Balancing Brightness and Photobasicity: Modulating Excited-State Proton Transfer Pathways in Push-Pull Fluorophores for Biological Two-Photon Imaging.

IF 4.6 Q2 MATERIALS SCIENCE, BIOMATERIALS
Adam M McCallum, Jiyao Yu, S Sumalekshmy, Abigail Hagwood, Christoph J Fahrni
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Abstract

Push-pull fluorophores with donor-π-acceptor architectures are attractive scaffolds for the design of probes and labels for two-photon microscopy. Such fluorophores undergo a significant charge-delocalization in the excited state, which is essential for achieving a large two-photon absorption cross-section and brightness. The polarized excited state may, however, also facilitate excited-state proton transfer (ESPT) pathways that can interfere with the probe response. Herein, we employed steady-state and time-resolved spectroscopic studies to elucidate whether ESPT is responsible for the pH-dependent emission response of the Zn(II)-selective fluorescent probe chromis-1. Composed of a push-pull architecture with a pyridine ring as the acceptor, the chromis-1 fluorophore core acts as a photobase that promotes ESPT upon acidification. Although the pKa of the pyridine acceptor increases more than six orders of magnitude upon excitation, the photobasicity is not sufficient to deprotonate solvent water molecules under neutral conditions. Rather, the pH-dependent emission response is caused by the pendant bis-isonicotinic acid chelating group which upon protonation facilitates an excited-state intramolecular proton transfer to the pyridine acceptor. A simple permutation of the core pyridine nitrogen from the para- to the ortho-position relative to the thiazole substituent was sufficient to reduce the excited-state basicity by two orders of magnitude without compromising the two-photon excited brightness. These results highlight the importance of choosing the appropriate fluorophore core and chelating moiety for minimizing pH-dependent responses in the design of fluorescent probes for biological imaging.

平衡亮度与光碱度:调节推拉荧光团中的激发态质子转移途径,实现生物双光子成像。
具有供体-π-受体结构的推拉荧光团是设计双光子显微镜探针和标签的极具吸引力的支架。这类荧光团在激发态会发生显著的电荷异化,这对于获得较大的双光子吸收截面和亮度至关重要。然而,极化激发态也可能促进激发态质子转移(ESPT)途径,从而干扰探针的响应。在此,我们采用稳态和时间分辨光谱研究来阐明 ESPT 是否是 Zn(II)选择性荧光探针 chromis-1 的 pH 依赖性发射响应的原因。chromis-1 的荧光团核心由一个吡啶环作为受体的推拉结构组成,在酸化时可作为光碱促进 ESPT。虽然吡啶受体的 pKa 在激发时会增加六个数量级,但在中性条件下,光碱度不足以使溶剂水分子去质子化。相反,与 pH 值相关的发射反应是由悬垂的双异烟酸螯合基团引起的,该基团在质子化时会促进激发态分子内质子向吡啶受体的转移。相对于噻唑取代基,只需将核心吡啶氮从对位变为正位,就足以将激发态碱性降低两个数量级,而不会影响双光子激发亮度。这些结果凸显了在设计用于生物成像的荧光探针时,选择适当的荧光团核心和螯合基团以尽量减少 pH 依赖性反应的重要性。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
ACS Applied Bio Materials
ACS Applied Bio Materials Chemistry-Chemistry (all)
CiteScore
9.40
自引率
2.10%
发文量
464
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