Excited state proton transfer based two color fluorescence: Perspectives and some biophysical applications

Photoinduced excited state intramolecular proton transfer (ESIPT) reactions comprise an important and extensively explored class of reactions in photochemistry. Till date, plant flavonols are one of the most widely known class of naturally abundant organic molecules exemplifying ESIPT and ‘two color’ fluorescence. From a bio-medical perspective, flavonols and related polyphenols, which are powerful antioxidants, have attracted significant interest as novel drugs (of high potency and low cyto-toxicity) for the prophylaxis and therapy of free radical induced and other important diseases. This article presents perspectives on proton transfer in photoexcited organic molecules from a historical context, emphasizing ESIPT reactions, in particular. Highlights of representative research findings are discussed, exemplifying the promising potential of plant flavonols as their own ESIPT based ‘fluorescence sensors’ for exploring their interactions with proteins, DNA (possessing duplex as well as higher order structures), and biomembranes, which represent the targets underlying the various pharmacological actions of flavonols. The usefulness of such approach for studying the confinement of intrinsically fluorescent flavonols in nano-vehicles for drug encapsulation, is also demonstrated.     

水杨醛衍生物激发态下的质子转移过程及在荧光探针领域的应用

从电子结构控制理论出发,通过在酚羟基对位引入吸电子取代基团稳定水杨醛中激发态的酮式构象,制备了目标化合物5-对氰基苯基-水杨醛(CN-SA).光谱测试结果显示,CN-SA表现出典型的ESIPT态荧光分子特性,而且辐射跃迁过程的酮式分配比例显著提高,荧光强度和颜色变化明显.CN-SA的荧光光谱不但能够对外围溶剂环境进行选择性识别,而且对溶解和聚集过程(聚集效应)及外围氢键形成能力的变化(pH效应和阴离子效应)等具有特异性响应,其变化可以定量表达.CN-SA仅通过结构微调即实现醇-酮构象的显著变化,可作为一个简单的多重刺激响应型荧光探针.     

Charge Separation in Excited States of Decoupled Systems—TICT Compounds and Implications Regarding the Development of New Laser Dyes and the Primary Process of Vision and Photosynthesis

The understanding of the dual fluorescence of certain aromatic systems has greatly advanced in recent years. The accompanying large charge separation has been shown to be linked to a twisted (or small overlap) arrangement of the chromophores. Recent theoretical models are able to describe the excited-state twisting of both single bonds (TICT compounds) and double bonds (olefins) in a unified picture. These models can help to elucidate the photophysical behavior of many organic, inorganic, and biologically relevant compounds, and their application to laser dyes and fluorescent probes provides a route to new “tailor-made” fluorescent materials. Applied to the primary processes of vision and photosynthesis, these models can lead to a deeper understanding of basic photobiological processes.     

Different ESIPT Mechanisms for Angular‐Shaped Quinacridone in Toluene and Dimethyl Formamide (DMF) Solvents: A Theoretical Study

Adopting density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we investigat and present two different excited‐state intramolecular proton transfer (ESIPT) mechanisms of angular‐quinacridone (a‐QD) in both toluene and DMF,theoretically. Comparing the primary structural variations of a‐QD involved in the intramolecular hydrogen bond, we conclude that N1–H2⋯O3 should be strengthened in the S₁ state, which may facilitate the ESIPT process. Particularly, in toluene, the S₁‐state‐stable a‐QD enol* could not be located because of the non‐barrier ESIPT process. Concomitantly, infrared vibrational spectral analysis further verified the stability of the hydrogen bond. In addition, the role of charge–transfer interaction has been addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. The potential energy curves according to variational N1–H2 coordinate demonstrates that the proton transfer process should occur spontaneously in toluene; however, in DMF, a low potential energy barrier of 0.493 kcal/mol is needed to complete the ESIPT reaction. Although this barrier of 0.493 kcal/mol is too low to make an important impact on the ESIPT reaction, just because of the existence of barrier, ESIPT mechanisms in toluene and DMF are different.     

Ultrafast dynamics in a nanocage of enzymes: solvation and fluorescence resonance energy transfer in reverse micelles

In this contribution we report studies of the nature of solvation and resonance energy transfer processes in a reverse micelle (RM) upon encapsulation of a digestive enzyme, alpha-chymotrypsin (CHT). We have used one donor, Coumarin 500 (C500), and three acceptors Rhodamine 123 (R123, cationic), ethidium bromide (EtBr, cationic), and Merocyanine 540 (MC540, anionic). By selectively exciting the donor at the surface of the RM with a proper excitation wavelength we have examined solvation dynamics in the microenvironment. The solvation correlation function in the RM without CHT exhibits single-exponential decay with time constant approximately 660 ps, which is similar to that of the CHT-included RM. However, in the case of CHT-included RM (w(0)=10), the time-resolved anisotropy and spectral linewidth analysis of the surface-bound donor reveal the existence of an annular aqueous channel of thickness approximately 2.5 A between the enzyme surface and the inner surface of the RM. The aqueous channel is a potential host for the water-soluble substrate and also is involved in maintaining the proper functionality of RM encapsulated CHT. The studies use both steady-state and time-resolved fluorescence resonance energy transfer (FRET) techniques to measure donor-acceptor distances in the RM and also emphasize the danger of using steady-state fluorescence quenching as a method in careful estimation of the distances. The local geometrical restriction on the donor and acceptor molecules was estimated from time-resolved polarization (anisotropy) measurements. The time-resolved anisotropy of the donor and acceptor molecules also revealed significant randomization of the relative orientation of transition dipoles of the donor and acceptor, justifying the use of 2/3 as the value of the orientation factor kappa2. These studies attempt to elucidate the excellence of the RM as a nanohost of biological macromolecules.