School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK.
J Phys Chem A. 2010 Nov 18;114(45):12028-41. doi: 10.1021/jp106473e. Epub 2010 Oct 26.
A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor groups are connected to electron-accepting diquaternized 2,2'-bipyridyl (diquat) units. The main aim is to combine large quadratic and cubic nonlinear optical (NLO) effects in potentially redox-switchable molecules with 2D structures. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all of the compounds show two reversible or quasireversible diquat-based reductions and partially reversible Dpap oxidations. Single crystal X-ray structures have been obtained for one salt and for the precursor compound (E)-4-(diphenylamino)cinnamaldehyde, both of which adopt centrosymmetric space groups. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and Stark (electroabsorption) spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large and generally increased substantially for the bis-Dpap derivatives when compared with their monosubstituted analogues. Polarized HRS studies show that the NLO responses of the disubstituted species are dominated by "off-diagonal" β(zyy) components. Lengthening the diquaternizing alkyl unit lowers the electron-acceptor strength and therefore increases the ICT energies and decreases the E(1/2) values for diquat reduction. However, compensating increases in the ICT intensity prevent significant decreases in the Stark-based β(0) responses. Cubic NLO properties have been measured by using the Z-scan technique over a wavelength range of 520-1600 nm, revealing relatively high two-photon absorption cross-sections of up to 730 GM at 620 nm for one of the disubstituted chromophores.
已制备了一系列发色盐，其中 4-（二苯胺基）苯基（Dpap）电子给体基团连接到电子受体双季铵化 2,2'-联吡啶（二季铵）单元。主要目的是将大的二次和三次非线性光学（NLO）效应结合到具有 2D 结构的潜在氧化还原开关分子中。这些生色团已通过各种技术（包括电子吸收光谱和循环伏安法）表征为其 PF(6)(-)盐。可见吸收光谱主要由强烈的π→π*分子内电荷转移（ICT）带主导，所有化合物均显示两个可逆或准可逆的二季铵还原和部分可逆的 Dpap 氧化。获得了一个盐和前体化合物（E）-4-（二苯胺基）肉桂醛的单晶结构，两者均采用中心对称空间群。使用 800nm 激光的超瑞利散射（HRS）测量了一阶超极化率β，并通过 ICT 带的斯塔克（电吸收）光谱估算了静态一阶超极化率β(0)。直接和间接得出的β值很大，并且当与它们的单取代类似物相比时，双 Dpap 衍生物的β值通常会大大增加。偏振 HRS 研究表明，取代物种的 NLO 响应主要由“非对角”β(zyy)分量主导。延长季铵化烷基单元会降低电子受体强度，从而增加 ICT 能量并降低二季铵还原的 E(1/2)值。然而，ICT 强度的补偿增加阻止了 Stark 基于β(0)响应的显着降低。使用 Z 扫描技术在 520-1600nm 的波长范围内测量了三阶 NLO 性质，揭示了一个取代生色团在 620nm 处高达 730GM 的相对高的双光子吸收截面。
