The inverted solvatochromism of protonated ferrocenylethenyl-pyrimidines: the first example of the solvatochromic reversal of a hybrid organic/inorganic dye

Matías Vidala, Camila Pastenesab, Marcos Caroli Rezendea, Carolina Aliagaab and Moisés Domínguez*a
aFacultad de Química y Biología, Universidad de Santiago de Chile, Av. B.O'Higgins, 3363, Santiago, Chile. E-mail: moises.dominguez@usach.cl
bCentro para el Desarrollo de la Nanociencia y la Nanotecnología, CEDENNA, Chile

Received 22nd August 2019 , Accepted 29th September 2019

First published on 30th September 2019


Five new solvatochromic 2,6-diaryl-4-ferrocenylethenylpyrimidines were synthesized and their spectral variations in solution investigated in twenty-seven solvents of variable polarity. Their observed positive solvatochromism in HBD- and non-HBD-solvents changed to an inverted behavior upon protonation of the pyrimidine ring by the addition of trifluoroacetic acid. Multiparametric analysis of the solvatochromic band of the protonated dyes showed that the solvent acidity and polarizability were mainly responsible for their inverted behavior, which arose from an internal charge-transfer from the ferrocene fragment to the pyrimidinium acceptor, confirmed by theoretical calculations.


1 Introduction

Solvatochromic dyes are useful sensors for the study of the microstructure of a vast number of chemical and biological systems, such as pure solvents,1–3 solvent mixtures,4–7 cyclodextrin inclusion complexes,8–10 micelles,11–14 or proteins.15 They can be constructed by connecting an electron-donor/electron-acceptor pair by a π-electronic system. The combination of purely organic groups, such as a phenolate or aminophenyl donor with a pyridinium,16–18 quinolinium,19 acridinium,20 or nitrophenyl21–24 acceptor has given rise to a variety of organic dyes displaying negative (increased transition energies of the solvatochromic band with the increased solvent polarity), positive (decreased transition energies of the solvatochromic band with the increased solvent polarity), and inverted (transition from a positive to a negative behavior at a specific polarity value) solvatochromism.

Metal-containing solvatochromic complexes are also suitable for applications as sensors. Some examples include the positive solvatochromic dyes [CuII(acac)(tmen)]ClO4,25 and [FeIII(CN)5imidazol]Na3,26 which are the basis for scales of solvent polarity.27,28 There are also some examples of metal-containing dyes exhibiting negative solvatochromism, such as the Mo0(CO)4(bipy)3,29 and the FeII(phen)2(CN)2[thin space (1/6-em)]30 complexes, which are the basis of the Ek,29 and EMLCT29 solvent polarity scales, respectively.

Hybrid dyes, with an organometallic fragment conjugated with an organic moiety, are also found in the literature. Among them, ferrocene has been employed as an electron-donor organometallic group in conjugation with organic fragments such as pyrimidines,31–34 pyridines, and N-alkylpyridinium derivatives.35 Some of these dyes have exhibited interesting nonlinear optical properties.36–38 The ferrocenyl donor group, conjugated with various electron-withdrawing groups,39,40 has given rise to solvatochromic derivatives, though with little regularity of their charge-transfer values in different solvents.

Despite the initial debate on the real existence of the phenomenon of inverted solvatochromism,21,41–44 several purely organic-based sensors exhibiting this spectral behavior have been reported.6,21,45,46 Nevertheless, after 40 years from the discovery of the inverted solvatochromism, there are no reports on metal-containing dyes exhibiting inverted solvatochromism. We have been interested in structure/behavior relationships of inverted solvatochromic dyes,21,45–49 therefore, in continuation of our effort, in the present work we describe a series of alkenylpyrimidines 1 (Fig. 1) with the ferrocenyl as the electron-donor group, which, upon protonation, give rise to pyrimidinium derivatives 2 exhibiting an inverted solvatochromism.


image file: c9qo01043b-f1.tif
Fig. 1 Solvatochromic ferrocenylethenylpyrimidine dyes 1a–e, and their protonated derivatives 2a–e studied in the present work.

The analysis of their UV-Vis absorptions in a variety of solvents with a wide range of polarities revealed two types of solvatochromism for the neutral family 1a–e and their protonated analogs 2a–e. The neutral compounds 1a–e showed a positive behavior with a moderate solvatochromic range, while the protonated series 2a–e exhibited inverted solvatochromism. As far as we are concerned, this last series constitute the first example of an inverted solvatochromic dye with an organometallic moiety.

2 Results and discussion

2.1 Synthesis of the dyes

The synthesis of the ferrocenylpyrimidine dyes 1a–e (Fig. 2) starts with the Palladium-catalyzed Suzuki–Miyaura cross-coupling reaction of the tosylate 3 with various aryl boronic acids in water, and under microwave irradiation50 to provide the methylpyrimidines 4a–e in very good to excellent yields (75–97%). Aldol-type condensation of the methylpyrimidines with ferrocenecarboxaldehyde 5 in aqueous sodium hydroxide, employing Aliquat 336 as a phase-transfer catalyst and 16 h heating in a sealed tube provided dyes 1a–e in good to excellent yields (54–91%).
image file: c9qo01043b-f2.tif
Fig. 2 Synthesis of the ferrocenylethenylpyrimidine dyes 1a–e.

The protonated dyes 2a–e were obtained in situ by the addition of 1–5 μL of trifluoroacetic acid to solutions of 1a–e in every solvent before recording their UV-Vis spectra, as is presented in Fig. 3.


image file: c9qo01043b-f3.tif
Fig. 3 Ferrocenylethenylpyrimidine dyes 1a–e and their protonation to generate 2a–e.
2.1.1 UV/Vis spectroscopic studies. The spectral behavior of the ferrocenylpyrimidine dyes was investigated in a collection of solvents of a wide range of solvent polarity values ENT.1 Twenty-seven solvents were evaluated for the neutral dyes 1a–e, and twenty-one for protonated dyes 2a–e. Electronic transition energies in kcal mol−1 of both series in the investigated solvents are listed in Table 1.
Table 1 Normalized solvent polarity values ENT and electronic transition energies ET of the solvatochromic bandsa of neutral 1a–e, and the protonated 2a–e in various solvents
Entry Solvent ENT 1a 1b 1c 1d 1e 2a 2b 2c 2d 2e
a ET calculated from the corresponding λnm values (nm), from the relationship ET = 28.590/λnm. 1 kcal mol−1 = 4.184 kJ mol−1.b Not recorded due to incomplete protonation of dyes 1a–e (see main text).
1 1,2-Ethanediol 0.790 58.50 58.61 58.75 58.42 58.57 49.12 49.32 49.58 49.27 49.05
2 Methanol 0.760 58.70 58.74 58.88 58.57 58.68 48.80 49.00 49.32 48.93 48.74
3 Ethanol 0.650 59.02 59.08 59.25 58.85 59.03 48.34 48.61 49.02 48.42 48.22
4 1-Propanol 0.620 59.13 59.10 59.34 58.91 59.06 48.16 48.35 48.74 48.10 47.98
5 1-Butanol 0.590 59.17 59.28 59.48 59.05 59.23 47.94 47.85 48.76 47.88 47.98
6 1-Pentanol 0.586 59.32 59.44 59.44 58.95 59.17 47.97 48.11 48.79 48.30 47.87
7 1-Hexanol 0.560 59.27 59.32 59.44 58.95 59.27 47.85 47.86 48.66 47.57 47.89
8 2-Propanol 0.550 59.44 59.32 59.69 59.07 59.35 47.71 47.34 48.54 47.87 47.76
9 1-Octanol 0.540 59.40 59.48 59.71 59.13 59.36 47.56 47.70 48.42 47.51 47.62
10 2-Pentanol 0.488 59.42 59.49 59.59 59.21 59.39 47.49 47.22 48.05 46.95 47.42
11 3-Pentanol 0.463 59.49 59.59 59.86 59.19 59.43 47.13 47.30 47.89 46.87 47.31
12 Dimethylsulfoxide 0.440 58.11 58.23 58.19 57.99 58.35 b b b b b
13 N,N-Dimethylformamide 0.390 58.35 58.61 58.47 58.23 58.47 b b b b b
14 2-Propanone 0.360 58.50 58.47 58.78 58.65 58.65 46.85 46.84 47.35 46.51 46.98
15 1,2-Dichloroethane 0.330 58.77 58.71 58.95 58.59 58.59 46.74 46.67 47.24 46.37 46.82
16 Dichloromethane 0.310 58.80 59.03 59.14 58.64 58.88 46.79 46.57 47.05 46.37 46.83
17 Trichloromethane 0.260 59.05 59.13 59.19 58.85 58.97 46.69 46.67 47.21 46.38 46.92
18 Ethyl acetate 0.230 59.34 59.56 59.51 59.32 59.22 46.82 46.89 47.01 46.69 46.94
19 Tetrahydrofuran 0.210 59.65 59.32 59.44 59.21 59.19 b b b b b
20 1,4-Dioxane 0.160 59.52 59.69 59.86 59.32 59.42 b b b b b
21 Methyl-tertbutyl ether 0.124 59.69 60.07 60.07 59.69 59.64 b b b b b
22 Diethyl ether 0.120 60.07 59.94 60.32 59.69 59.54 b b b b b
23 Benzene 0.110 59.86 59.96 59.96 59.83 59.72 47.05 47.00 47.30 46.94 47.22
24 Toluene 0.100 59.82 60.08 59.96 60.01 59.86 46.83 47.06 47.54 46.72 47.13
25 n-Octane 0.012 60.20 60.47 60.89 60.40 60.11 47.08 47.18 47.58 47.48 47.42
26 n-Hexane 0.010 60.41 60.57 60.70 60.19 60.15 47.25 47.10 47.71 47.41 47.57
27 Cyclohexane 0.006 60.45 60.45 60.57 60.21 60.19 47.18 47.10 47.81 47.18 47.65


Comparison of the two families of dyes, reveals the effect of increasing the electron-accepting ability of the pyrimidine ring on the solvatochromism of the resulting dyes. Protonation of solutions of the neutral pyrimidines facilitates the charge transfer from the ferrocenyl donor group to the pyrimidine ring, resulting in significant bathochromic shifts (to longer wavelengths) to cause changes in the color of the solutions, as is illustrated in Fig. 4, where the solvatochromic absorption band of dyes 1a and 2a differs in 127 nm (ca. −12.30 kcal mol−1).


image file: c9qo01043b-f4.tif
Fig. 4 Solutions of neutral 1a and protonated 2a in dichloromethane (c = 5 × 10−5 M).

The addition of 1–5 μL of trifluoroacetic acid was enough to achieve the complete protonation of the pyrimidine ring of dyes 1a–e to provide dyes 2a–e in solution exclusively. The presence of the protonated species was confirmed in every solvent recorded by the complete disappearance of the absorption band of the neutral dyes 1a–e. Nevertheless, results in very basic solvents, such as dimethylsulfoxide (SB = 0.647), N,N-dimethylformamide (SB = 0.613), tetrahydrofuran (SB = 0.591), 1,4-dioxane (SB = 0.444), methyl-tertbutyl ether (SB = 0.567), diethyl ether (SB = 0.562), were discarded because the complete protonation of dyes 1a–e was not achieved due to a protonation competition with the solvent.

The dyes 2a–e proved to be very stable in solution after the addition of TFA. None degradation of the dyes nor spectra deformation was observed during the experiments, even after 180 minutes of the initial addition of TFA, and even at 40 times of TFA excess in relation to the original experiments. The results of stability experiments are presented in the ESI.

Variations of the transition energies of the solvatochromic band in the investigated solvents were quite different for both series (Table 1). Neutral pyrimidines 1a–e showed a positive-solvatochromic behavior in both Hydrogen-Bond Donor (HBD)- and in non-HBD solvents, with linear dependences of their ET on the solvent polarity parameter ENT (Fig. 5a), exhibiting correlation coefficients of r2 = 0.945, and r2 = 0.971, respectively. By contrast, in the same set of solvents, the protonated series 2a–e showed an inverted solvatochromic behavior (Fig. 5b).


image file: c9qo01043b-f5.tif
Fig. 5 Variations of the electronic transition energy ET of dyes 1a/2a as a function of normalized solvent polarity values ENT. (a) Neutral dye 1a in HBD-(blue) and non-HBD-solvents (red) in twenty-seven solvents; (b) protonated dye 2a in twenty-one different solvents.

The inverted solvatochromic behavior of dyes 2a–e with the varying solvent polarity ENT was fitted with the hyperbolic fitting shown in eqn (1), where the parameter p provides the polarity value where the solvatochromic inversion takes place.46 All the hyperbolic fitting exhibited good correlation coefficients (r2 = 0.91–0.97), and the inversion polarity point for dyes 2a–e occurs at ENT = 0.189–2.263, close to the polarity of trichloromethane, and ethyl acetate, such as other conspicuous inverted solvatochromic dyes.51,52

 
ET = ET0 + n(ETp)2 (1)

The solvatochromism of dyes 1a–e and 2a–e are similar in non-HBD solvents, exhibiting both positive solvatochromism, nevertheless, whereas in HBD-solvents dyes 1a–e still display positive solvatochromism, dyes 2a–e exhibit negative-solvatochromic behavior.

The decreased transition energies observed with the increased solvent polarity values in non-protic solvents (positive solvatochromism) can be attributed for both series to the effect of the increased solvent dipolarity, and solvent polarizability of those solvents. These solvent properties should be responsible for the better stabilization of a polarized excited state, arising from an extended charge-transfer from the ferrocenyl donor to the pyrimidine (or pyrimidinium) acceptor. However, the opposite behavior of the two series in protic solvents is in principle a bit puzzling. It is clear from Fig. 4 that protonation of the pyrimidine ring leads to smaller transition energies for the solvatochromic band of the neutral dyes 1a–e.

Pyrimidine protonation should promote the internal charge-transfer to a better electron-accepting moiety, thus reducing the corresponding transition energies. Nevertheless, the same argument can be invoked to explain the positive solvatochromism of the neutral dyes in the presence of alcohols of increasingly HBD acidity (Fig. 5a). For the protonated series 2a–e, however, the solvent HBD acidity seems to produce the opposite effect (Fig. 5b).

An explanation for this apparent inconsistency can be sought in the effect of hydrogen-bond interactions between the solvent and the cyclopentadienyl moiety of the ferrocenyl fragment. The fully protonated pyrimidinium ring of dyes 2a–e is no longer available for hydrogen-bond interactions with acidic alcohols. The electron-rich region of these molecules is now the cyclopentadienyl moiety, which, by interaction with HBD-solvents, reduces the electron-donating ability of the ferrocenyl group, thus increasing the corresponding transition energy of the solvatochromic band of the dye in these media. This explanation finds support in the observations made by Perepichka et al.,39 that replacement of the ferrocenyl donor by the more inferior electron-donating ruthenocene unit led to hypsochromic shifts of the solvatochromic band of the resulting dye. Either by complete replacement of the donor metallocene for a less electron-donor unit or by reduction of its electron-donating ability through interaction with HBD-solvents, the effect is the same: increased transition energy of the solvatochromic band of the dye.

In search of a quantitative support to the above qualitative rationalizations, a multiparametric regression analysis of the transition energies of protonated dyes 2a–e was performed employing eqn (2) and Catalán's parameters for solvent HBD acidity (SA), Hydrogen-Bond Acceptor (HBA) basicity (SB), polarizability (SP) and dipolarity (SdP).53

 
ECalcT = ET0 + aSA + bSB + cSP + dSdP (2)

The magnitude and sign of the resulting coefficients a, b, c, and d describe the effect of each of these solvent properties on the solvatochromic transition energy of the protonated dyes. Their effects are depicted visually in Fig. 6.


image file: c9qo01043b-f6.tif
Fig. 6 Percentile contribution of Catalán's solvent acidity (SA, blue), basicity (SB, orange), dipolarity (SdP, red) and polarizability (SP, green) to the experimental solvatochromism of dyes 2a–e.

As anticipated in the qualitative rationalization of their inverted behavior, the solvatochromism of dyes 2a–e is predominantly affected by the solvent HBD acidity (SA) and polarizability (SP). They have opposite effects on the solvatochromic transition energies: in non-HBD solvents, where acidity contributions are absent, or nearly absent, increased solvent polarizability reduces energy values ET, leading to a positive solvatochromism. In protic solvents, acidity contributions become increasingly important, leading to the inversion of the positive solvatochromism to negative solvatochromic behavior.

2.2 Quantum-mechanical calculations

Protonation of dyes 1a–e can take place on the two different nitrogens (N1 or N3) of the pyrimidine ring. Both, the product resulting from protonation at N1 and N3 were optimized at the TPSS/def2-TZVP level in dichloromethane, employing the SMD model.54 The energy difference between the two isomeric pyrimidinium derivatives obtained B3LYP/def2-QZVP method as single point calculations was rather small (2.63 kcal mol−1), suggesting that both species should be in equilibrium in solution.

In order to shed light on the nature of the solvatochromic band of the neutral and the protonated dyes, spectrum calculations with TDDFT method at the B3LYP/def2-QZVP level of theory, employing the SMD = dichloromethane were conducted.

In agreement with previous theoretical studies on the electronic transitions of ferrocenyl dyes,39,55 a complex picture emerged of the origin of the solvatochromic bands of dyes 1a and 2a, with the involvement of several low-intensity transitions from metal d-orbitals to the organic acceptor fragment. Fig. 7 shows the main molecular orbitals involved in the longest-wavelength absorption bands of the neutral dye 1a and 2a. The electronic transition in both cases involved a charge transfer from the HOMO, which is a molecular orbital resulting from the mixture of the metal d-orbitals and the cyclopentadienyl orbitals to the LUMO or the LUMO+1 for dyes 1a and 2a, respectively.


image file: c9qo01043b-f7.tif
Fig. 7 Frontier molecular orbitals of dyes 1a and 2a in dichloromethane, obtained by the TDDFT B3LYP/def2-QZVP SMD method.

The computational calculations confirmed the experimental trend of a bathochromic shift of the solvatochromic band upon protonation (see Fig. 4). The absorption band at 53.44 kcal mol−1 (535 nm), calculated for neutral 1a shifts to 47.73 kcal mol−1 (599 nm) upon protonation of the N1 pyrimidine atom. When compared with the data of Table 1, the absolute value of the calculated transition energies was excellent (47.73 vs. 46.76 kcal mol−1) for the protonated dye 2a, and departs in ca. 5 kcal mol−1 (53.44 vs. 58.80 kcal mol−1) for the neutral pyrimidine derivative 1a. Finally, both transitions could be described as internal charge-transfer processes from the ferrocenyl iron d-orbitals, with the inclusion of the cyclopentadienyl π-system, to the heterocyclic pyrimidine (or pyrimidinium) acceptor.

3 Conclusions

Two new families of solvatochromic dyes with a ferrocenyl unit as the electron-donor group and a pyrimidine or a protonated pyrimidinium acceptor group were synthesized and their spectral behavior recorded in twenty-seven different solvents of varying polarity for the neutral dyes 1a–e, and in twenty-one for the protonated dyes 2a–e.

The two series showed the same positive solvatochromic behavior in non-HBD solvents. However, in HBD solvents, their behavior was significantly different, with the neutral series 1a–e showing positive solvatochromic behavior, while the protonated series 2a–e exhibiting negative solvatochromism. The observed inverted solvatochromism in the series 2a–e was rationalized in terms of the opposing effects of the medium HBD acidity and polarizability on the longest-wavelength transition of theses dyes.

Although previous examples are found in the literature of solvatochromic (positive and negative) dyes with a hybrid structure comprising an organometallic unit conjugated with an organic moiety, series 2a–e constitute, to our knowledge, the first example of such hybrid dyes that exhibit inverted solvatochromism.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

M. V. thanks project 021941CR-POSTDOC from DICYT-USACH. C. A. and C. P. thank to CEDENNA PB0807 project.

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Footnote

Electronic supplementary information (ESI) available: Experimental procedures, copies of 1H NMR and 13C NMR spectra, computational details, acid and time stability experiments of dye 2a. See DOI: 10.1039/c9qo01043b

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