了从电偶极子允许的上激子态到下禁止激子态的光引发动力学，时间分辨率接近10fs。对298K和77K的MB和MB2 2DES谱带的线型分析表明，二聚体的对角/对角线宽比没有明显差异，而MB的均匀线宽明显减小。这一结果被解释为超快布居驰豫，对均匀的线宽施加了限制，而不是像单体那样纯粹的退相。在目标分析的帮助下，进行了窄带瞬时吸收，以模拟较长时间的动力学。MB的动力学由一个顺序模型描述，其特征是振动弛豫(1-10ps)，然后是系间交叉和内部转换(tau类似于370ps)，留下MB三重态物种。或者，二聚体动力学在内部完全熄灭，类似于10ps，产生3-4ps的基态恢复时间。这种对基态的快速而完全的驰豫表明，当单体接近时，浓度猝灭的效果。激子态的形成引入了初始能量漏斗，最终导致布居弛豫到基态，即使内部能量远高于其结合能，也防止了二聚体的解离。
The photochemistry and aggregation properties of methylene blue (MB) lead to its popular use in photodynamic therapy. The facile formation of strongly coupled "face-to-face" H-aggregates in concentrated aqueous solution, however, significantly changes its spectroscopics the simplest MB aggregate, MB2, was investigated over femtosecond to nanosecond time scales revealing sequential internal conversion events that fully relax the excited 1 4 population. MB monomer dynamics were analyzed in tandem for a direct comparison. First, ultrafast internal conversion properties and photophysics. The photoinitiated dynamics of from the electric-dipole allowed upper exciton state to the lower forbidden exciton state was evaluated by use of broadband transient absorption (BBTA) and two-dimensional electronic spectroscopy (2DES) with a time resolution of similar to 10 fs. Lineshape analysis of MB and MB2 2DES bands at 298 and 77 K show effectively no difference in-the diagonal/antidiagonal line width ratio for the dimer, in marked contrast to the distinct reduction of the homogeneous line width for MB. This result is interpreted as ultrafast population relaxation imposing a limitation to the homogeneous line width, instead of pure dephasing as in the case of the monomer. Narrowband transient absorption was performed with the aid of target analysis, to model the dynamics at longer times. The MB dynamics were described by a sequential model featuring vibrational relaxation (1-10 ps) followed by intersystem crossing and internal conversion (tau similar to 370 ps) leaving behind MB triplet species. Alternatively, the dimer dynamics were entirely quenched within,similar to 10 ps, yielding a ground state recovery time of 3-4 ps. Such fast and complete relaxation to the ground state demonstrates the effect of concentration quenching when monomers are brought into close proximity. The formation of exciton states introduces an initial energy funnel that eventually leads to population relaxation to the ground state, preventing even the dissociation of dimers despite having internal energies well above its binding energy.