发色团Chromophore

发色团含 π 电子或孤对电子，可发生 π→π* 和 n→π* 跃迁。多个发色团共轭时，HOMO-LUMO 能隙减小，吸收峰向长波方向移动（红移）、强度增大（增色效应），共轭足够长时吸收进入可见区，物质显色。助色团的孤对电子与发色团的 π 体系共轭（p-π 共轭），也使吸收红移。溶剂极性也会影响：π→π* 跃迁在极性溶剂中红移，n→π* 跃迁蓝移。

Chromophores contain π electrons or lone pairs and undergo π→π* and n→π* transitions. When several are conjugated the HOMO–LUMO gap shrinks, shifting absorption to longer wavelength (a red, or bathochromic, shift) and raising its intensity (hyperchromic effect); with enough conjugation absorption enters the visible and the substance is coloured. An auxochrome's lone pair conjugates with the chromophore's π system (p–π conjugation), also red-shifting the band. Solvent polarity matters too: π→π* bands red-shift and n→π* bands blue-shift in polar solvents.

1876 年德国化学家奥托·维特在研究偶氮染料时提出发色团（chromophore，希腊语“携带颜色”）和助色团（auxochrome）理论，认为染料颜色来自发色团，助色团加深颜色并使染料能固着在纤维上。这一理论指导了 19 世纪末合成染料工业的发展，后被分子轨道理论解释。

In 1876 the German chemist Otto Witt, studying azo dyes, proposed the theory of chromophores (Greek for 'colour-bearing') and auxochromes, holding that dye colour comes from the chromophore while auxochromes deepen it and help the dye bind to fibres. The theory guided the synthetic-dye industry of the late nineteenth century and was later explained by molecular orbital theory.

天然发色团包括视网膜中的视黄醛（视觉）、叶绿素和类胡萝卜素（光合作用）、血红素（血液红色）、花青素（花果颜色）；合成染料如偶氮染料、蒽醌染料、三苯甲烷染料。生物学中荧光蛋白（GFP）的发色团由蛋白质自身三个氨基酸环化生成。发色团设计用于染料、颜料、光敏剂、荧光探针和有机光伏材料。

Natural chromophores include retinal in the eye (vision), chlorophylls and carotenoids (photosynthesis), haem (the red of blood) and anthocyanins (flower and fruit colours); synthetic dyes include azo, anthraquinone and triphenylmethane dyes. The chromophore of green fluorescent protein forms from three of the protein's own amino acids by cyclisation. Chromophore design underlies dyes, pigments, photosensitisers, fluorescent probes and organic photovoltaics.