中央研究院化學研究所－學術研究

This study explores how solid-state photoisomerization can still occur in crystals where molecules are tightly packed. By examining a library of salicylhydrazone derivatives, we obtained single crystals for sixteen compounds and found that even planarly packed structures can show clear photochromic behavior when there is enough local steric freedom around the reactive bonds. Through systematic analysis of π–π stacking patterns and quantification of the free volume available along the isomerization pathway, we identified a strong and direct correlation between local free space and photoreactivity. These findings enabled us to develop a crystal-structure based descriptor called the “pedal space,” which summarizes the steric environment into a single parameter and defines the minimum spatial requirement for solid-state photoisomerization. We refer to this behavior as topochemical photoisomerization, highlighting how subtle differences in packing, such as slipped versus co-facial stacking, can determine the photochemical outcome. This framework provides a structure-guided approach for designing solid-state photoswitchable materials with potential applications in future optoelectronic technologies.

這項研究探討一個有趣的問題：分子在固態中緊密排列，看似沒有活動空間，但有些化合物在照光後仍會「變形」並改變顏色。為了理解這個現象，我們分析了一系列水楊醛腙衍生物，並成功取得 16 種單晶結構。我們發現，只要反應位置附近保有些許「可活動空間」，即使晶體整體呈現平坦、緊密的堆疊，仍能發生光異構化。此外，我們透過系統評估 π–π 堆疊方式並量化反應途徑中的自由體積，清楚顯示局部空間大小與光反應性之間具有關鍵的正相關。 基於這些觀察，我們提出一項新的結構參數「pedal space」，用來衡量分子在晶體中是否有足夠空間完成光反應，並能預測哪些晶體具有光致變色潛力。我們將這種反應行為稱為「拓撲化光異構化」，強調即使是微小的堆疊差異，例如分子間的細微滑移，都會影響光反應是否能順利進行。這些概念可作為未來開發固態可光切換材料的設計基礎，應用於光電元件、記憶材料與智慧薄膜等領域。