Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan.
J Phys Chem B. 2009 Nov 19;113(46):15212-24. doi: 10.1021/jp9072922.
To develop a molecular design concept for highly fluorescent polyimides (PIs), ultraviolet-visible optical absorption spectra and fluorescent excitation/emission spectra of PI thin films were extensively investigated with the aid of density functional theory calculations. It was clarified that fully aromatic PIs (Ar-PIs) have strong charge transfer (CT) interactions, and their lowest excited states are attributable to CT(pi-pi*) states which emit weak fluorescence. The CT interactions in PIs are effectively suppressed by using alicyclic diamines as the source material, regardless of the dianhydride structure. This is due to the nonaromatic nature and weak electron-donating property of alicyclic diamines. The lowest excited states of Al-PIs derived from aromatic dianhydrides having delocalized pi conjugation and an alicyclic diamine are attributable to LE(pi-pi*) states which emit strong fluorescence. In particular, Al-PIs having ether (-O-) linkages in the dianhydride moiety demonstrated the strongest blue photoluminescence with a quantum efficiency of 11%. This is explainable by the fact that the bent and rotatable diphenyl ether structure prevents dense chain packing which effectively weakens the fluorescence. However, the lowest excited states of Al-PIs derived from dianhydrides with localized pi conjugation and an alicyclic diamine are attributable to LE(n-pi*) states which emit quite weak fluorescence. These results indicate that the Al-PIs synthesized from alicyclic diamines and aromatic dianhydrides having delocalized conjugation with nonplanar structures are potential candidates for thermally stable polymers which emit strong fluorescence in the visible region, i.e., "highly fluorescent polyimides".
为了开发高荧光性聚酰亚胺（PI）的分子设计概念，我们借助密度泛函理论计算，广泛研究了 PI 薄膜的紫外可见光吸收光谱和荧光激发/发射光谱。结果表明，全芳香族 PI（Ar-PI）具有较强的电荷转移（CT）相互作用，其最低激发态归因于 CT（pi-pi*）态，其发射较弱的荧光。通过使用脂环族二胺作为原料，可以有效地抑制 PI 中的 CT 相互作用，而与二酐的结构无关。这是由于脂环族二胺具有非芳香性和较弱的给电子性。由具有离域 π 共轭和脂环族二胺的芳香族二酐衍生的 Al-PI 的最低激发态归因于发射强荧光的 LE（pi-pi*）态。特别是，在二酐部分具有醚（-O-）键的 Al-PI 表现出最强的蓝色光致发光，量子效率为 11%。这可以解释为弯曲和可旋转的二苯醚结构阻止了密集的链堆积，从而有效地减弱了荧光。然而，由具有局域 π 共轭和脂环族二胺的二酐衍生的 Al-PI 的最低激发态归因于发射相当弱荧光的 LE（n-pi*）态。这些结果表明，由脂环族二胺和具有非平面结构的离域共轭的芳香族二酐合成的 Al-PI 是具有强荧光的热稳定聚合物的潜在候选材料，即“高荧光性聚酰亚胺”。
