Ehlers Florian, Scholz Mirko, Schimpfhauser Jens, Bienert Jürgen, Oum Kawon, Lenzer Thomas
Georg-August-Universität Göttingen, Institut für Physikalische Chemie, Tammannstr. 6, 37077 Göttingen, Germany.
Phys Chem Chem Phys. 2015 Apr 28;17(16):10478-88. doi: 10.1039/c4cp05600k.
In recent work, we demonstrated that the S* signal of β-carotene observed in transient pump-supercontinuum probe absorption experiments agrees well with the independently measured steady-state difference absorption spectrum of vibrationally hot ground state molecules S0* in solution, recorded at elevated temperatures (Oum et al., Phys. Chem. Chem. Phys., 2010, 12, 8832). Here, we extend our support for this "vibrationally hot ground state model" of S* by experiments for the three terminally aldehyde-substituted carotenes β-apo-12'-carotenal, β-apo-4'-carotenal and 3',4'-didehydro-β,ψ-caroten-16'-al ("torularhodinaldehyde") which were investigated by ultrafast pump-supercontinuum probe spectroscopy in the range 350-770 nm. The apocarotenals feature an increasing conjugation length, resulting in a systematically shorter S1 lifetime of 192, 4.9 and 1.2 ps, respectively, in the solvent n-hexane. Consequently, for torularhodinaldehyde a large population of highly vibrationally excited molecules in the ground electronic state is quickly generated by internal conversion (IC) from S1 already within the first picosecond of relaxation. As a result, a clear S* signal is visible which exhibits the same spectral characteristics as in the aforementioned study of β-carotene: a pronounced S0 → S2 red-edge absorption and a "finger-type" structure in the S0 → S2 bleach region. The cooling process is described in a simplified way by assuming an initially formed vibrationally very hot species S0** which subsequently decays with a time constant of 3.4 ps to form a still hot S0* species which relaxes with a time constant of 10.5 ps to form S0 molecules at 298 K. β-Apo-4'-carotenal behaves in a quite similar way. Here, a single vibrationally hot S0* species is sufficient in the kinetic modeling procedure. S0* relaxes with a time constant of 12.1 ps to form cold S0. Finally, no S0* features are visible for β-apo-12'-carotenal. In that case, the S1 → S0 IC process is expected to be roughly 20 times slower than S0* relaxation. As a result, no spectral features of S0* can be found, because there is no chance that a detectable concentration of vibrationally hot molecules is accumulated.
在最近的工作中，我们证明了在瞬态泵浦 - 超连续谱探测吸收实验中观察到的β - 胡萝卜素的S信号，与在高温下记录的溶液中振动热基态分子S0的独立测量稳态差分吸收光谱非常吻合（Oum等人，《物理化学化学物理》，2010年，12卷，8832页）。在此，我们通过对三种末端醛取代的类胡萝卜素β - 阿朴 - 12'- 胡萝卜醛、β - 阿朴 - 4'- 胡萝卜醛和3'，4'- 二脱氢 - β，ψ - 胡萝卜 - 16'- 醛（“球孢红醛”）进行实验，扩展了对这种S的“振动热基态模型”的支持，这些类胡萝卜素通过350 - 770 nm范围内的超快泵浦 - 超连续谱探测光谱进行了研究。阿朴类胡萝卜醛具有增加的共轭长度，导致在溶剂正己烷中S1寿命分别系统地缩短为192、4.9和1.2 ps。因此，对于球孢红醛，在弛豫的第一皮秒内，通过从S1的内转换（IC），在基电子态中迅速产生大量高振动激发分子。结果，出现了清晰的S信号，其表现出与上述β - 胡萝卜素研究中相同的光谱特征：明显的S0 → S2红边吸收和S0 → S2漂白区域中的“手指型”结构。冷却过程通过假设最初形成的振动非常热的物种S0*来简化描述，随后它以3.4 ps的时间常数衰减形成仍然热的S0物种，该物种以10.5 ps的时间常数弛豫形成298 K下的S0分子。β - 阿朴 - 4'- 胡萝卜醛的行为非常相似。在这里，在动力学建模过程中单个振动热的S0物种就足够了。S0以12.1 ps的时间常数弛豫形成冷的S0。最后，对于β - 阿朴 - 12'- 胡萝卜醛没有观察到S0特征。在这种情况下，预计S1 → S0内转换过程比S0弛豫慢约20倍。结果，找不到S0*的光谱特征，因为没有机会积累可检测浓度的振动热分子。
