Control of luminescence in conjugated polymers through control of chain microstructure
作者:W. James Feast、Franco Cacialli、Alex T. H. Koch、Rusli Daik、Christine Lartigau、Richard H. Friend、David Beljonne、Jean-Luc Brédas
DOI:10.1039/b615408p
日期:——
The development of semiconducting polymers with high solid-state luminescence efficiencies has enabled the fabrication of efficient polymer light-emitting diodes. Luminescence is often quenched in well-ordered molecular solids, as a result of inter-molecular dipolar coupling, and the general observation of efficient luminescence in semiconducting polymers is unexpected. We report here the synthesis and characterisation of a series of model ‘glassy’ poly(arylenevinylene)s where we control the cis to trans ratio about the vinylene linkage and also the phenylene linkage geometry. Photoluminescence efficiency is enhanced for more disordered materials, with highest values for 50 : 50 cis : trans ratios, when it exceeds 50%. We also find that the free volume associated with these glassy disordered polymers allows conformational relaxation of the excitonic state, via ring rotation at the vinylene linkage, causing a large Stokes' shift of the emission. We propose that high luminescence efficiency in these glassy polymers is due to emission from the more disordered regions, and that two effects due to disorder are simultaneously required: firstly that these regions are luminescent (prevention of aggregation by disorder), and, secondly, that the electronic excited state (exciton) can lower its energy below its value in more ordered regions by means of the ring-rotational coupling.
具有高固态发光效率的半导体聚合物的开发使得高效聚合物发光二极管的制造成为可能。由于分子间偶极耦合作用,有序分子固体中的发光通常会被淬灭,而在半导体聚合物中普遍观察到的高效发光现象却出乎意料。我们在此报告了一系列模型 "玻璃状 "聚(芳基乙烯)的合成和表征,在这些聚(芳基乙烯)中,我们控制了乙烯链的顺反比以及亚苯基链的几何形状。对于更无序的材料,光致发光效率会提高,当顺反比为 50 : 50 时,光致发光效率最高,当超过 50%时,光致发光效率最低。我们还发现,与这些玻璃状无序聚合物相关的自由体积允许激发态通过乙烯基连接处的环旋转发生构象弛豫,从而导致发射发生较大的斯托克斯偏移。我们认为,这些玻璃态聚合物的高发光效率是由于较无序区域的发射所致,而无序同时需要两种效应:首先,这些区域是发光的(防止无序聚集);其次,电子激发态(激子)可以通过环转耦合将其能量降低到低于较有序区域的值。