Imidazole as a parent π-conjugated backbone in charge-transfer chromophores

Research activities in the field of imidazole-derived push–pull systems featuring intramolecular charge transfer (ICT) are reviewed. Design, synthetic pathways, linear and nonlinear optical properties, electrochemistry, structure–property relationships, and the prospective application of such D-π-A organic materials are described. This review focuses on Y-shaped imidazoles, bi- and diimidazoles, benzimidazoles, bis(benzimidazoles), imidazole-4,5-dicarbonitriles, and imidazole-derived chromophores chemically bound to a polymer chain.


Introduction
Over the past three decades, great progress has been made in the development and the investigation of new organic push-pull systems. In contrast to inorganic materials, the advent of dipolar (hetero)organic materials with readily polarizable structure was stimulated by their relative ease of synthesis, well-defined structure, chemical and thermal robustness, possibility for further modification, and facile property tuning. Hence, heteroaromatic push-pull chromophores have been targeted and investigated as active components of optoelectronic devices, organic light-emitting diodes (OLED), photovoltaic cells, semiconductors, switches, data-storage devices, etc [1][2][3]. A typical one-component organic D-π-A chromophore consists of a π-conjugated system end-capped with strong electron donors D (e.g. NR 2 or OR groups) and strong electron acceptors A (e.g. NO 2 or CN groups). This D-π-A arrangement assures efficient intramolecular charge transfer (ICT) between the donor and acceptor moieties and generates a dipolar push-pull system featuring low-energy and intense CT absorption ( Figure 1). The polarizability and the respective optical linear and nonlinear (NLO) properties of these systems depend primarily on their chemical structure, in particular, the electronic behavior of the appended donors and acceptors and the character and length of the π-conjugated linker [4][5][6][7]. thermal robustness, good solubility in common organic solvents, and should be available in reasonable quantities. Hence, various five-and six-membered heterocycles were utilized as suitable π-conjugated chromophore backbones. Moreover, heteroatoms may act as auxiliary donors or acceptors and improve the overall polarizability of the chromophore. In this respect, five-membered diazoles, in particular imidazole, seem to be suitable parent π-conjugated backbones. Imidazole possesses two nitrogen atoms of different electronic nature, represents a robust and stable heterocycle, and can easily be synthesized and further functionalized at positions C2, C4, and C5 in addition to N1. On the imidazole backbone, two principal orientations of the substituents are possible, and these are most frequently used to generate Y-shaped chromophores as shown in Figure 2. The donor appended through an additional π-linker to the imidazole C2, completed with two peripheral acceptors linked at the imidazole C4/C5 positions, generates the first class of chromophores (D-π-IM-(π-A) 2 systems). The second class (A-π-IM-(π-D) 2 systems) possesses one acceptor and two donors in the reversed orientation. A nonsymmetrical orientation of the donors and acceptors is scarce, most likely due to a more difficult synthesis.
The purpose of this article is to review the recent progress in the design, development, and investigation of imidazole-derived charge-transfer chromophores. Synthetic pathways, linear and nonlinear optical properties, electrochemistry, and the prospective application of such organic materials are described. Metal complexes and metal sensitizers are not covered in this review.

Review Synthesis of imidazole-derived chromophores
A condensation of α-diketones and aldehydes in the presence of ammonia or ammonium salts (Debus-Radziszewski synthesis) is one of the oldest, most versatile, and most frequently employed methods used for the construction of imidazole (glyoxaline) derivatives [8][9][10]. This simple synthetic pathway is also widely employed for the construction of variously substituted 2,4,5-triarylimidazole-derived chromophores (lophines), as shown in Scheme 1 [11][12][13][14][15]. A similar synthetic strategy is used for the construction of diimidazole-type push-pull systems bearing two imidazole rings, which serve as donor and acceptor moieties [16][17][18][19]. A sequential construction of the chromophore backbone by modern cross-coupling reactions represents another synthetic approach used for the synthesis of superior diimidazole chromophores [20]. Benzimidazole D-π-A derivatives are a well-investigated class of charge-transfer chromophores. Although many synthetic approaches are known to date [10,21,22], the most popular ones involve the condensation of appropriately substituted arylenediamines or o-nitroanilines with an aldehyde or carboxylic acid, as well as Debus-Radziszewski synthesis as shown in Scheme 1 [23][24][25][26].
(non)linearities can be ascribed to Moylan, Miller, and co-workers as early as 1993 [31,32]. Donor-acceptor-substituted imidazoles, oxazoles, and thiazoles were synthesized, and their properties were compared within the individual series of substituents as well as across the three heterocyclic rings ( Figure 3, Table 1). These A-π-IM-(π-D) 2 systems possess exceptional thermal stabilities, respectable dipole moments, and significant nonlinearities. It was found that the chromophore nonlinearity depends primarily on the type of substituents A/D and secondarily on the nature of the conjugating heterocyclic ring (Table 1).   More recently, Bu and co-workers also contributed significantly to the field of imidazole-derived CT chromophores for NLO. The first class of studied compounds 5a-c resembles those chromophores reported by Moylan et al.: The parent π-conjugated backbone of N-methyllophine end-capped with two donors and one acceptor [33]. Bu's further efforts were focused on (i) the incorporation of an additional, readily polarizable heterocycle, such as thiophene or thiazole; (ii) the improvement of the electron-withdrawing ability of the used acceptor; and (iii) the elongation of the π-conjugated pathway. Thus, the first series of chromophores (5a-c) was completed with the thiophene-derived system 6 [14] with a tricyanovinyl acceptor moiety and chromophores 7a-c [33] featuring a thiophene π-linker and a nitrostyryl acceptor. The molecular structures of compounds 6 and 5c were also confirmed by X-ray analysis [34]. The last series of investigated compounds involved donor 4,5-disubstituted imidazoles 8a-d [35] with acceptors at C2 linked through a thiazole-styryl π-linker ( Figure 4). Whereas the series 5a-c and 7a-c showed promising optical nonlinearities, high thermal stability, excellent solubility, and good transparency, molecules 8a-d were investigated as two-photon absorbing chromophores (Table 2). Bu and co-workers also investigated the fluorescence properties of this family of imidazoles [36].
In addition to the work of Moylan and Bu, several other groups, mainly from Asia, reported the synthesis and application of Y-shaped imidazole-derived CT chromophores. Wang and co-workers investigated simple tripodal chromophores 9a-d with nitro, dialkylamino, and hydroxy groups as acceptor and donors [13]. Whereas the imidazole-and thiazole-based chromophores 10a,b possess two extended π-linkers with the imino  spacers at the imidazole C4/C5 and nitro and dimethylamino groups as acceptor and donor [15], chromophore 11 (VPDPI) represents a polarizable blue-light-emitting material [37]. The newly synthesized chromophores were investigated in terms of their absorption and emission properties, molecular first-order hyperpolarizability β, measured by solvatochromic method at 1907 nm, and thermal stability determined by TGA or DTA ( Figure 5, Table 3). Imidazoles 12 (DIYSP, δ = 41 GM, [38,39]) and 13 (FD3, δ = 1556 GM, [40]) were developed as two-photon absorbing and fluorescent A-π-A' chromophores, which undergo photopolymerization or can be applied as fluorescent sensors for (homo)cysteine. Donor 4,5-disubstituted imidazole derivatives 14 bearing a cyanoacrylic moiety connected to imidazole C2 by thiophene or thiazole π-linkers were recently utilized as dye-sensitized solar cells with an efficiency up to 6.3% [41].  Several similar classes of imidazole-derived push-pull compounds can be found in the literature. They were mainly investigated in terms of their synthesis and basic (non)linear optical properties [42][43][44][45][46].
The nonlinear optical properties of donor-and acceptor-substituted five-membered heterocycles, such as imidazole, oxazole, and thiazole, were also investigated by DFT calculations [48,49]. These theoretical results confirmed, in general, the experimental data and trends discussed above.

Diimidazole-derived chromophores
The aforementioned charge-transfer chromophores 4-26 consist of a 1,2,4,5-tetrasubstituted imidazole ring, which may act as either a donor or acceptor moiety depending on the orientation of substituents. A π-conjugated backbone end-capped with donor-and acceptor-substituted imidazole rings constitutes a diimidazole-derived push-pull, push-push, and pull-pull charge-transfer chromophore. The most common synthetic approach to diimidazoles, with the rings connected at C2, is shown in Scheme 1. Typical diimidazole chromophores in D-π-A arrangement ( Figure 9) were investigated by Wu and Ye et al. Within the last ten years, diimidazole D-π-D systems were extensively studied, in particular for their easy synthesis and unique properties [19,[54][55][56]. Their general structure is shown in Figure 10 and selected properties are summarized in Table 6.   [19,[54][55][56] and photochromic diimidazoles 32,33 [57,58].

Benzimidazole-derived chromophores
In contrast to imidazoles, benzimidazoles possess fused benzene or higher (hetero)aromates, generally appended at C4/C5. This arrangement enables (i) an extension of the chromophore π-conjugated system; (ii) a planarization of the molecule; (iii) facile functionalization of the fused aromate by known methods; and (iv) a straightforward synthesis starting from inexpensive and readily available compounds (Scheme 1).

Scheme 3:
Oxidation of 1H-diimidazoles to 2H-diimidazoles (quinoids). Typical representatives of benzimidazole-derived D-π-A systems are shown in Figure 11. In 2004, Carella, Centore, and co-workers [25] reported the synthesis and further application of nitrobenzimidazole-derived anilines 35 and 36. These two compounds were further used for the construction of various charge-transfer chromophores 37-43, in particular by simple diazotation and subsequent azo-coupling of the terminal NH 2 group [62][63][64][65][66]. Chromophores 37-43 found wide application as polymer dopants, cross-linkable organic glasses or inorganic-organic hybrid materials and showed high, stable, and tunable NLO performances, very good thermal stability, and, last but not least, easy synthesis from low-cost commercial precursors (Table 7).
Raposo and co-workers investigated benzimidazole derivatives 44-47 with either a donor-or acceptor-substituted benzene ring, whose π-conjugated pathways comprise thiophene and pyrrole subunits [24]. This series of chromophores was further extended by arylthienylimidazole phenanthrolines 48-52 and oligothienylimidazole phenanthrolines 53-57 ( Figure 12; [23,67]). The benzo [d]imidazole core in compounds 46-57 behaves as an electron acceptor and, when substituted with electron donors at C2, an efficient ICT can be achieved. Consequently, the measured hyperpolarizabilities β increase with the rise in donating ability of the appended donors or extension of the π-conjugated path. Thiophene, used as a part of the π-linker, particularly in chromophores 53-57, caused β enhancement up  to 320 × 10 −30 esu (Table 8). This clearly demonstrates the beneficial role of the thiophene as a polarizable unit and auxiliary electron donor. A combination of fused phenanthroline-imidazole acceptor moiety, N,N-dimethylamino donor, and arylthienyl π-linker, as in 50, resulted in a CT chromophore with β = 189 × 10 −30 esu. It should also be noted that all chromophores showed exceptionally high thermal stability with T D up to 470 °C.
In 2007, Liu et al. reported a very nice example of D-π-A system 63 based on benzimidazole as a parent π-conjugated backbone fused with TCAQ (tetracyanoanthraquinodimethane) and TTF (tetrathiafulvalene) as acceptor and donor moieties, respectively [71]. This molecule was investigated in terms of absorption spectroscopy, X-ray analysis, and electrochemistry and showed remarkable responses as a function of pH. Unfortunately, no NLO properties were investigated.
Benzimidazole-derived compounds were recently also used as chromophores with switchable properties. Benzimidazolo Similar to diimidazole compounds 27-34, two benzimidazole cores may also be incorporated into the chromophore backbone. The molecular structures of recently investigated bis(benzimidazole)-derived chromophores 67-71 are shown in Figure 13. All these bis(benzimidazole) systems were primarily studied as fluorescent compounds. Polymeric chromophores 67 and 68 showed blue fluorescence with emission maxima at 410-515 nm [75]. A-π-D-π-A molecules 69 featuring a central phenothiazine donor moiety and two peripheral benzimidazole acceptor units were investigated by Ahn et al. [76]. These ambipolar molecules possess energy levels that are wellmatched with the Fermi levels of the electrodes to facilitate the electron or hole injection and transfer in OLED devices. 2,5-Bis(benzimidazol-2-yl)pyrazine derivatives 70 (BBIP), with improved solubility through N,N'-dialkylation, exhibited high fluorescence intensity even in protic solvents, as well as interesting solvatochromic properties [77]. Terphenyl-bridged bis(benzimidazolium) salts 71, soluble in water and common organic solvents, emit blue light with λ max,em at 420-441 nm in thin films [78]. This feature makes them potentially applicable as blue-light emitters in OLEDs.
Benzimidazole-based push-pull systems were studied also theoretically. Abe et al. studied pyridinium betaines of general formula 72 consisting of negatively charged benzimidazolate and a positively charged pyridinium ion ( Figure 13; [79]). Moreover, the π-conjugated system was systematically enlarged and either donor-or acceptor-substituted in order to generate D-π-A-π-D and D-π-A-π-A systems. The performed ab initio and INDO/S MO calculations of ground-state dipole moments and first-order hyperpolarizabilities β revealed that the latter chromophore arrangement resulted in significantly enhanced nonlinearities. The benzimidazolate anion as a donor moiety was quantum-chemically studied also by Xu, Su, and co-workers [80]. Structurally highly similar chromophores to 44-47 ( Figure 12), reported by Raposo [24], were investigated by means of molecular geometry optimization, absorption/emission spectra, first-order hyperpolarizability calculations, and simulation of NH proton abstraction by using a fluoride anion. Remarkably large differences between the β values of protonated/deprotonated forms showed that benzimidazoles are potent molecules for a new type of NLO molecular switching.
The chemistry of 4,5-dicyanoimidazole was reviewed in 1987 by Donald and Webster [100] and its application in liquidcrystal media and devices was again summarized in a Merck patent in 2004 [101].
In 2004 and 2005, Carella, Centore, and co-workers utilized 2-amino-4,5-dicyanoimidazole 83 (for X-ray structure analysis, see [102]) in the synthesis of chromophores 84-86 featuring central phenylazo π-linker, 4,5-dicyanoimidazole as acceptor, and N,N-dialkylamino donor ( Figure 15; [103,104]). The nonlinear optical properties of these three chromophores were investigated by EFISH experiment ( Table 9). The molecular structure of chromophore 84 was also confirmed by X-ray analysis. These chromophores, with free terminal OH-functions, were further used as monomers for copolymerization with polyester, polyuretane, and polymethacrylate (see below). Structurally very similar chromophore 87 (R = H; R 1 = CH 2 CH 2 OH; R 2 = Et) was used for incorporation into the sol-gel hybrid  films based on alkoxysilanes [105,106]. This new material is to be applied as an electro-optic modulator. The π-conjugated path was systematically varied and enlarged in order to study its influence on the chromophore polarizability. The chromophores were primarily investigated by electronic-absorption spectra, electrochemistry, X-ray analysis, and quantum-chemical calculations. The resulting data set was further processed by factor analysis to deduce the structure-property relationships. The most important structural factors affecting the (non)linear optical properties and electrochemical behavior are (i) the presence of a strongly conjugating donor and (ii) the length and (iii) planarity of the π-conjugated system. In this respect, chromophores 90c, 92c, and 93c seem to possess one of the better balances between performance and practicality within the studied series.
The photoinduced absorption, birefringence, and secondharmonic generation of chromophores 88c-93c (D = NMe 2 ) embedded within polymethylmethacrylate matrices were  studied and complimented by quantum-chemical calculations. These doped polymer films showed very efficient and tunable nonlinearities with β av ranging from 899 to 25798 au (Table 10; [108]).
Moreover, the N,N-dimethylamino donor in 88c-93c can easily be protonated. Whereas in the unprotonated form (88c-93c), an efficient ICT from the donor to the acceptor exists (D-π-A system), in the protonated forms (88cH + -93cH + ) only dimin- Figure 17: pH-triggered NLO switches 88c-93c [109]. ished ICT between the π-linker and the peripheral acceptors A and A + takes place (Figure 17; [109]). This results in a high contrast in the nonlinearities between both forms (Table 11) as well as in a raised energy and character of the HOMO ( Figure 17). Hence, chromophores 88c-93c proved to be very efficient pH-triggered NLO switches.
The fluorescent and photophysical properties of chromophores 88-93 were further studied [110,111]. The fluorescence was studied in various solvents and polymer matrices and at various temperatures. Intense fluorescence with quantum yields of 0.05 to 0.98 was observed in nonpolar solvents and polymer matrices within the range of 320 to 528 nm (Table 10).
The first set of 4,5-dicyanoimidazole-derived chromophores 88-93 possessed only one donor at the imidazole C2. Hence, our further synthetic efforts were focused on the synthesis of branched chromophores 95-100 ( Figure 18; [112]). The synthesis of this series of chromophores involved two-fold Suzuki-Miyaura and Sonogashira cross-coupling reactions on dibromoolefin 94 (for X-ray structure see [113]). This compound proved to be a very useful, fully planar precursor for the construction of a chromophore π-conjugated backbone. In contrast to 88-93, the presence of two (or four) N,N-dimethylamino donors and the systematic extension of the π-linkers in 95-100 resulted in a bathochromically shifted CT-band, lowered electrochemically measured and calculated HOMO-LUMO gaps, and enhanced first-order hyperpolarizability up to 70 × 10 −30 esu (Table 12).
Organic π-conjugated materials based on 4,5-dicyanoimidazole were recently developed as opto-electronic materials with a practical application. For instance, in 2002 Yang et al. [114] reported a fairly simple organic-electrical bistable device (OBD) based on amine 83 ( Figure 15). Yang's OBD consisted of organic material based on 83 with a built-in thin aluminum active layer. The OBD's conductivity in the two electric states was considerably different, and, moreover, the OBD showed remarkable stability without significant device degradation over a million write-erase cycles. Hence, the performance of this device makes OBD attractive for application in rewritable memory cells. In 2007, Sellinger et al. became very interested in the Heck coupling of N-alkyl vinazenes with various (hetero)aromates [115]. This synthetic interest resulted in four new diimidazole compounds 112-115 ( Figure 20). This series of basic π-conjugated compounds was significantly extended in 2009 by a library of various π-linkers [116]. As a materials researcher, Sellinger applied these n-type conjugated materials as small-molecule electron acceptors. The combination of V-BT (114) with polyhexylthiophene donor (P3HT) in an initial organic solar cell showed high external quantum efficiencies exceeding 14%. Sellinger's further efforts were focused on improving optical, photovoltaic, and charge-transport properties as well as efficiencies of V-BT derived solar cells. Thus, he studied new processing techniques for solar cells, the use of Figure 19: Imidazole as a donor-acceptor unit in CT-chromophores 101-111 [20].   [115,116].

Imidazole chromophores incorporated into the polymer
Recently, imidazole-derived CT chromophores found wide application either as polymer dopants (guest-host systems) or in polymers with chemically bonded NLO-phores (side-chain, main-chain, and cross-linked). An incorporation of the chromophore into the polymer backbone brings with it a higher and facile polarizability, higher thermal stability, and NLO responses as well as prospective applicability in modern materials chemistry. The second-order susceptibilities of nonlinear optical polymers are historically referred to as "d ij " coefficients (1/2 of the respective χ ij (2) values). The electro-optic coefficient r ij , indicating the degree of the refractive index change caused by a unit increase in the voltage applied across the polymer film, is another important feature of the nonlinear optical polymer waveguides. The relationship between the d and r coefficients can be simplified according to the following equation (1) where n is the index of refraction. However, only two components of the d and r coefficients that are parallel and perpendicular to the average dipolar chromophore axis are important and investigated (d 33 , d 31 and r 33 , r 31 ). The physical stability of the nonlinear optical polymers refers to the stability of alignment of the chromophore. The glass transition temperature (T g ) and the decomposition temperature (T D ) are the most widely provided parameters of polymer physical stability. The polar order of the polymer (centrosymmetry removal) is usually achieved by the electric-field, thermal (T p ) and optical poling procedures [121].
Only the polymer systems with covalently attached imidazole CT chromophores will be discussed in the following section.
Tang et al. showed another approach to producing nonlinear optical polymers. The synthetically easily available hydroxy lophine 124 was covalently bonded to the polyphosphazene backbone and subsequently modified by post-azo coupling with variously substituted benzenediazonium salts to afford systems 125-130 ( Figure 22; Table 15; [125][126][127]). These systems possess good optical transparency, high T g , and large d 33 (SHG) and photoinduced birefringence values relative to those known  for polyphosphazenes to date. Last but not least, this simple synthetic pathway opens space for manifold elaboration and functionalization of various prepolymers in order to enhance their nonlinearities. Recently, Müllen et al. [19] as well as Koszykowska et al. [128] contributed to the field of nonlinear optical polymers ( Figure 22). Müllen's imidazole-functionalized poly(p-phenylene) 131 proved to be a promising hole-transporting emissive material, which can be oxidized to quinoid (Scheme 3) with an additional low-wavelength absorption at 655 nm (lightabsorbing material for solar cells Typical representatives of benzimidazole CT chromophores 37-43, intended as reactive monomers for incorporation into the polymer backbone, were investigated by Carella, Centore et al. [62,63] and Cross et al. [65,66] and are shown in Figure 11. The   terials exhibited their longest absorption maxima λ max at 490-515 and third-order NLO susceptibility χ (3) (3ω;ω,ω,ω) within the range of 1.5 to 2.6 × 10 −13 esu (measured by THG at 1064 nm).

Conclusion
This review has attempted to show that 1,3-diazole, imidazole, may act as a robust and stable parent π-conjugated backbone for organic chromophores with intramolecular charge transfer. This synthetically readily accessible five-membered heteroaromate and its push-pull derivatives are currently of high interest for materials chemists due to their unique and tunable properties. In general, the imidazole-derived chromophores may possess two Y-shaped arrangements: One electron donor at C2 and two electron acceptors at C4/C5 or vice versa. Hence, according to the C4/C5 substitution, the entire imidazole moiety may behave as an electron acceptor or donor. Taking our series of structurally similar chromophores 21-26 and 88-93 as an example, which primarily differ in the orientation of the substituents along the imidazole ring, C4/C5 donor-substituted imidazole derivatives showed higher nonlinearities. This implies that imidazole is more polarizable in the direction C4/C5→C2. However, two imidazole units that are differently C4/C5 substituted and connected at C2 may be employed as acceptor or donor moieties. It was shown that this diimidazole arrangement (e.g., in 101-111) represents very powerful chromophore with high nonlinearities. Push-pull benzimidazoles feature more-planar π-conjugated systems due to the fused benzene ring. This fact further improves the polarizability of the entire D-π-A chromophore (e.g., compare chromophores 5-8 with 37-40). The structure and the length of the π-linker connecting both acceptor and donor moieties play a crucial role. It was shown that polarizable subunits, such as olefins and thiophenes, increase the chromophore (hyper)polarizability significantly. Thus, the most important structural factors affecting D-A interaction responsible for the linear and nonlinear optical properties are (i) the strength of the appended donors and acceptors; (ii) the length and electronic nature of the π-conjugated path; and (iii) chromophore overall planarity. These three features mainly dictate the chromophore properties and, therefore, are mainly used to finely tune the desired (non)linearities. Imidazole-derived chromophores have found also a wide range of practical applications in OLEDs, OPVCs, switches, memories, and polymers. A combination of all of these properties makes imidazole a very promising scaffold for materials chemistry.