White light emission from single natural molecules: Solvatochromic properties of 10-hydroxycamptothecin

Abstract

With the development of display systems, artificial lighting, and fluorescent sensor, white light emission (WLE) materials have received great attention in recent years. Various organic and inorganic compounds have been synthesized to generate WLE. Natural products with WLE ability are attractive for their easy availability and cost effective. However, WLE from natural products are very rare and mainly realized by a complicated combination of two or three natural dyes emitting complementary lights. A single natural product molecule that can generate WLE has not yet been reported. Camptothecin (CPT) and its derivative 10-hydroxycamptothecin (HCPT) are natural products with anti-tumour activity and unique fluorescence properties. Here we demonstrated that CPT showed pH-dependent WLE properties. CPT can produce WLE in an acetic acid-ethanol mixture with 35% acetic acid fraction (V/V) under excitation at 360 nm. HCPT showed solvatochromism-dependent WLE properties and can easily generate WLE by adjusting the ratios of water/ethanol solution and showed an extremely close pure white fluorescence in a water-ethanol mixture with 5.7% water fraction (V/V). The corresponding Commission Internationale d'Eclairage (CIE) coordinates is (0.32, 0.34). Due to the remarkable excited-state acidity properties, the WLE efficiency of HCPT was extraordinarily high with a 31.72% quantum yield. HCPT also can produce WLE in triethylamine-ethanol solution and showed great CIE coordinates (0.31, 0.34) in 9.1% triethylamine fraction (V/V). Besides, we performed preliminary white fluorescent application of HCPT in polyvinyl alcohol films with good results. The WLE mechanisms of CPT and HCPT were also discussed. We hope our findings could inspire more WLE studies about CPT and its derivatives. Meanwhile, more single natural molecules with WLE properties could be discovered.