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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Relation between two-photon absorption and dipolar properties in a series of fluorenyl-based chromophores with electron donating or electron withdrawing substituents
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Relation between two-photon absorption and dipolar properties in a series of fluorenyl-based chromophores with electron donating or electron withdrawing substituents

机译：一系列带有给电子或吸电子取代基的芴基生色团的双光子吸收与偶极性质之间的关系

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摘要

We investigate two-photon absorption (2PA) in a series of fluorenyl-based 9,9-diethyl-2-ethynyl-7-((4-R-phenyl)ethynyl)-9,9a-dihydro-4aH-fluorene chromophores with R being various electron donating (ED) and electron withdrawing (EW) groups. We use wavelength-tunable femtosecond laser pulses to measure the 2PA cross sections in the lowest dipole-allowed transition and show that the substituents with stronger ED or EW character enhance the peak 2PA cross section (up to α_2 ～ 60 80 GM) while the neutral substituents lead to smaller cross sections, α_2 < 10 GM. We apply two-level approximation to establish a quantitative relation between the 2PA in the pure electronic transition (0 0) and the corresponding change of the permanent electric dipole moment upon the excitation (δμ). This relation is elucidated by comparing δμ values obtained from the 2PA measurements with quantum-chemical calculations and with measurements of solvatochromic shifts in a series of solvents. We show that the calculated δμ correlate well with the values obtained from the 2PA spectroscopy. The δμ values obtained from the solvatochromic shifts agree well with the above two methods for the chromophores with neutral or weak EW or ED substituents. On the other hand, stronger EW or ED end groups give much larger Stokes shifts, which lead to an overestimation of the δμ values. We tentatively attribute this effect to the excitation-induced electronic density change occurring predominantly at the substituent side of the molecule, which causes the effective point dipole associated with the δμ to interact more strongly with the surrounding solvent.
机译：我们调查了一系列基于芴基的9,9-二乙基-2-乙炔基-7-（（4-R-苯基）乙炔基）-9,9a-二氢-4aH-芴发色团的双光子吸收（2PA） R是各种给电子（ED）和吸电子（EW）基团。我们使用波长可调飞秒激光脉冲来测量最低偶极子跃迁中的2PA截面，结果表明具有较强ED或EW特性的取代基会增强2PA峰的截面（高达α_2〜60 80 GM），而中性取代基导致较小的截面，α_2<10 GM。我们应用两级近似来建立纯电子跃迁中的2PA（0 0）与激发时的永久电偶极矩的相应变化（δμ）之间的定量关系。通过比较从2PA测量获得的δμ值与量子化学计算以及一系列溶剂中溶剂化变色的测量值，可以阐明这种关系。我们表明，计算出的δμ与从2PA光谱获得的值具有很好的相关性。从溶剂化变色获得的δμ值与上述两种中性或弱EW或ED取代基发色团的方法非常吻合。另一方面，较强的EW或ED端基会产生更大的斯托克斯位移，从而导致δμ值的高估。我们暂时将此效应归因于激发诱导的电子密度变化，该变化主要发生在分子的取代基侧，这导致与δμ相关的有效点偶极子与周围的溶剂发生更强烈的相互作用。

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