Kopczynski Matthäus, Ehlers Florian, Lenzer Thomas, Oum Kawon
Institut für Physikalische Chemie, Universität Göttingen, Tammannstrasse 6, D-37077 Göttingen, Germany.
J Phys Chem A. 2007 Jun 28;111(25):5370-81. doi: 10.1021/jp0672252. Epub 2007 Jun 6.
The ultrafast excited-state dynamics of two carbonyl-containing carotenoids, 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al, have been investigated by transient absorption spectroscopy in a systematic variation of solvent polarity and temperature. In most of the experiments, 12'-apo-beta-caroten-12'-al was excited at 430 nm and 8'-apo-beta-caroten-8'-al at 445 or 450 nm via the S0 --> S2 (11Ag- --> 11Bu+) transition. The excited-state dynamics were then probed at 860 nm for 12'-apo-beta-caroten-12'-al and at 890 or 900 nm for 8'-apo-beta-caroten-8'-al. The temporal evolution of all transient signals measured in this work can be characterized by an ultrafast decay of the S2 --> SN absorption at early times followed by the formation of a stimulated emission (SE) signal, which subsequently decays on a much slower time scale. We assign the SE signal to a low-lying electronic state of the apocarotenals with intramolecular charge-transfer character (ICT --> S0). This is the first time that the involvement of an ICT state has been detected in the excited-state dynamics of a carbonyl carotenoid in nonpolar solvents such as n-hexane or i-octane. The amplitude ratio of ICT-stimulated emission to S2 absorption was weaker in nonpolar solvents than in polar solvents. We interpret the results in terms of a kinetic model, where the S1 and ICT states are populated from S2 through an ultrafast excited-state branching reaction (tau2 < 120 fs). Delayed formation of a part of the stimulated emission is due to the transition S1 --> ICT (tau3 = 0.5-4.1 ps, depending on the solvent), which possibly involves a slower backward reaction ICT --> S1. Determinations of tau1 were carried out for a large set of solvents. Especially in 12'-apo-beta-caroten-12'-al, the final SE decay, assigned to the nonradiative relaxation ICT --> S0, was strongly dependent on solvent polarity, varying from tau1 = 200 ps in n-hexane to 6.6 ps in methanol. In the case of 8'-apo-beta-caroten-8'-al, corresponding values were 24.8 and 7.6 ps, respectively. This indicates an increasing stabilization of the ICT state with increasing solvent polarity, resulting in a decreasing ICT-S0 energy gap. Tuning the pump wavelength from the blue wing to the maximum of the S0 --> S2 absorption band resulted in no change of tau1 in acetone and methanol. Additional measurements in methanol after excitation in the red edge of the S0 --> S2 band (480-525 nm) also show an almost constant tau1 with only a 10% reduction at the largest probe wavelengths. The temperature dependence of the tau1 value of 12'-apo-beta-caroten-12'-al was well described by Arrhenius-type behavior. The extracted apparent activation energies for the ICT --> S0 transitions were in general small (on the order of a few times RT), which is in the range expected for a radiationless process.
通过瞬态吸收光谱法，在系统改变溶剂极性和温度的条件下，研究了两种含羰基类胡萝卜素12'-脱辅基-β-胡萝卜素-12'-醛和8'-脱辅基-β-胡萝卜素-8'-醛的超快激发态动力学。在大多数实验中，12'-脱辅基-β-胡萝卜素-12'-醛通过S0→S2（11Ag-→11Bu+）跃迁在430nm处被激发，8'-脱辅基-β-胡萝卜素-8'-醛在445或450nm处被激发。然后在860nm处探测12'-脱辅基-β-胡萝卜素-12'-醛的激发态动力学，在890或900nm处探测8'-脱辅基-β-胡萝卜素-8'-醛的激发态动力学。在这项工作中测量的所有瞬态信号的时间演化可以通过早期S2→SN吸收的超快衰减来表征，随后形成受激发射（SE）信号，该信号随后在慢得多的时间尺度上衰减。我们将SE信号归因于具有分子内电荷转移特征（ICT→S0）的脱辅基类胡萝卜素的低电子态。这是首次在正己烷或异辛烷等非极性溶剂中羰基类胡萝卜素的激发态动力学中检测到ICT态的参与。非极性溶剂中ICT受激发射与S2吸收的振幅比低于极性溶剂。我们根据动力学模型解释结果，其中S1和ICT态通过超快激发态分支反应（τ2<120fs）从S2填充。受激发射的一部分延迟形成是由于S1→ICT跃迁（τ3=0.5-4.1ps，取决于溶剂），这可能涉及较慢的反向反应ICT→S1。对大量溶剂进行了τ1的测定。特别是在12'-脱辅基-β-胡萝卜素-12'-醛中，归因于非辐射弛豫ICT→S0的最终SE衰减强烈依赖于溶剂极性，从正己烷中的τ1=200ps到甲醇中的6.6ps不等。在8'-脱辅基-β-胡萝卜素-8'-醛的情况下，相应的值分别为24.8和7.6ps。这表明随着溶剂极性的增加，ICT态的稳定性增加，导致ICT-S0能隙减小。将泵浦波长从蓝翼调至S0→S2吸收带的最大值，在丙酮和甲醇中τ1没有变化。在S0→S2带的红边（480-525nm）激发后在甲醇中的额外测量也显示τ1几乎恒定，在最大探测波长处仅降低10%。12'-脱辅基-β-胡萝卜素-12'-醛的τ1值的温度依赖性可以用阿伦尼乌斯型行为很好地描述。提取的ICT→S0跃迁的表观活化能通常较小（约为RT的几倍），这在无辐射过程预期的范围内。
