Abstract
Fluorescence sensing plays an important role in high sensitivity, selectivity, and real-time monitoring of biological and environmentally relevant species. Several classes of fluorescent dyes (fluorophores) including rhodamine, BODIPY, 1,8-naphthalimide, and coumarin-among others−when conveniently functionalized with reactive pyridyl receptors, have emerged as effective sensors to detect and quantify chemical species with high accuracy through fluorescent imaging and spectroscopy. Among the sensing targets, monitoring of harmful chemical species, e.g., metal ions (zinc, copper, iron, mercury, cadmium, lead, etc.) and anions (chloride, fluoride, sulfide, thiocyanate, etc.) can be used to understand their physiological and pathological role in live-cells and tissues, as well as to protect human health. This chapter focuses on recent advances in the molecular design of pyridyl-substituted fluorophores, their photophysical properties, and sensing applications.
Keywords
- molecular design
- fluorescent dyes
- pyridyl receptors
- photophysical properties
- sensing behavior
1. Introduction
Fluorescence detection techniques have become of paramount importance for monitoring biochemical and biological processes, allowing the detection and quantification of levels of chemical species in the human body and in the surrounding environment. Indeed, fluorescence sensing is a highly sensitive technique having numerous parameters that can serve as analytical information, including decay time, energy transfer, and quenching efficiency, in addition to the more conventional measurement of fluorescence intensity or polarization. Through the design of fluorescent dyes (fluorophores), it is possible to obtain molecules and materials that respond to the presence of a target analyte through changes in its physicochemical properties, presenting typically high sensibility and selectivity, quick response time and simplicity of measurement, and quantification of the analyte [1].
When combined with specific receptor units, fluorescent dyes can be extremely useful in several applications such as detection and quantification of chemical species, as well as in understanding their physiological and pathological role in cells and tissues. Receptors based on the pyridyl group are of major importance in ligand design for many of the above applications. The pyridine ring possesses a dipole moment found to be 2.22 D; therefore, it exhibits greater electronegativity as compared with the phenyl ring [2]. The pyridyl groups, such as di-(2-picolyl)amine (DPA), are excellent metal ion binding sites for the construction of fluorescent probes and can be attached to specific fluorophores or integrated into the fluorophore as part of the metal binding group, as found in quinolines. The principal fluorescence mechanisms involved in the design of the chemosensors are schematized in Figure 1 and include:
Photoinduced electron transfer (PET, Figure 1a): Originally proposed by A. Prasanna de Silva and coworkers [3], PET involves the use of fluorophore-spacer-receptor-type structures. The spacer is used to separate the fluorophore from the receptor at a certain distance while allowing the intramolecular electron transfer causes the interruption of the fluorophore’s fluorescence. The interaction of the analyte with the receptor causes a change in the redox potential of the receptor and the electron transfer became energetically unfavorable, which leads to the re-establishment of fluorophore’s fluorescence;
Intramolecular charge transfer (ICT, Figure 1b): In ICT, the fluorophore can integrate the receptor unit and is characterized by a donor and an electron acceptor group, forming a push−pull system. When the analyte, in particular charged species, interacts with the receptor, causes the strengthening or weakening of the push−pull character, leading to a change in the emission band. This is a characteristic process of ratiometric sensors [4];
Resonance energy transfer (FRET, Figure 1c). This mechanism involves the energy transfer from the excited state of a “donor” fluorophore to an “acceptor” fluorophore. In most cases, FRET occurs between two distinct fluorophores with overlapped emission spectrum of the “donor” and the absorption spectrum of the “acceptor” [4].
Such fluorescence mechanisms have inspired the development of new fluorescent structures and materials for the preparation of optical sensors for analyte detection in real scenarios. This chapter will focus precisely on recent advances in the molecular design of pyridyl substituted fluorophores, their photophysical properties, and sensing applications.
2. Pyridyl groups in fluorescent dyes
2.1 Rhodamine dyes
2.1.1 Molecular design
Xanthene is a heterocyclic organic compound with yellow coloration that contains two benzene rings connected through an oxygen atom and a methylene group (Figure 2). This class of dyes comprises fluorescein, rhodamine, and rhodol derivatives. Rhodamines were first produced in the late nineteenth century. They can be distinguished from other dyes by the presence of
The derivatization reaction of the carboxylic group at position 2′ of the xanthene leads to spirocyclic derivatives (closed form);
Modification at positions 3, 4, 5, and 6. In some cases, the alkylation at positions 3 and 6 can promote a bathochromic shift, which increases with the increase in the degree of alkylation, while in other cases, the functionalization of the amino groups of xanthene moiety can cause the total loss of fluorescence [5];
Modifications in the periphery of the phenyl ring at positions 4′ and/or 5′ are difficult to perform, especially when aiming to prepare isomerically pure derivatives from the sequential Friedel–Crafts reaction of an aminophenol with an asymmetric anhydride. This reaction usually led to a mixture of two isomers often difficult to separate and purify. Some of these derivatives are used for labeling molecules of interest [5];
Modifications at position 9 are used for the synthesis of dihydro derivatives;
Substitution of the xanthene heteroatom (O), for example, by Si can potentiate the absorption and emission capacity in the near-infrared region, fluorescence quantum yield, or fluorescence intensity [6].
Some studies also focus on the influence of the positional isomers of the pyridine’s nitrogen (
2.1.2 Photophysical properties
The excellence of the photophysical properties of rhodamines is one of the main reasons for their success and wide application in several areas. Rhodamines possess high molar absorptivity coefficient (
One of the most interesting features of rhodamine derivatives is the existence of two isomeric forms-spirolactone (closed form) and quinoid (opened form) (Figure 2)-with very different optical properties. The spirolactone form is colorless and nonfluorescent, while the open form is highly fluorescent and has a pink coloration. The open form owes its properties to its extended π-conjugation and the interconversion from the closed to open form allows the rhodamine derivatives to possess an
2.1.3 Sensing applications
Rhodamines are frequently used in the preparation of highly selective, fast response, and sensitive sensing tools, employed in the detection of contaminants and environmental parameters in air, water, and waste [9]. Figure 2 shows a series of selected examples of rhodamine derivatives/probes, those structural and photophysical features and sensing behaviors will be discussed in the next paragraphs.
One of the most explored rhodamine-based dyes for conjugation with pyridyl derivatives is rhodamine B hydrazide (
In 2012, a study related to the influence of the number, nature, and size of coordinating entities was reported [12]. The synthesized probe
A similar dye with a longer spacer (
Kan and co-workers reported two probes (
Some probes are designed to incorporate selected receptor groups, such as sulfur derivatives-thiourea, sulfonyl, or thiol groups. Sarkar and co-workers [20] prepared a rhodamine-linked pyridyl thiourea probe (
In 2015, Fu and co-workers prepared three novel rhodamine-triazine aminopyridine derivatives, in which the
In 2014, a study based on the influence of different substituents attached to the
Another strategy for the design of rhodamine-pyridine probes is by conjugation with other dyes or aromatic rings. In 2016, a rhodamine derivative incorporating a 2-[(1H-pyrrol-2-ylmethyl)-(2-pyridinyl-methyl) amino]- tripodal receptor was reported (
In 2019, our work group designed a series of pyridyl analogs of rosamines (rhodamine derivatives lacking the carboxylic group at position 2′ of the benzenic ring) and studied the influence of solvent and charge on their photophysical properties [30]. It was found that the structural variation involving the position of the
Many other examples of rhodamine-pyridyl derivatives can be found in literature for selectively sensing several analytes, such as picric acid [32].
2.2 BODIPY dyes
2.2.1 Molecular design
The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, also known as boron dipyrrin or boron dipyrromethene (BODIPY), is one of the most popular families of organic fluorophores that have found numerous practical applications as fluorescence probes for bioimaging and sensing, laser dyes, and as bright pigments in various fields of technology,
From the molecular design point of view, the BODIPY dye (Figure 3) can be functionalized at the pyrrolic ring, at the central
2.2.2 Photophysical properties
The BODIPY typically exhibits a weak absorption band in 350–450 nm region and a strong absorption band in the 450–580 nm, corresponding to
2.2.3 Sensing applications
Several BODIPY derivatives having very attractive photophysical properties and photochemical stability have found very useful applications as fluorescent platforms for sensing applications. The introduction of the pyridyl or polypyridyl groups at the periphery of the BODIPY core can lead to a large variety of chemosensors for detecting anions, cations, amino acids, etc. Figure 3 shows a series of selected examples of these BOPIDY derivatives with different sensing behaviors.
Y. Wu and co-workers [37] reported one of the most notable examples of
Another example of a
Developed by G. T. Sfrazzetto and co-workers [39], probe
Through a benzyl pyridinium cleavable unit at
The dyad
In a similar approach, dyad
The functionalization of the BODIPY dye at the
Probe
2.3 1,8-Naphthalimide dyes
2.3.1 Molecular design
1,8-Naphthalimide (
2.3.2 Photophysical properties
The spectroscopic properties of 1,8-naphthalimides are strongly dependent on the C-4 substituent group. To increase the fluorescent quantum yield, the substituent group at the 4-position should be an electron-donating group. Other features that contribute for 1,8-naphthalimides extensive use are related with their extraordinary thermal and chemical stability.
2.3.3 Sensing applications
Several
The first example, probe
In the next example, a fluorescent ion-imprinted probe (
The third example, probe
The fourth example represents a ratiometric and selective fluorescent probe (
In the last example, Lee and co-workers designed a pyrene-appended naphthalimide, probe
2.4 Coumarin dyes
2.4.1 Molecular design
Coumarins are a large family of compounds containing the 2
2.4.2 Photophysical properties
Although the coumarin unit exhibits a very weak fluorescence, the introduction of proper substituents originates new coumarin derivatives with significant fluorescence in the visible light range. Hundreds of coumarin dyes have been developed as active components due to their improved quantum yields, tunable emission wavelengths, and the fact that they are very responsive to the polarity of their microenvironments. The previously published results on the photophysical properties of fluorescent coumarins have revealed important structure-property relationships, which have also been important to guide the design of fluorescent chemosensors.
2.4.3 Sensing applications
A vast variety of coumarin-derived fluorescent chemosensors were built by combining the coumarin moiety with other functional receptors. Herein we present a series of coumarin derivatives in which the receptor is a pyridyl moiety (Figure 5).
L. Wang and co-workers reported two ratiometric probes,
J. Portilla and co-workers reported the coumarin probe (
Another example of a simple coumarin-pyridyl probe was presented by K. Xu and co-workers. The study presents two probes, but we will focus on probe
Probe
The next examples were designed by F. Yu and co-workers and represent two-photon fluorescence probes, probes
2.5 Other pyridyl-based fluorophores
In quinoline dyes, for example, the pyridyl group is part of the fluorophore, as well as an integral part of the metal-binding group. This fluorophore can be conveniently functionalized with several substituent groups for sensing essentially Zn2+ including 6-methoxy-(8-
3. Conclusions
The optical properties of dyes as well as their sensitivity and selectivity toward analytes are highly dependent not only on the fluorophore backbone but also on its substituents and the solvent in which the detection occurs.
Throughout the chapter, several classes of fluorescent dyes-rhodamines, BODIPY’s, 1,8-naphthalimides, and coumarins-functionalized with reactive pyridyl receptors were examined. The presented examples explored the strategies used for structural optimization to improve sensing abilities using the principal fluorescence sensing mechanisms. In coming years, new developments are expected toward better sensitivity and selectivity of the probes, to improve their application in the detection and quantification of important analytes in the fields of health and environment.
Acknowledgments
This work received financial support from PT national funds (FCT/MCTES, Fundação para a Ciência e a Tecnologia, and Ministério da Ciência, Tecnologia e Ensino Superior) through the project UIDB/50006/2020, UIDP/50006/2020, PTDC/QUI-QIN/28142/2017, EXPL/QUI-OUT/1554/2021 and PARSUK for the Portugal-UK Bilateral Research Fund (BRF 2022). A. M. G. Silva and A. Leite thank FCT for funding through program DL 57/2016 - Norma transitória.
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