Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.
J Chem Phys. 2013 Sep 21;139(11):114903. doi: 10.1063/1.4819906.
The ratio of the 0-0 to 0-1 peak intensities in the photoluminescence (PL) spectrum of red-phase poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], better known as MEH-PPV, is significantly enhanced relative to the disordered blue-phase and is practically temperature independent in the range from T = 5 K to 180 K. The PL lifetime is similarly temperature independent. The measured trends are accounted for by modeling red-phase MEH-PPV as disordered π-stacks of elongated chains. Using the HJ-aggregate Hamiltonian expanded to include site disorder amongst electrons and holes, the absorption and PL spectra of cofacial MEH-PPV dimers are calculated. The PL 0-0/0-1 line strength ratio directly responds to the competition between intrachain interactions which promote J-aggregate-like behavior (enhanced PL ratio) and interchain interactions which promote H-aggregate-like behavior (attenuated PL ratio). In MEH-PPV aggregates, J-like behavior is favored by a relatively large intrachain exciton bandwidth--roughly an order of magnitude greater than the interchain bandwidth--and the presence of disorder. The latter is essential for allowing 0-0 emission at low temperatures, which is otherwise symmetry forbidden. For Gaussian disorder distributions consistent with the measured (inhomogeneous) line widths of the vibronic peaks in the absorption spectrum, calculations show that the 0-0 peak maintains its dominance over the 0-1 peak, with the PL ratio and radiative lifetime practically independent of temperature, in excellent agreement with experiment. Interestingly, interchain interactions lead only to about a 30% drop in the PL ratio, suggesting that the MEH-PPV π-stacks--and strongly disordered HJ-aggregates in general--can masquerade as single (elongated) chains. Our results may have important applications to other emissive conjugated polymers such as the β-phase of polyfluorenes.
红相聚[2-甲氧基-5-(2-乙基己氧基)-1,4-亚苯基乙烯]，即 MEH-PPV 的光致发光（PL）谱中 0-0 峰与 0-1 峰的强度比相对于无序蓝相显著增强，并且在 5 K 到 180 K 的温度范围内几乎与温度无关。PL 寿命也与温度无关。红相 MEH-PPV 被模拟为拉长链无序 π 堆叠，这一模型可以解释所测量的趋势。使用扩展到包括电子和空穴中站点无序的 HJ 聚集体哈密顿量，计算了面对面 MEH-PPV 二聚体的吸收和 PL 光谱。PL 0-0/0-1 线强比直接响应于促进 J-聚集体样行为（增强的 PL 比）的链内相互作用与促进 H-聚集体样行为（衰减的 PL 比）的链间相互作用之间的竞争。在 MEH-PPV 聚集体中，J 样行为受到相对较大的链内激子带宽的青睐——大约比链间带宽大一个数量级，以及无序的存在。后者对于在低温下实现 0-0 发射至关重要，否则 0-0 发射是对称性禁止的。对于与吸收光谱中振动态峰的测量（非均匀）线宽一致的高斯分布无序，计算表明 0-0 峰保持对 0-1 峰的主导地位，PL 比和辐射寿命几乎与温度无关，与实验结果非常吻合。有趣的是，链间相互作用仅导致 PL 比下降约 30%，这表明 MEH-PPV π 堆叠——以及一般的强无序 HJ 聚集体——可以伪装成单个（拉长）链。我们的结果可能对其他发射共轭聚合物（如聚芴的 β 相）具有重要应用。
