Solvatochromic shift mechanisms of centrosymmetric chromophores in polar liquids

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• 作者：Indrek Renge ; ^{indrek.renge@ut.ee}
• 关键词：Absorption spectrum ; Dielectric theory ; Quadratic Stark effect ; Polyene ; Random field ; Solvent shift
• 刊名：Journal of Photochemistry and Photobiology A ; Chemistry
• 出版年：2012
• 期刊代码：66_10106030
• 类别：ch
• 出版时间：1 July, 2012
• 卷：239
• 期：Complete
• 页码：7-16
• 文件大小：799 K

摘要

Origins of absorption shifts in polar liquids are considered for centrosymmetric chromophores devoid of dipole moments. The refractive index (n) dependent bathochromism shows that the excited state is always more polarizable than ground state. With increasing solvent dielectric constant (蓻), mainly red shifts occur, but there are exceptions. Hypsochromism of n-蟺* transition in s-tetrazine is ascribed to a decrease of quadrupole moment in the excited state. In this case the upper level is pushed up in the reaction field of the ground-state quadrupole. Red shifts of intense bands in polyenes (1¹B_u or S₂) and polyarenes (¹B_b or 尾), reaching 鈭尖垝250 cm^鈭? are assigned to local polarization in the reaction fields of chromophore bond dipoles. The hyperbolic-like 蓻-dependence of solvent shift, approximated to an empirical function (蓻 鈭?#xA0;n²)/(蓻 鈭?#xA0;n² + c), is much steeper (with 鈥渟tretching鈥?factor c = 3 卤 0.5) in this case, as compared to that for the linear effect (for s-tetrazine, c = 9). Polarization mechanism also causes less band broadening in polar environments. Contrary to a common view, the spectral shift is probing local reaction fields of chromophores, rather than random fields in the cavities accommodating the dye. Negligible polarization in solvent 鈥渃avity鈥?fields is in accordance with earlier hole-burning study of matrix induced dipole moments in tetrapyrrolic pigments that depend surprisingly little on the polarity of polymeric hosts (R.B. Altmann, I. Renge, L. Kador, D. Haarer, J. Chem. Phys. 97 (1992) 5316 ).