Imperial College London, Division of Molecular Biosciences, South Kensington campus, SW7 2AZ, London, UK.
Phys Chem Chem Phys. 2012 Dec 5;14(45):15752-64. doi: 10.1039/c2cp41718a. Epub 2012 Oct 23.
The quantum yield of photoisomerisation of the photoactive yellow protein (PYP) strongly depends on peak power and wavelength with femtosecond optical excitation. Using systematic power titrations and addition of second order dispersion resulting in 140, 300 and 600 fs pulse durations, the one and multi-photon cross-sections at 400, 450 and 490 nm have been assessed from transient absorption spectroscopy and additionally the Z-scan technique. Applying a target model that incorporates photoselection theory, estimates for the cross-sections for stimulated emission and absorption of the first excited state, the amount of ultrafast internal conversion and the underlying species associated dynamics have been determined. The final quantum yields for photoisomerisation were found to be 0.06, 0.14-0.19 and 0.02 for excitation wavelengths 400, 450 and 490 nm and found to increase with increasing pulse durations. Transient absorption measurements and Z-scan measurements at 450 nm, coinciding with the maximum wavelength of the ground state absorption, indicate that the photochemical quantum yield is intrinsically limited by an ultrafast internal conversion reaction as well as by stimulated emission cross-section. With excitation at 400 nm photoisomerisation quantum yield is further significantly limited by competing multi-photon excitation into excited state absorption at 385 nm previously proposed to result in photoionisation. With excitation at 490 nm the photoisomerisation quantum yield is predominantly limited further by the significantly higher stimulated emission cross-section compared to ground state cross-section as well as multi-photon processes. In addition to photoionisation, a second product of multi-photon excitation is identified and characterised by an induced absorption at 500 nm and a time constant of 2 ps for relaxation. With power densities up to 138 GW cm(-2) the measurements have not provided indication for coherent multi-photon absorption of PYP. In the saturation regime with 450 nm excitation, the limit for the photoisomerisation quantum yield was found to be 0.14-0.19 and the excited state absorption cross-section 6.1 × 10(-17) cm(2) or 0.36 times the ground state cross-section of 1.68 × 10(-16) cm(2) per molecule. This places a fundamental restriction on the maximum populations and sample penetration that may be achieved for instance in femtosecond pump-probe experiments with molecular crystals of PYP.
光致变色蛋白（PYP）的光异构化量子产率强烈依赖于皮秒光激发的峰值功率和波长。使用系统的功率滴定法，并添加导致 140、300 和 600 fs 脉冲持续时间的二级色散，已经从瞬态吸收光谱和另外的 Z 扫描技术评估了 400、450 和 490nm 的单光子和多光子截面。应用包含光选择理论的目标模型，估计了第一激发态的受激发射和吸收截面、超快内转换的量以及相关的基态动力学相关的量。发现 400nm、450nm 和 490nm 激发波长下的光异构化最终量子产率分别为 0.06、0.14-0.19 和 0.02，并且发现它们随脉冲持续时间的增加而增加。在与基态吸收的最大波长 450nm 相吻合的瞬态吸收测量和 Z 扫描测量表明，光化学量子产率本质上受到超快内转换反应以及受激发射截面的限制。在 400nm 激发下，光异构化量子产率进一步受到多光子激发到先前提出导致光离化的 385nm 激发态吸收的竞争的显著限制。在 490nm 激发下，光异构化量子产率主要受到与基态截面相比显著更高的受激发射截面以及多光子过程的限制。除了光离化之外，还通过在 500nm 处的感应吸收和 2ps 的弛豫时间来识别和表征多光子激发的第二个产物。在高达 138GW/cm²的功率密度下，测量结果没有表明 PYP 的相干多光子吸收。在 450nm 激发的饱和区域，发现光异构化量子产率的极限为 0.14-0.19，激发态吸收截面为 6.1×10^(-17)cm²，或每分子 0.36 倍于基态截面 1.68×10^(-16)cm²。这对例如在 PYP 分子晶体的飞秒泵浦探测实验中可能实现的最大种群和样品穿透性施加了基本限制。
