9,10-Anthraquinones Disubstituted with Linear Alkoxy Groups: Spectroscopy, Electrochemistry, and Peculiarities of Their 2D and 3D Supramolecular Organizations

Spectroscopic,
electrochemical, and structural properties of 2,6-dialkoxy-9,10-anthraquinones
(Anth-OCn, n = 4, 6, 8, 10, and
12) of increasing alkoxy substituents length were investigated. UV–vis
spectroscopy showed a substitution-induced bathochromic shift of the
least energetic band from 325 nm in the case of unsubstituted anthraquinone
to ca. 350 nm for the studied derivatives. Similarly as unsubstituted
anthraquinone, the studied compound showed two reversible one electron
reductions to a radical anion and spinless anions, respectively. The
first reduction was affected by electron-donating properties of the
substituents, its potential being shifted to ca. −1.5 V (vs
Fc/Fc+), i.e., by 80 to 95 mV as compared to the case of
unsubstituted anthraquinone. This corresponded to a decrease of |EA|
from 3.27 to 3.19–3.17 eV. The experimental spectroscopic and
electrochemical data were in full agreement with the DFT calculations.
The introduction of the alkoxy substituent improved solution processibility
of the studied compounds and facilitated the formation of their ordered
supramolecular 2D aggregation on HOPG as well as single crystal growth
from solutions. Comparative structural investigations carried out
on single crystals and monolayers deposited on HOPG revealed two,
mutually related, effects of the substituent length on the resulting
supramolecular organization. The first one concerns both the 2D organization
in the monolayers and 3D molecular arrangement in crystals: increasing
substituent length evolution of the structure occurs from herringbone-type
to lamellar. The second effect, observed in monolayers of the derivatives
with longer substituents, concerns gradual evolution of their lamellar
structures with increasing substituent length. This evolution is induced
by the structure of the graphite substrate and involves increasing
correlation of the molecules orientation (anthraquinone cores as well
as alkoxy substituents) with the symmetry of the graphite substrate.
As a result, their 2D and 3D structures become dissimilar.

本研究对烷氧基取代基链长递增的2,6-二烷氧基-9,10-蒽醌（Anth-OCn，n=4、6、8、10、12）的光谱学、电化学及结构性质展开了系统探究。紫外-可见（UV–vis）光谱分析结果显示，取代基引发了最低能量吸收带的红移：未取代蒽醌的该吸收带位于325 nm，而本研究涉及的衍生物则约为350 nm。与未取代蒽醌类似，目标衍生物均可发生两次可逆单电子还原反应，分别生成自由基阴离子与无自旋阴离子。首次还原过程受取代基给电子性质影响，其还原电位相较于未取代蒽醌偏移了约80~95 mV，最终达到约-1.5 V（相对于Fc/Fc+）。该变化对应电子亲和能|EA|从3.27 eV降至3.19~3.17 eV。实验测得的光谱学与电化学数据与密度泛函理论（DFT）计算结果完全吻合。烷氧基取代基的引入提升了目标化合物的溶液加工性能，同时促进了其在高定向热解石墨（HOPG）表面形成有序超分子二维聚集体，以及从溶液中生长单晶。针对单晶与沉积于HOPG表面的单分子层开展的对比结构研究表明，取代基链长对最终超分子组装结构存在两种相互关联的调控效应。第一种效应同时体现在单分子层的二维组装与晶体的三维分子排布中：随着取代基链长增加，分子堆积结构从鱼骨型逐渐转变为层状。第二种效应则在长取代基衍生物的单分子层中被观测到：随着取代基链长增加，其层状组装结构会发生逐步演化。该演化过程由石墨基底的结构所诱导，分子（包括蒽醌母核与烷氧基取代基）的取向与石墨基底的对称性之间的关联性会逐步增强。最终，这类衍生物的二维与三维组装结构出现差异。