Rebane Aleksander, Drobizhev Mikhail, Makarov Nikolay S, Wicks Geoffrey, Wnuk Pawel, Stepanenko Yuriy, Haley Joy E, Krein Douglas M, Fore Jennifer L, Burke Aaron R, Slagle Jonathan E, McLean Daniel G, Cooper Thomas M
Physics Department, Montana State University , Bozeman, Montana 59717, United States.
National Institute of Chemical Physics and Biophysics , 12618 Tallinn, Estonia.
J Phys Chem A. 2014 May 29;118(21):3749-3759. doi: 10.1021/jp5009658. Epub 2014 May 15.
We study instantaneous two-photon absorption (2PA) in a series of nominally quasi-centrosymmetric trans-bis(tributylphosphine)-bis-(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl) ethynyl)-R)-platinum complexes, where 11 different substituents, R = N(phenyl)(NPh), NH, OCH, t-butyl, CH, H, F, CF, CN, benzothiazole, and NO, represent a range of electron-donating (ED) and electron-withdrawing (EW) strengths, while the Pt core acts as a weak ED group. We measure the 2PA cross section in the 540-810 nm excitation wavelength range by complementary femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods and compare the obtained values to those of the Pt-core chromophore and the corresponding noncentrosymmetric side group (ligand) chromophores. Peak 2PA cross sections of neutral and ED-substituted Pt complexes occur at S → S transitions to higher energy states, above the lowest-energy S → S transition, and the corresponding values increase systematically with increasing ED strength, reaching maximum value, σ ∼ 300 GM (1 GM = 10 cm s), for R = NPh. At transition energies overlapping with the lowest-energy S → S transition in the one-photon absorption (1PA) spectrum, the same neutral and ED-substituted Pt complexes show weak 2PA, σ < 30-100 GM, which is in agreement with the nearly quadrupolar structure of these systems. Surprisingly, EW-substituted Pt complexes display a very different behavior, where the peak 2PA of the S → S transition gradually increases with increasing EW strength, reaching values σ = 700 GM for R = NO, while in the S → S transition region the peak 2PEF cross section decreases. We explained this effect by breaking of inversion symmetry due to conformational distortions associated with low energy barrier for ground-state rotation of the ligands. Our findings are corroborated by theoretical calculations that show large increase of the permanent electric dipole moment change in the S → S transition when ligands with strong EW substituents are twisted by 90° relative to the planar chromophore. Our NLT results in the S → S transition region are quantitatively similar to those obtained from the 2PEF measurement. However, at higher transition energy corresponding to S → S transition region, the NLT method yields effective multiphoton absorption stronger than the 2PEF measurement in the same systems. Such enhancement is observed in all Pt complexes as well as in all ligand chromophores studied, and we tentatively attribute this effect to nearly saturated excited-state absorption (ESA), which may occur if 2PA from the ground state is immediately followed by strongly allowed 1PA to higher excited states.
我们研究了一系列名义上准中心对称的反式双（三丁基膦）-双（4-（（9,9-二乙基-7-乙炔基-9H-芴-2-基）乙炔基）-R）-铂配合物中的瞬时双光子吸收（2PA），其中11种不同的取代基，R = N（苯基）（NPh）、NH、OCH、叔丁基、CH、H、F、CF、CN、苯并噻唑和NO，代表了一系列给电子（ED）和吸电子（EW）强度，而铂核作为一个弱给电子基团。我们通过互补的飞秒双光子激发荧光（2PEF）和非线性透射（NLT）方法，在540 - 810 nm激发波长范围内测量了2PA截面，并将所得值与铂核发色团和相应的非中心对称侧基（配体）发色团的值进行了比较。中性和给电子取代的铂配合物的2PA截面峰值出现在从S态到更高能态的S→S跃迁处，高于最低能量的S→S跃迁，并且相应的值随着给电子强度的增加而系统地增加，对于R = NPh，达到最大值σ ∼ 300 GM（1 GM = 10⁻⁵² cm⁴ s⁻¹）。在与单光子吸收（1PA）光谱中最低能量的S→S跃迁重叠的跃迁能量处，相同的中性和给电子取代的铂配合物显示出较弱的2PA，σ < 30 - 100 GM，这与这些体系的近四极结构一致。令人惊讶的是，吸电子取代的铂配合物表现出非常不同的行为，其中S→S跃迁的2PA峰值随着吸电子强度的增加而逐渐增加，对于R = NO达到σ = 700 GM的值，而在S→S跃迁区域，2PEF截面峰值减小。我们通过与配体基态旋转的低能垒相关的构象畸变导致反演对称性的破坏来解释这种效应。我们的发现得到了理论计算的证实，理论计算表明，当具有强吸电子取代基的配体相对于平面发色团扭转90°时，S→S跃迁中永久电偶极矩变化大幅增加。我们在S→S跃迁区域的NLT结果与从2PEF测量中获得的结果在定量上相似。然而，在对应于S→S跃迁区域的较高跃迁能量处，NLT方法在相同体系中产生的有效多光子吸收比2PEF测量更强。在所有研究的铂配合物以及所有配体发色团中都观察到了这种增强，我们初步将这种效应归因于近乎饱和的激发态吸收（ESA），如果从基态的2PA紧接着是强烈允许的到更高激发态的1PA，就可能发生这种情况。
Zotero 插件