McDonough Thomas J, Zhang Lushuai, Roy Susmit Singha, Kearns Nicholas M, Arnold Michael S, Zanni Martin T, Andrew Trisha L
Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave, Madison, WI 53706, USA.
Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Ave, Madison, WI 53706, USA.
Phys Chem Chem Phys. 2017 Feb 8;19(6):4809-4820. doi: 10.1039/c6cp06454j.
We compare the ultrafast dynamics of singlet fission and charge generation in pentacene films grown on glass and graphene. Pentacene grown on graphene is interesting because it forms large crystals with the long axis of the molecules "lying-down" (parallel to the surface). At low excitation fluence, spectra for pentacene on graphene contain triplet absorptions at 507 and 545 nm and no bleaching at 630 nm, which we show is due to the orientation of the pentacene molecules. We perform the first transient absorption anisotropy measurements on pentacene, observing negative anisotropy of the 507 and 545 nm peaks, consistent with triplet absorption. A broad feature at 853 nm, observed on both glass and graphene, is isotropic, suggesting hole absorption. At high fluence, there are additional features, whose kinetics and anisotropies are not explained by heating, that we assign to charge generation; we propose a polaron pair absorption at 614 nm. The lifetimes are shorter at high fluence for both pentacene on glass and graphene, indicative of triplet-triplet annihilation that likely enhances charge generation. The anisotropy decays more slowly for pentacene on graphene than on glass, in keeping with the smaller domain size observed via atomic force microscopy. Coherent acoustic phonons are observed for pentacene on graphene, which is a consequence of more homogeneous domains. Measuring the ultrafast dynamics of pentacene as a function of molecular orientation, fluence, and polarization provides new insight to previous spectral assignments.
我们比较了在玻璃和石墨烯上生长的并五苯薄膜中单线态裂变和电荷产生的超快动力学。生长在石墨烯上的并五苯很有趣，因为它形成了大晶体，分子的长轴“平躺”（平行于表面）。在低激发通量下，石墨烯上并五苯的光谱在507和545 nm处包含三重态吸收，在630 nm处没有漂白，我们表明这是由于并五苯分子的取向所致。我们对并五苯进行了首次瞬态吸收各向异性测量，观察到507和545 nm峰的负各向异性，这与三重态吸收一致。在玻璃和石墨烯上均观察到的853 nm处的宽峰是各向同性的，表明是空穴吸收。在高通量下，还有其他特征，其动力学和各向异性不能用加热来解释，我们将其归因于电荷产生；我们提出在614 nm处有极化子对吸收。对于玻璃和石墨烯上的并五苯，高通量下的寿命都较短，这表明三重态-三重态湮灭可能增强了电荷产生。石墨烯上并五苯的各向异性衰减比玻璃上的并五苯慢，这与通过原子力显微镜观察到的较小畴尺寸一致。在石墨烯上的并五苯中观察到了相干声子，这是更均匀畴的结果。测量并五苯作为分子取向、通量和极化函数的超快动力学，为先前的光谱归属提供了新的见解。
