Dayal Smita, Burda Clemens
Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, USA.
Photochem Photobiol Sci. 2008 May;7(5):605-13. doi: 10.1039/b800040a. Epub 2008 Mar 20.
Due to the increased use of quantum dots (QDs) in diverse laser microscopies, it is interesting to study the excitation pump power and excitation wavelength dependence of QD-based energy transfer (ET) processes. The ET in QD conjugates with phthalocyanines (Pcs) was studied with femtosecond time-resolved pump-probe spectroscopy upon one- and two-photon excitation. At the used excitation wavelengths only the QDs are excited and become the energy donors. Due to the matched spectral overlap of QD photoluminescence and Pc absorption, the ET occurs on a picosecond time scale. The ET process shows strong pump power dependence whereby an increase in excitation power results in multiple QD excitations and in shorter excited state lifetimes on the QDs due to Auger relaxation. As a result, high excitation pump power leads also to an accelerated ET to the acceptor molecules from the initially multiply excited states of the QDs. Excited state quenching studies as function of pump power suggest that ET occurs mainly from the lowest one-exciton state (n = 1) and only to a minor extent from the multiply excited states (n > 1). For the short-lived, multiply excited states the ET competes inefficiently with Auger recombinations and energy transfer efficiencies of phi(ET)(n=1>) approximately 20%, phi(ET)(n=2>) approximately 7%, phi(ET)(n=3>) < or = 2% were obtained. Also after two-photon excitation the ET efficiency is highest from the one-exciton state. The experimentally determined ET efficiencies were compared with theoretical ET efficiencies upon multiple excitations. In both cases the ET efficiency decreases with the increase in excitation pump power.
由于量子点（QD）在各种激光显微镜中的使用增加，研究基于量子点的能量转移（ET）过程对激发泵浦功率和激发波长的依赖性很有意思。利用飞秒时间分辨泵浦 - 探测光谱对量子点与酞菁（Pc）共轭物在单光子和双光子激发下的能量转移进行了研究。在所使用的激发波长下，只有量子点被激发并成为能量供体。由于量子点光致发光与酞菁吸收的光谱重叠匹配，能量转移在皮秒时间尺度上发生。能量转移过程显示出强烈的泵浦功率依赖性，即激发功率的增加会导致多个量子点激发，并且由于俄歇弛豫，量子点上的激发态寿命会缩短。结果，高激发泵浦功率也导致从量子点最初的多重激发态到受体分子的能量转移加速。作为泵浦功率函数的激发态猝灭研究表明，能量转移主要发生在最低的单激子态（n = 1），仅在较小程度上发生在多重激发态（n> 1）。对于短寿命的多重激发态，能量转移与俄歇复合的竞争效率较低，得到的能量转移效率分别为：φ(ET)(n = 1)>约20%，φ(ET)(n = 2)>约7%，φ(ET)(n = 3)≤2%。双光子激发后，单激子态的能量转移效率也是最高的。将实验测定的能量转移效率与多重激发时的理论能量转移效率进行了比较。在这两种情况下，能量转移效率都随着激发泵浦功率的增加而降低。
