An overview of the recent developments in functional dyes, which are useful for hi-tech applications for those based on optoelectronics, such as dye sensitized solar cells, photochromic dyes and biomedical applications, such as photodynamic therapy for the treatment of cancer and fluorescent sensors is presented.
Functional dye term appeared for the first time in 1981 by Japanese scholars [
Hi-tech applications based on optoelectronics, such as dye sensitized solar cells, photochromic materials, liquid crystal displays, and the newer emissive displays such as organic light emitting devices; electronic materials, such as organic semiconductors; imaging technologies, such as electrophotography (photocopying and laser printing), thermal printing, and especially ink-jet printing; “invisible” imaging by using infrared absorbers in optical data storage, computer-to-plate and security printing; biotechnology as dye-affinity chromatography for the purification of proteins and enzymes; biomedical applications, such as fluorescent sensors and anticancer treatments such as photodynamic therapy, created the need for novel dyes to meet new and demanding criteria.
Dyes, and related ultraviolet and particularly infrared active molecules, which have been specifically designed for these hi-tech applications, are generally called functional dyes.
The present state of the art article attempts to provide an overview of the recent developments in functional dyes. Emphasis will be paid to the recent design of dye molecules, which are useful for hi-tech applications for those based on optoelectronics, such as dye-sensitized solar cells (DSCs), photochromic dyes, and biomedical applications, such as photodynamic therapy (PDT) for the treatment of cancer and fluorescent sensors.
The forecasts for the next 50 years predict that human energy needs are likely to double while fossil energy reserves are shrinking. At the same time, on earth, per year, plants and bacteria capture and convert light into a quantity of biological material greater than 100 times the food needed for mankind [
The natural light harvesting systems of plants and photosynthetic bacteria are one of the most fascinating functional molecular assemblies, which inspired many scientists for mimicking such process to harvest the solar energy. The first requirement for a light-harvesting system is, of course, its capacity to absorb light. If the system has to be used for artificial solar energy conversion, the absorption spectrum of its components should cover a substantial part of the visible spectral region. Another essential property of the light-absorbing units of an antenna system is their chemical and photochemical stability. As schematized in Figure
Schematic representation of an artificial antenna for light harvesting. Absorption of light by an array of chromophores is followed by vectorial energy transfer that leads excitation energy to a reaction center indicated by
The conversion of sunlight into electricity is a clean, abundant, and renewable energy source. The efficiency of conventional solar cells, or photovoltaic devices, made from inorganic materials reached up to 24
New ways of manufacturing solar cells that can scale up to large volumes and low cost are required. In this interest a new generation of solar cells called dye-sensitized solar cells (DSCs) has been reported in 1991 by O’Regan and Grätzel [
A dye-sensitized solar cell of Grätzel type [
Grätzel type DSC.
Light absorption is performed by a monolayer of dye (D) adsorbed chemically at the semiconductor surface and excited by a photon of light (
For efficient operation of the cell, the rate of electron injection must be faster than the decay of the dye-excited state. Also, the rate of reduction of the oxidized sensitizer (dye cation) by the electron donor in the electrolyte (
Dye sensitizers serve as the solar energy absorber in DSC, whose proprieties will have much effect on the light harvesting efficiency and the overall photoelectric conversion efficiency. The ideal sensitizer for DSCs should absorb all light below a threshold wavelength of about 920 nm. In addition, it should be firmly grafted to the semiconductor oxide surface and inject electrons to the conduction band with a quantum yield of unity. Its redox potential should be sufficiently high that it can be regenerated rapidly via electron donation from the electrolyte or a hole conductor. Finally, it should be stable enough to sustain at least
The sensitizers used in DSC were divided into two types: organic and inorganic dyes according to the structure. Inorganic dyes include metal complex, such as polypyridyl complexes of ruthenium and osmium, metal porphyrin, phthalocyanine, and inorganic quantum dots, while organic dye includes natural and synthetic organic dyes. To date, DSCs with Ru bipyridyl complexes (N3 and N719) and the black ruthenium dye (Figure
Chemical structure of N3, N719, and the black ruthenium dye.
However, the noble metal Ru is a limited resource and is expensive. In order to obtain even cheaper photosensitizer for DSCs, metal-free organic photosensitizers are strongly desired. Metal-free organic dyes offer superior molar extinction coefficients, low cost, and a diversity of molecular structures. Recently, novel photosensitizers such as coumarin [
For a higher photovoltage and higher conversion efficiency, the charge recombination process at the interface should be retarded [
An efficient dye is usually designed to be of donor-conjugated bridge-acceptor structure called donor-acceptor architecture. Most dye molecules have only one donor group [
Chemical structure of designed dyes for DSCs.
The structural design of the dyes 1–8 is shown in Figure
Light stability of organic dye sensitizers is another factor to be considered in the structural design of the dye molecules, since the main disadvantage of organic dye sensitizers in comparison with inorganic ones is their stability, which is generally lower than that of metal complexes, because of side reactions such as formation of excited triplet states and unstable radicals. In this interest, the design features indicated in dye
It has been reported that dye aggregation on the semiconductor surface is the main factor in decreasing the efficiency of sensitized photocurrent generation [
Unlike artificial dyes, the natural ones are available, easy to prepare, low in cost, nontoxic, and environmentally and fully biodegradable. In this interest the use of anthocyanins such as cyanin dye (Figure
Chemical structures of cyanin dye.
The structure of the dye has the design needed for DSCs as it has two othro hydroxyl groups as donors in conjugation with oxonium moiety. This feature would lead to the formation of orthoquinoide form upon anchoring onto
Schematic representation of anchoring cyanin dye (quinonodial form) onto
Photochromism refers to a reversible phototransformation of a chemical species between two forms having different absorption spectra [
An important class of organic photochromes at the present time is the spiropyrans and the closely related spirooxazines. As shown in Figure
Photochromism mechanism of a typical spiropyran or spirooxazine.
Another promising family of photochromic compounds is provided by the synthetic flavylium salts, benzopyrylium derivatives, which have the same basic structure as anthocyanins mentioned above. Photochromism based on flavylium systems arises from the photoinduced
Highly transparent photochromic gels in the visible region can be prepared by the incorporation of flavylium compounds in Pluronic matrixes (ABA-type tri-block copolymers composed of poly(ethylene oxide) (PEO) (A) and poly(propylene oxide) (PPO) units (B)). The chromophore 7-(N,N-diethylamino) 4-hydroxyflavylium switches from its yellow chalcone form to the red flavylium cation upon irradiation and reverts back to the chalcone form in the dark.
Recent progress in information technologies toward increasing capacity of optical information carriers and information processing rates makes urgent the development of new lightsensitive media for use in super high-capacity optical memory. Compared to the thermal principle of data recording on modern optical disks, the photochemical principle of optical data recording on photochromic multilayer optical disks can be expected to ensure a higher density of data recorded on the pico- or femtosecond scale.
Therefore, photonic devices that incorporate photochromic molecules represent the future of digital optical storage where the recording of information is an all photomode recording method that allows the read, write, and erase functions to be controlled by light energy [
One promising approach is the development of photochromic materials. These materials can interconvert between two distinct isomeric states when stimulated by two different wavelengths of light, where each isomer of the photochromic materials can represent “0” and “1” of a digital mode. Molecules with this behavior are the best promising candidate optical storage materials as opposed to heat-mode recording employed with the optical media currently in use.
The most well-known organic photochromic switches are based on the ring opening/ring closure reactions in photoinduced electrocyclic reactions. There are six main groups of compounds that have found numerous applications as switching molecules: dihydroazulenes and vinylheptafulvenes [
Structures of opened (left) and closed (right) forms of various representatives of the photochromic switch families. From top to bottom: dihydroazulene, dihydroindolizine, diarylethene, dihydropyrene, spiropyran, spirooxazine, naphthopyran, and fulgide. Colored forms of photochromic compounds are marked with shading.
There are many other examples of photoswitchable compounds, but these six classes of compounds are the most widely studied ones. Photoswitching of all of them involves reversible formation and opening of rings, and closed-ring forms of naphthopyrans, spiropyrans, spiroxazines, and fulgides are colorless, while the opened forms are strongly colored; in the case of diarylethenes and vinylheptafulvenes, the opened forms are colorless, and the closed forms are strongly colored. Many molecules have been designed to achieve thermal stability, and they prospectively can be applied in molecular memories as optical storage materials. However, numerous molecules undergo thermal back reactions. This process is disadvantageous from the point of view of long-term information storage, but for short-term storage and modeling of logic gates, it does not constitute any obstacle [
It has been reported that photochromic compounds suitable for photonic applications must meet the following requirements: high cross section for two-photon absorption, high efficiency of photochromic transformations, thermal stability and fatigue resistance to irreversible photo- and thermoinduced transformations of both forms (colorless and colored), and nondestructive readout by any suitable method (via fluorescence, refraction, reflection, polarization, etc.) [
On the other hand, the inclusion of photochromic molecules such as naphthopyrans in a rigid matrix of poly(
Photochromic dyes, such as spirooxazines, are disperse dyes. Structurally, there are similarities with traditional disperse dyes for textiles in that they are small- to medium-size neutral molecules with a balance of hydrophilic and hydrophobic character. The major difference, however, is that they are nonplanar in their ring-closed form because the two ring systems of the molecules are connected through a spiro linkage (
Since the color change of these organic photochromic dyes is triggered by UV light, these dyes also have a UV protection function. Hence, applying photochromic dyes to a fabric can impart to the fabric, enhanced UV protection, smart fashion effects, functional effects such as camouflage, security printing, and for use in “smart” textile applications in general. In this prospective, recent research activity is growing for the application of photochromic compounds on textiles [
The application of spirooxazines on polyamide fibers by exhaustion method at high temperature has been reported. However, the exhaustion percentage values of spiroxazine dyes were very low [
In an attempt to increase the exhaustion percentage and to better have wash fastness, disperse-reactive spirooxazine dyes containing dichlorotriazine were used for dyeing polyamide fabrics (Figure
Reversible photoconversion structures between ring-closed colorless and ring-opened colored spiroxazine forms.
Conventionally dyed photochromic fabric suffers from less dye uptake and low photochromic performance as a consequence of the rigid polymer matrix physically restricts the photochromic transformation. On the other hand, surface coating of photochromic microcapsules imparts a relatively harsh handle to the coated fabric.
Therefore, Sol-gel technique is thus the choice as it provides nanosized pores which can provide free space for photochromic transformations. In this interest, a thin film coating onto wool fabrics with spirooxazine photochromic dyes encapsulated in silica nanoparticles by sol-gel method resulted in better photochromic performance, good durability, and minimal influence on the fabric handle [
Photodynamic therapy (PDT) is a light-activated treatment which is used in the clinic to destroy diseased tissues [
Dyes and pigments selectively absorb or transmit/reflect wavelengths of visible light and thus allow the perception of color in everyday life. In terms of light absorption, this depends on electronic transitions within the dye chromophore; thus certain dye types may only absorb specified wavelengths of light, depending on the relative dispositions of the electronic singlet ground and excited state energy levels.
Promoted electrons generally revert to the ground state via a mixture of nonradiative processes. Those which do not follow this route include fluorescent and phosphorescent dyes which lose energy, in part, by light emission and photosensitizing dyes which can use the excitational energy for reaction. Electronically, the main difference between photosensitiser molecules and other dye types is that the absorbed light energy can be passed on efficiently to other molecules in the vicinity or be utilised for photochemical reaction. The electronically excited photosensitizer molecule (singlet excited state) is relatively stable and can undergo an electronic rearrangement (to the excited triplet state,
Photosensitization pathways:
Excitational energy transfer from the triplet state to other molecules facilitates photodynamic action through two types.
Photosensitizer action thus is aided by a relatively long-lived singlet excited state, since this is required for intersystem crossing to occur and populate the triplet excited state. However, this must also be sufficiently stable to allow
The quantity of singlet oxygen generated by a photosensitizer in type II is regulated by the efficiency of a spin-forbidden electronic transition from a singlet to a triplet state (ISC). Chemically this entails the inclusion in the chromophoric moiety of heavy atoms and is normally achieved by the substitution of chromophoric protons by lower group halogens (usually bromine or iodine) or lower group replacement of heteroatoms (e.g., sulphur for oxygen, selenium for sulphur, etc.). The introduction of a heavy atom into a molecule is known to have an influence over the rates of the ISC and is termed the heavy-atom effect [
On the other hand, it would be advantageous to have control over absorbance wavelength, as it could be envisaged that different tissue types would require different excitation wavelengths dependent upon tissue location (deep-seated tumors versus skin lesions), size, density, and pigmentation. The depth of light penetration through tissue is directly related to wavelength [
The depth that light penetrates into tissue is dependent upon the optical properties of the tissue and the wavelength of the light. At wavelengths longer than 900 nm water starts to absorb, whereas at wavelengths less than 700 nm scattering by macromolecules is significant, and absorption by endogenous chromophores, such as haemoglobin, becomes dominant at wavelengths below 650 nm [
Photofrin was the first and at the present time remains the most common, clinically used PDT agent. Although it has been approved for use in the United States, Canada, Japan, and Europe for the treatment of esophageal, endobroncheal, bladder, lung, stomach, cervical, and skin cancers [
Several inconvenients have been appeared over the last years upon using different photosensitizers as a PDT, and these may be due to the poor quantum yield of singlet oxygen generation, the weak absorption in the therapeutic window (700–950 nm), where optimal penetration of light into tissue is afforded, and/or the incompatibility of photosensitizer with patients cells.
Cyanine dyes have strong absorption (
Cyanine dyes consist of two heterocyclic nuclei containing nitrogen centers linked through an odd number of methine bonds, which are normally in the trans conuration, in such a way that resonance occurs through the conjugated system between the tertiary and quaternary nitrogen atoms. The carbon atoms of the methine groups may be attached to groups other than hydrogen, or may be part of carbocyclic or heterocyclic ring systems. Certain cyanine dyes such as merocyanines and ketocyanines have been studied extensively as potential photosensitizers for photodynamic therapy PDT [
In this respect, the chemical structure of merocyanine MC540 (Figure
The use of heavy atoms also stabilises the polymethine chain to photoisomerisation (a major deactivation pathway) [
Cyanine photosensitisers such as Indocyanine Green (Figure
The investigation of cyanine photosensitizers for the development of novel PDT agents has been growing. Recently, some new azo-cyanine (delocalized cationic azo dyes, as shown in Figure
It has been reported that the substitution of one of the oxygen atoms of the central four member ring with amino group enhanced the production of singlet oxygen [
The practical importance of functional dyes used in optical chemical sensor development has received increased attention in recent years. Fluorescent sensors represent a versatile tool for visualizing specific molecular targets and events in vitro and increasingly in vivo [
A fluorescence-based technique for sensor applications offers significant advantages over other techniques due to its generally nondestructive character, high sensitivity, and specificity. Chemosensors are molecules or molecular systems changing their optical or electrical properties upon interaction with small anions, cations, or neutral molecules.
Chemically driven molecular switches usually molecules comprise three main building blocks: receptor moiety, linker (spacer), and the reporter moiety. Receptor moieties are specially designed binding sites for triggering ions or molecules. They should exhibit desirable selectivity and sensitivity toward selected triggers. The linker, in turn, should provide electronic communication between the receptor and reported moieties.
There are three main ways of providing sufficient electronic communication: (i) bridge providing overlap of
The most chemical switches with optical readout are usually based on the following photophysical phenomena: photoinduced electron transfer (PET) and photoinduced charge transfer (PCT).
The principle of the PET-based chemical switch is shown in Figure
Principle of the PET chemically driven luminescent molecular switch. The HOMO level of the unbound receptor acts as an electron donor and effectively quenches the fluorescence of the reporter moiety (a). Upon coordination of the substrate, the energy of the HOMO level of the receptor is decreased because of electrostatic interaction with cationic species, and PET quenching is no longer possible (b).
Importantly, the rate of electron transfer is much faster than the luminescence when PET is thermodynamically allowed (Figure
PET-based luminescent switches may be triggered by various chemical and physical stimuli: protons, metal cations, anions, neutral organic molecules, and even nanoparticles. Depending on the desired spectral properties and lifetime of the fluorescent switch, the molecular assembly may include various organic (anthracene, pyrene, naphthalimides, pyromellitimide, coumarins, fluoresceins, pyrazolobenzothiazoles, and diphenylpyrazoles) or inorganic fluorophores (polypyridine RuII complexes and lanthanide complexes) [
If the trigger ion is an open shell transition metal ion, the quenching occurs upon binding as PET involves the
Two anthracene-based switches show this phenomenon. Tertiary aliphatic amines are efficient quenchers of anthracene emission, upon the protonation of which the fluorescence is observed, as in the case of molecule SWITCH-1. Exchange of the aliphatic amine for pyridine moiety yields the ON/OFF switch: protonated amine becomes an efficient quencher (SWITCH-2) [
The fluorescence can also be easily switched on with lithium and other alkali metal cations (SWITCH-3) [
The opposite effect (ON/OFF switching on ion binding) was observed in the case of the fluorescent switch based on quinacridone skeleton with two diamine pendant arms (SWITCH-6) [
Extremely intense (
Another large category of chemically driven optical switches encompasses the PCT systems. The principle of the PCT-based chemical switch is shown in Figure
Principle of the PCT chemically driven luminescent molecular switch based on the donor-spacer-acceptor architecture. Binding of cationic trigger to the donor (green) moiety results in the hypsochromic shift of absorption (emission) band (a), and binding of the same trigger to the acceptor moiety (red) results in bathochromic shift of the corresponding transition (b). The multireceptor system may exhibit both bathochromic and hypsochromic shifts upon binding different trigger ions (c), which results in a multistate molecular switch.
In such push-pull systems, excitation leads to redistribution of electron density and generation of dipole moment. If the receptor binds a charged trigger species, the additional charge interacts with the photogenerated dipole, thus modifying the fluorescence spectrum. Repulsive interaction results in the hypsochromic shift (Figure
As shown in Figures
The naphthyridine-based SWITCH-9 shows a strong absorption band at 450 nm. Binding of one mercury equivalent increases the asymmetry of electron distribution within the molecule, which in turn results in the bathochromic shift of the lowest energy absorption band. Binding of the second mercury ion results in turn in a hypsochromic shift of the low-energy band due to interaction of bound cations with photoinduced dipole moments. This switch, due to low binding constant, requires high concentrations of triggering cation [
Dithiazacrown receptors are characterized by a very high affinity toward mercury ions, and even very low concentrations are sufficient to trigger optical changes of the squaraine-based switch (SWITCH-10). Binding of
Fluorescent marker technology is a process that fluorescent compound is combined with the labeled-object by physical or chemical action to form a complex. By detecting the fluorescence intensity, the message of the labeled object is transferred qualitatively or quantitatively. As one of the widely used fluorescent labeling compounds (fluorescence dye sensor), fluorescent dyes have many advantages, such as fast detection speed, good repetitiveness, low dosage, and nonradiation [
Determination of nucleic acids is required in many fields, such as molecular biology, biotechnology, and medical diagnosis. As mentioned above fluorescence-based technique for the detection of nucleic acids offers significant advantages over other techniques due to its high sensitivity and specificity. Nucleic acids by themselves are nonfluorescent and thus they need to be bound with certain fluorescent probes such as cyanine dyes. Due to their excellent staining properties, cyanine dyes have been utilized in biological applications such as the detection of DNA [
The excitation and emission spectra of cyanine fluorescent sensors span the visible and NIR spectra from 450 nm, where autofluorescence from biomolecules is greatly reduced, up to 800 nm. Biochemical processes frequently involve protonation and deprotonation of biomolecules with concomitant changes in the pH of the medium. Consequently, there has been an increase in the use of pH-sensitive fluorescent probes for intracellular studies [
Recently, new cyanine fluorescent pH sensors have been reported [
Excitation and emission spectra of dye
The natural light harvesting complexes of plants are one of the most fascinating functional molecular assemblies. Scientists are yet to challenge and to mimic the nature in harvesting the solar energy. As we have seen, solar energy conversion using DSCs is possible; however future research work in this field is required to better have the suitable molecular design of sensitizers.
The investigations presented in this review clearly indicate that information processing at the molecular level is possible using photochromic dyes and remained to be developed in the thermal stability of the photochromes so as to have all light responsive function.
The use of photochromic dyes in the coloration of textile is in its fancy stage and it is anticipated that this trend in combination with nanotechnology will get more effective in the production of smart textiles.
Also, some success in the field of photodynamic therapy has been made using functional dyes and future development is needed to design photosenitizers able to selectively interact with the tissue and with high productivity of singlet oxygen.
Last but not least, known small-molecule fluorophores have a wide range of spectral and chemical properties. Elaboration of these core structures has provided numerous sensors for analytes and for assaying biological systems. Nonetheless, extraordinary opportunities remain, because delving deeper into biochemical and biological phenomena will require ever more sophisticated and tailored sensors.