Scholz Mirko, Flender Oliver, Lenzer Thomas, Oum Kawon
Physikalische Chemie, Universität Siegen , Adolf-Reichwein-Str. 2, 57076 Siegen, Germany.
J Phys Chem A. 2017 Nov 9;121(44):8380-8388. doi: 10.1021/acs.jpca.7b08252. Epub 2017 Nov 1.
The C carotenoid capsanthin is of photophysical interest because it belongs to the family of terminally carbonyl-substituted apocarotenes. These have the potential to exhibit intramolecular charge transfer (ICT) character in the excited state. We studied its ultrafast dynamics in different solvents using broadband transient absorption spectroscopy in the 260-1600 nm range. Photoexcitation initially populated the S(1B) state which decayed to the S(2A) state in 120-150 fs. The lifetime of the S state decreased from 10.3 ps (isooctane) to 9.1 ps (methanol). Together with the absence of stimulated emission this suggested a weak ICT character of S. A steric influence of the five-membered ring in capsanthin was identified based on a comparison with the sister apocarotenone citranaxanthin which features methyl substitution at the keto group. While both apocarotenones exhibit the same S lifetime in isooctane, the decrease in lifetime in polar solvents is weaker for capsanthin because presumably the five-membered ring sterically perturbs stabilization of ICT character by the solvent. For the S state of capsanthin we further observed intramolecular vibrational redistribution and collisional energy transfer to the solvent with time constants of 340-420 fs and 8.5-9.1 ps, respectively. No evidence for excited-state photoisomerization of capsanthin was found.
C类胡萝卜素辣椒红素具有光物理研究价值，因为它属于末端羰基取代的类胡萝卜素家族。这些化合物在激发态有可能表现出分子内电荷转移（ICT）特性。我们使用260 - 1600 nm范围内的宽带瞬态吸收光谱研究了其在不同溶剂中的超快动力学。光激发最初使S(1B)态布居，该态在120 - 150 fs内衰减到S(2A)态。S态的寿命从10.3 ps（异辛烷）降至9.1 ps（甲醇）。再加上没有受激发射，这表明S态的ICT特性较弱。通过与在酮基处具有甲基取代的类胡萝卜素柠檬黄质进行比较，确定了辣椒红素中五元环的空间影响。虽然这两种类胡萝卜素在异辛烷中表现出相同的S态寿命，但辣椒红素在极性溶剂中寿命的降低较弱，这可能是因为五元环在空间上干扰了溶剂对ICT特性的稳定作用。对于辣椒红素的S态，我们还分别观察到了分子内振动重新分布以及与溶剂的碰撞能量转移，时间常数分别为340 - 420 fs和8.5 - 9.1 ps。未发现辣椒红素激发态光异构化的证据。
