Becker K, Da Como E, Feldmann J, Scheliga F, Csanyi E Thorn, Tretiak S, Lupton J M
Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität, Amalienstr. 54, 80799 München, Germany.
J Phys Chem B. 2008 Apr 24;112(16):4859-64. doi: 10.1021/jp800870p. Epub 2008 Mar 29.
Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support pi-electron conjugation across approximately 80 degrees curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.
单个聚对苯撑乙烯分子构成了无序共轭聚合物中发色团的模型体系，并且能够阐明单个发色团而非整个聚合物链的实际构象是如何控制其光物理性质的。单个低聚物和聚合物链展现出相同范围的光谱特性。正如激发过程中的偏振各向异性所揭示并得到量子化学计算支持的那样，即使是七聚体也能在大约80度的曲率上支持π电子共轭。随着链变得更加变形，低温下的光谱线宽（通常被解释为聚集的标志）由于分子光物理稳定性的降低和随机光谱波动的增加而增加多达30倍。这些结论有助于解释单链斯塔克光谱学的结果，在该光谱学中，仅在窄跃迁线的情况下才观察到大型静态偶极子。这些窄跃迁源自延伸的发色团，其中主链取代基诱导产生的偶极子不会相互抵消。共轭聚合物中的发色团通常被认为是单个线性跃迁偶极子，它们的总和构成了聚合物的光学性质。我们的结果表明，至少对于聚对苯撑乙烯来说，主导光谱性质的是单个发色团的实际形状，而非聚合物链的整体发色团排列和形式。
