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Seven Chromisms Associated with Dithizone Lumanyano L. A. Ntoi, Blenerhassitt E. Buitendach, and Karel Grobler von Eschwege J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09490 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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The Journal of Physical Chemistry

Seven Chromisms Associated with Dithizone Lumanyano L. A. Ntoi, Blenerhassitt E. Buitendach, Karel G. von Eschwege* *Department of Chemistry, University of the Free State, PO Box 339, Bloemfontein, 9300, South Africa

S Supporting Information

ABSTRACT: Variations in the electromagnetic wavelengths of absorption and reflection of molecules do not only make life colorful, but is often of central importance in solar energy conversion and opto-electronic information processing and transfer.

Dithizone and its

derivatives and complexes are, to our knowledge, probably the most “colorful” compound, being responsive to no less than seven different external stimuli that effects color change. Apart from a more detailed discussion of the cyclic voltammetry and observed electrochromism

in

concentratochromism,

the

SCH3-substituted

solvatochromism,

free

ligand

halochromism,

and

mercury

complex,

thermochromism

and

chronochromism in the ligand, and photochromism in the carboxy-functionalized phenylmercury(II) complex are also presented here. Color changes are either associated with proton transfer or loss, or isomerization.

INTRODUCTION Intensely dark-green colored diphenylthiocarbazone (dithizone, H2Dz) originally earned its place in analytical chemistry because of the colorful complexes it forms with numerous metals.

Pi-electron delocalization along the geometrically flat dithizone backbone (see

Scheme 1, left)1 gives rise to a HOMO that stretches along the entire ligand, including the two phenyl end groups.2 This elongated HOMO persists in the ligand also when complexed to metals.3 In 1977 Irving published an extensive overview on this century-old compound.4,5 First signs of chromism were noticed by him6 and Webb,7 independently discovering photochromism in the orange bis-dithizonatomercury(II) complex. 8

Meriwether reported similar observations for several metals.

Fifteen years later

The geometry of the photo-

induced blue isomer (see Scheme 1, right) and the reaction mechanism that proceeds via a conical intersection were only recently clarified by means of DFT calculations9 and ultra-fast laser studies.10 At the same time both solvatochromism and concentratochromism were observed in the free ligand.2 By means of time-dependant DFT calculations the green to 1

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orange color change was conclusively ascribed to intramolecular proton transfer, i.e. the standard green thione form existing at higher concentrations in non-polar organic solvents, while converting to the orange thiol tautomer at low concentrations in polar methanol (Scheme 1, left). The absorbance spectrum of the orange tautomer is very similar (λmax = ca 470 nm) to that of the singly deprotonated anionic species, HDz−. The latter characteristic classifies dithizone as additionally also being halochromic, i.e. changing color as a consequence of change in pH. As opposed to the first proton, the second imine proton may not readily be removed. Tests in our laboratory did however show that this may indeed be accomplished, namely by sodium metal in nonprotic media, resulting in the magenta-colored disodium salt, Na2Dz .

Ph

H

thiol orange

S N

C N

Hg

H

orange

N

S

N

N

C N

R

R

H N N

R

R

hν

∆T Ph

thione green

H

S

N

C N

R

Hg

H

blue

N

N

C N

N

-H

S

R

+

R

N H

N

R

anion orange

S N

C N

R

H N N R

Scheme 1. Left: Chromism in the dithizone free ligand is attributed to intramolecular proton transfer as seen in the orange thiol and green thione species, and also to proton loss that yields the orange anionic species. Right: Photochromism in dithizonato metal complexes is attributed to isomerization around the C−N bond. R: –COOH, H, –CH3 or –SCH3 are substituents relevant to this report.

UV-visible and cyclic voltammetry studies led to spectro-electrochemistry investigations of the orange dithizonatomercury(II) and purple cobalt(III) complexes, which revealed these compounds as being distinctly electrochromic by nature.11,12

The extended unsaturated

molecular backbone and the presence of two imine protons allow for the possibility of a series of consecutive oxidations and reductions, mostly accompanied by distinct color changes. 2


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A recent initiative to synthesize water-soluble alternatives to these traditionally lipophilic class of compounds led to extended chromic responses observed in especially the para-carboxy functionalized dithizone and its complexes. Apart from this derivative being more electron deprived, for the sake of a broadened scope electron enriched derivatives (para-CH3 and –SCH3 substituents) were also included in experiments recorded here. To our knowledge no other compound (with its derivatives and complexes) exhibits such a variety of chromisms;

two or three perhaps, but not seven chromisms.

Certain

carbocyanine dyes, for example, were recently observed to be both halochromic and solvatochromic,13

Pt

complexes

bearing

oligo-ethyleneoxide

pendants

are

both

14

mechanochromic and photochromic, dimetallic ruthenium amine conjugated complexes are electrochromic,15

novel fullerene materials are concentratochromic,16 trinuclear gold

imidazolate complexes are thermochromic,17 while bis(4-dimethylaminophenyl)ethylenes are chronochromic.18

EXPERIMENTAL METHODS Synthetic methods for derivatized dithizones19,20 and corresponding metal complexes11 were followed as reported elsewhere. All synthesis reagents and solvents (Sigma-Aldrich and Merck) were used without further purification. UV-visible measurements were performed on a Shimadzu UV-2550 spectrometer fitted with a Shimadzu CPS temperature controller, using an optical glass cuvette. Photo-excitation of the photochromic compounds was achieved with a 400 W mercury-halide lamp. Cyclic voltammetry was conducted on a BAS100B Electrochemical Analyzer, at 20.0 °C, under argon in 0.5 mM solutions in dry CH2Cl2 in the presence of 0.1 M [N(nBu)4][B(C6F5)4]. A three-electrode cell was used, consisting of a glassy carbon working electrode, and Pt-wire counter and reference electrodes.

All potentials were referenced

+

against Fc/Fc . Spectroelectrochemistry measurements were conducted in an OTTLE cell in a Varian Cary 5000 NIR spectrophotometer. The OTTLE cell was in-house manufactured, consisting of a Pt-mesh as working electrode, a platinum auxiliary electrode and a silver wire reference electrode contained between two KBr windows. The OTTLE cell was charged with DCM solutions of 0.5 mM for each compound in the presence of 0.1 M [N(nBu)4][B(C6F5)4]. At every 25 mV interval potentials were kept constant while obtaining UV-visible scans.

3



RESULTS AND DISCUSSION 3.1. Electrochromism. Comparative cyclic voltammetry (CV) and spectro-electrochemistry studies of the H2Dz and p-SCH3−H2Dz ligands, and the PhHg(HDz) and PhHg(p-SCH3−HDz) complexes are presented here. Scheme 2 corresponds to the positive (oxidation) and negative (reduction) CV scans of H2Dz and p-SCH3−H2Dz as presented in Figure 1, with corresponding data listed in Table 1. As reported earlier,21 H2Dz undergoes two oxidations, at 624 and 1026 mV, the former being representative of disulphide formation.

SCH3

substituents on the two dithizone phenyl rings are electron-donating and, as expected, bring about a less positive p-SCH3−H2Dz first oxidation potential of 430 mV. Instead of only the two backbone oxidations as seen in the former unsubstituted ligand, there are now two additional oxidation waves, which are ascribed to the two SCH3 groups. Apart from the first backbone oxidation at 430 mV, the three remaining waves, at 730, 884 and 1113 V, correspond to the second backbone and two SCH3-centered oxidations. Although the latter three oxidation waves may not be assigned unambigiously, for the sake of illustration Scheme 2 places the two substituent-centered oxidations in the third and fourth positions.

1 2 H2Dz

12 3 4

Current / µA

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p-SCH3−H2Dz H2Dz

II

IV

FcH

I

p-SCH3−H2Dz

III

II I

Potential / mV vs Fc/Fc+ Figure 1. Cyclic voltammograms at 0.500 V s-1 scan rates for 0.5 mM H2Dz and p-SCH3−H2Dz in dichloromethane, with 0.1 M [NBu4][B(C6F5)4] as supporting electrolyte, a glassy carbon working electrode and Pt reference and auxiliary electrodes. Positive and negative scans are indicated by arrows. The reversible redox 4


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wave centred on 0 mV is that of the internal reference standard, ferrocene (FcH). The H2Dz CV is previously reported, but inserted for comparison.21

OXIDATION: Wave 1

H2Dz -e-, -H+

Wave 2 -e-

½(HDz)2

HDz+

Wave 1

CH3S−H2Dz−SCH3 -e-, -H+ ½(CH3S−HDz−SCH3)2 Wave 2 -e-

CH3S−HDz+−SCH3

CH3S−HDz+−SCH3+

Wave 4 -e-

Wave 3 -e-

CH3S+−HDz+−SCH3+

REDUCTION: H2Dz

Wave I e-, H+

H3Dz Wave I e-

SCH3−H2Dz−SCH3 Wave II e-, H+

Wave II e-

H3Dz−

CH3S−H2Dz−SCH3− Wave III

CH3S−H3Dz−−SCH3 e-, H+

CH3S−H4Dz−−SCH3

Wave IV e-

CH3S−H4Dz−−SCH3−

Scheme 2. Electrochemical oxidations (top) and reductions (bottom) of H2Dz and p-SCH3−H2Dz. Wave numbers correspond to labels in Figure 1. For clarity sake both substituents are indicated in the schemes.

Table 1. Anodic and cathodic peak potentials vs Fc/Fc+ for H2Dz and p-SCH3−H2Dz, and its phenylmercury(II) complexes (within brackets) from a glassy carbon working electrode at 20 °C. Scan rate = 0.500 V s−1, in DCM. Arabic numerals indicate oxidation peak potentials and Roman numerals indicate reduction peak potentials. positive scans Peak 1 2 3 4

H2Dz, mV 624 (669) 1026

p-SCH3−H2Dz, mV 430 (538)

negative scans H2Dz, mV

p-SCH3−H2Dz, mV

730 (910) 884 (1164) 1113 -960 (-

I

1511)

-987 (-1279)

-2142 (II

2265)

-1273 (-1571)

5



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III

-2119 (-2389)

IV

-2307

Green unsubstituted dithizone has two characteristic absorption bands at ca 450 and 600 nm, with corresponding p-SCH3−H2Dz bands lying at ca 490 and 670 nm. Figures 3 and 4 show the color changes (changes in absorbance spectra) during oxidation and reduction of these compounds.

The first significant color changes are observed at about the same

oxidation potentials for these two species, namely at 625 and 600 mV. This potential is associated with formation of the orange disulphide. The second transitions occur at 850 mV, corresponding to the mono-cationic singly deprotonated species. p-SCH3−H2Dz shows an additional weak color change at 1275 mV. This is ascribed to the oxidation of one of the substituents, which is expected to have only a small effect on the color of the compound, as opposed to oxidation of the π-conjugated backbone.

Decomposition products that are

essentially colorless form at 1500 mV . Again, four reduction waves are observed for p-SCH3−H2Dz, as opposed to the two waves of the parent compound, see Figure 1. The redox path proposed in Scheme 2 is, apart from electrochemically, also chemically vindicated, with the structures of most of the unsubstituted species being confirmed by single crystal X-ray crystallography.2 The negative scans in Figure 3 illustrate electrochomic behaviour also during reduction. Green H2Dz, as with its first oxidation, also turns orange at the first reduction (-725 mV). The same is seen for the SCH3 derivative, however only at -875 mV. This lower potential is expected, since the electron donating substituent resists reduction along the ligand backbone. Final color changes are seen at -1800 and -1900 mV. p-SCH3-H2Dz has, as may also be seen on the CV (Figure 1, Wave II), an additional substituent reduction, which is reflected in the small spectral redshift at -1275 mV. It should be noted that spectro-electrochemical scans associated with color changes are often seen at potentials slightly different, however in the same region than what are seen in corresponding CV’s. The reason for this is the spectro-electrochemical scans being obtained at very low scan rates, as compared to fast rates performed during CV’s, which results in electrochemical response time variations.

Additionally, Pt wire was used as reference

electrode during cyclic voltammetry measurements, while a Ag wire is employed in the spectro-electrochemical OTTLE cell. 6


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1 PhHg(HDz)

1

Current / µA

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2 3

PhHg(p-SCH3−HDz) PhHg(HDz)

FcH I

II

PhHg(p-SCH3−HDz)

II

III

I

Potential / mV vs Fc/Fc+ Figure 2. Cyclic voltammograms at 0.500 V s-1 scan rates for 0.5 mM PhHg(HDz) and PhHg(p-SCH3−HDz) in dichloromethane, with 0.1 M [NBu4][B(C6F5)4] as supporting electrolyte, a glassy carbon working electrode, and Pt reference and auxiliary electrodes. Positive and negative scans are indicated by arrows. The reversible redox wave centred on 0 mV is that of the internal reference standard, ferrocene (FcH). The PhHg(HDz) CV is previously reported, but inserted for comparison.22

OXIDATION: PhHgHDz PhHgHDz(SCH3)2 Wave 2 -e-

Wave 1 -e-

PhHgHDz+

Wave 1 -e-

PhHgHDz+(SCH3)2

PhHgHDz+(SCH3)2+

Wave 3 -e-, -H+

PhHgDz+(SCH3)2+

REDUCTION: PhHgHDz

Wave I e-

PhHgHDz(SCH3)2 Wave II e-, H+

Wave II

PhHgHDz− e-, H+ Wave I e-

PhHgH2Dz−

PhHgHDz(SCH3)2−

PhHgH2Dz−(SCH3)2

Wave III e-

PhHgH2Dz−(SCH3)2−

7



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Scheme 3. Electrochemical oxidations (top) and reductions (bottom) of PhHg(HDz) and PhHg(p-SCH3−HDz). Wave numbers correspond to labels in Figure 2. For clarity sake both substituents are indicated in the schemes.

8


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0 mV

850 mV 625 mV 1500 mV

Absorbance

Absorbance

0 mV -725 mV

-1800 mV

Wavelength (nm)

Wavelength (nm)

Figure 3. H2Dz (1 mM in dichloromethane) positive scan (left, 0 to 1500 mV), and negative scan (right, 0 to −1800 mV).

1500 mV

-2000 mV

1275 mV 850 mV 600 mV

-1275 mV 0 mV

-875 mV

Absorbance

Absorbance

0 mV

-1900 mV

Wavelength (nm)

Wavelength (nm)

Figure 4. p-SCH3−H2Dz (1 mM in dichloromethane) positive scan (left, 0 to 1500 mV), and negative scan (right, 0 to −2000 mV).

1600 mV 0 mV

575 mV

Wavelength (nm)

Absorbance

-1050 mV Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 mV -2100 mV

Wavelength (nm)

Figure 5. PhHg(p-SCH3−HDz) (1 mM in dichloromethane) positive scan (left, 0 to 1600 mV), and negative scan (right, 0 to −1850 mV).

9



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As opposed to the unsubstituted free ligand with two protons and thus two associated oxidation waves, the dithizonatophenylmercury(II) complex has only one imine proton giving rise to one oxidation wave, at 669 mV, see Table 1, Figure 2 and Scheme 3. As in the free ligand the SCH3 substituents also here in the complex give rise to two additional oxidation waves, namely at 910 and 1164 mV. The first oxidation occurs at 538 mV, which is 131 mV lower than in the unsubstituted complex. Along the path of reduction two peaks are observed for PhHgHDz (-1511 & -2265 mV) and three for PhHg(p-SCH3−HDz) (-1279, -1571 & -2389 mV).

The first PhHgHDz

reduction at -1511 mV is paralleled by the second (-1571 mV) reduction in PhHg(pSCH3−HDz), as the latter would be less readily reduced. This led to the conclusion that the more positive reduction wave, at -1279 mV, is substituent centred. Figure 5 shows the spectro-electrochemistry of PhHg(p-SCH3−HDz) only, with corresponding PhHgHDz data already published elsewhere.11 During oxidation the first significant color change occurs at 575 mV, after which decoloration, most probably due to decomposition, takes place at 1600 mV. No color changes directly associated with the two substituent oxidations are seen, with oxidative electrochromism here largely similar to that of the unsubstituted complex.

The reduction path shows two major electrochromic color

changes, namely at -1050 mV where the orange complex becomes magenta, which then is followed by decoloration at -2100 mV.

Although more single-electron reductions do take

place (Figure 2), these are not all associated with color changes.

3.2. Solvatochromism.

A comparative solvatochromism study was done of dithizone

solutions (3.2 x 10-5 M) and its electron-poor p-COOH and electron rich p-SCH3 derivatives in various solvents, ranging from polar to non-polar. At this concentration the observed solvatochromic effect for dithizone itself is small, with only a small drift in λmax values (see Supporting Information, Figure 1S). In more polar solvents like THF, methanol and DMSO the longer wavelength peak decreases, resulting in both peaks being of almost equal height. As reported previously, polar solvents support intramolecular proton transfer at low concentrations, yielding the orange thiol form of dithizone.2

Of six possible tautomers

TDDFT spectral calculations yielded electronic oscillators of only the thiol geometry that corresponds to the experimentally obtained spectrum of this orange species. Computed oscillators of the standard green thione symmetrical geometry in turn, are in almost perfect agreement with experimental spectra in non-polar solvents. As may be seen from the three orange structures in Scheme 1, electron distribution along the backbones of these thiol and 10


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anionic species, as well as the corresponding complex, are all largely similar, thus yielding

Absorbance

the same color.

Wavelength (nm)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavelength (nm) Figure 6. UV-visible spectra of 3.2 x 10-5 M solutions of p-COOH−H2Dz (top) and p-SCH3−H2Dz (bottom) in various solvents.

As for p-COOH−H2Dz, the solvatochromic effect is most pronounced. Here, although being able to use only polar solvents due to solubility limitations, not just changes in molar absorptivity (ε) are seen, but also significant shifts in λmax values, see Figure 6 (top). In both methanol and DMSO the long wavelength peak (> 600 nm) is reduced to virtually zero. λmax in methanol lies broadly centered around 481 nm. Acetone yields the most blue-shifted peak, i.e. at 453 nm, while in DMSO 540 nm represents the peak that is most red-shifted, as also may be seen in its high energy region, at 350 nm. Therefore, by merely changing solvents, a large shift of 82 nm is obtained. Also in p-SCH3−H2Dz solvatochromism is more pronounced than in H2Dz, see Figure 6 (bottom). Hydrogen bonding to substituent atoms, O and S, is suggested to facilitate proton transfer during tautomerization reactions, thus the more pronounced solvatochromism observed for electronically altered –COOH and –SCH3 derivatives. As for the unsubstituted derivative, only small peak shifts occur, however, significant changes in ε of mostly the low energy absorption bands (long wavelengths) are observed. Less polar solvents do not support 11



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tautomerization to the orange form, with solutions thus staying green, corresponding to larger ε values at long wavelengths. In acetone, being the least polar solvent considered here, the green tautomer is seen to be most abundant, with the orange peak at ca 470 nm reduced to a small shoulder. As for the effect of groups that alter electronic charges on the dithizone backbone, the most electron-donating substituent, -SCH3, cause the largest redshift seen in series of dithizones.19,23

This holds for both non-polar DCM and polar DMSO.

Peaks of this

derivative are observed at 490 and 667 nm in DMSO, as opposed to corresponding H2Dz values at 447 and 615 nm.

3.3. Concentratochromism. Of the three ligands that were tested only H2Dz was found to be concentratochromic in methanol alone. This particular form of chromism is extremely rare, with dithizone being the species amongst a handful that exhibits this phenomenon most vividly.24,25,2

Low concentration (0.8 x 10-5 M) solutions are orange, with only one peak

absorbing at ca 470 nm.

During concentration increase transition to the green thione

tautomer is observed, as seen by the appearance of the 595 nm peak (see Supporting Information, Figure S2). p-COOH–H2Dz however expands the occurrence of this phenomenon, as color variation due to changed concentrations is observed also in other polar solvents (see Figure 7, top) like THF, ethanol, methanol and DMSO, although less pronounced in the latter. p-SCH3–H2Dz also exhibits concentratochromism in various solvents. As opposed to the carboxy derivative, in p-SCH3–H2Dz solutions the long wavelength absorption band becomes dominant at high concentrations. The color change is therefore more vivid. As in the case of solvatochromism where similar spectral changes are seen, concentratochromism is ascribed to the same equilibrium between the thione (green) and thiol (orange) tautomers.

Polar solvents are known to stabilize reaction intermediates where

charge separation occurs and are therefore also assisting intramolecular proton transfer between the dithizone imine and sulphur atoms.

In event of increased concentration,

stabilization is suggested to instead be attributed to adjoining dithizone molecules. This is consistent with the increased occurrence of concentratochromism in the –COOH derivative: through H-bonding an adjacent –COOH group may not only stabilize but also partake in the proton exchange reaction, i.e. the labile carboxy proton may bond with the dithizone sulphur atom, while in turn a dithizone imine proton fills the vacant position on –COO– .

12


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Absorbance

p-COOH–H2Dz in ethanol

Wavelength (nm) p-SCH3–H2Dz in ethanol

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavelength (nm) Figure 7. UV-visible spectra of solutions of p-COOH–H2Dz in tetrahydrofurane (top) and p-SCH3–H2Dz in ethanol (bottom), at concentrations: 0.8, 1.6, 3.2, 6.5, 9.7 and 13 x 10-5 M (from bottom to top).

3.4. Halochromism. This section involves H2Dz and the three derivatives; p-COOH–H2Dz, p-CH3–H2Dz and p-SCH3–H2Dz. Initial construction of Beer-Lambert curves confirmed linearity in absorbances even as high as 3, allowing for brightly colored concentrated solutions to be used during measurements. The dithizones were dissolved with the aid of an ultrasonic bath, preparing 9.7 x 10-5 M methanolic dithizone solutions. These solutions were titrated against methanolic KOH solutions of similar concentration. The spectra obtained after incremental additions of KOH to p-COOH−H2Dz are shown in Figure 8. The green pCOOH−H2Dz peaks at 443 and 620 nm disappear with the addition of KOH, while the orange peak of the anion appears at 480 nm. These halochromic (color changes resulting from changes in pH) reactions are chemically reversible, as confirmed by addition of dilute HCl to product solutions.

13


Wavelength (nm) Figure 8. UV-visible overlay spectra of the titration of 25 mL of p-COOH–H2Dz solution against KOH. Both concentrations were 9.7 x 10-5 M in methanol.

Absorbance

p-COOH–H2Dz, 620 nm

Volume of KOH (mL) H2Dz, 590 nm

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Absorbance


Volume of KOH (mL) Figure 9. Spectrophotometric curves of the titrations of 25 mL of p-COOH–H2Dz (top) and H2Dz (bottom) against KOH in methanol. Both concentrations were 9.7 x 10-5 M. Due to dilution of the dithizone solution during titration, absorbance corrections were made.

The spectrophotometric titration curves of H2Dz and p-COOH−H2Dz are shown in Figure 9. The H2Dz endpoint lies at 20 mL, 5 mL below the equivalence point that is expected for a 1:1 mole reaction (Figure 9, bottom). Apparent endpoints were found at ca 15 mL for the p-CH3 derivative, at ca 7 mL for p-SCH3 and at ca 5 mL for p-COOH (Figure 9, top). The amount of base added is consistently less than what is stoichiometrically required for complete mono-deprotonation of all dithizone molecules present.

Considering an

explanation for this observation in isolation would be problematic. However, by now having

14


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learned how extremely sensitive dithizone is to a variety of external stimuli, and the spectra of the thiol and anionic forms overlapping, clearly more than just the usual acid-base reaction is involved during formation of the orange solutions. Another additional form of chromism, ionochromism, is suggested to also come into play here. The mere generation of orange dithizonate anions during addition of base results in these anions facilitating proton transfer in the parent green thione tautomer, forming the orange thiol. Addition of more base, beyond the apparent “endpoint”, will of course continue the process of dithizone monodeprotonation, however without being visible due to spectral overlap. In addition to the above it should be noted that the apparent endpoints (within brackets, in mL) follow the trend of decreasing group electronegativities of the respective substituents: H (20) > CH3 (15) > SCH3 (7) > COO− (5) This trend is consistent with the above ionochromism hypothesis, where the most electron-rich species are expected to assist or catalyze the intramolecular proton transfer reaction most readily, i.e. the mono-, di- or tri-anions of p-COOH−H2Dz effecting a complete color change already at 5 mL. Lastly, it was observed that: - green methanolic solutions of p-COOH–H2Dz change to orange simply by being sucked up into a new pasteur pipette. The solutions however remain green if the pipette has prior been used, though cleansed before this test, and - the same colour change occur when this solution is passed through neutral silica gel (see Figure S3 under Supporting Information). These colour changes are suggested to also be halochromic by nature, as it is known that new glass surfaces and silica gel (both being silica oxides) are not pH neutral. Consequently the extreme sensitivity of p-COOH–H2Dz to external stimuli of diverse kinds is hereby shown to also serve as a very sensitive sensor for visibly indicating surfaces that are, in this case, slightly basic.

3.5. Thermochromism. Of the dithizone ligands that were studied, p-SCH3–H2Dz was the only one that showed reversible thermochromism, and only in ethanol. Methanol, acetone, THF and DMSO were also tested. Colour change due to temperature variation took place only at low concentrations (1.6 x 10-5 M) of the ligand. Absorbance spectra were recorded while raising the temperature by 10 ºC increments, with 10 minute intervals allowed in between measurements. Heating the solution from 10 to 70 ºC resulted in a colour change from orange to green, i.e. the higher temperature shifting the equilibrium towards the green 15



tautomer. Figure 10 shows the orange solution (λmax at 495 nm) disappearing, while the shoulder at 655 nm grows to almost 4 times its original height. The isosbestic point at ca 550 nm indicatives one single species going to another, without the presence of intermediates. (Overlay spectra may be seen under Supporting Information, Figure S4.)

Heating and

cooling cycles were replicated three times; the solution consistently changed from orange to green and back, thus confirming repeatability and reversibility. Based on previous arguments pertaining to solvato- and concentratochromism, the conclusion is that the geometry change here is again from thiol (orange) to thione (green) at high temperature. 495 nm

Absorbance

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655 nm

Wavelength (nm) Figure 10. UV-visible spectra of p-SCH3–H2Dz in ethanol, heated from 10 ºC (orange) to 70 ºC (green).

3.6. Photochromism. The phenylmercury complex of the new carboxy-dithizone, PhHg(pCOOH-HDz), exhibits photochromicity similar to that of its unsubstituted parent compound,19 see Scheme 1 (right). The complex is insoluble in non-polar solvents, where-in the lifetime of the photo-induced blue isomer traditionally is the longest (up to several minutes), and thus readily measurable.

Photochromism in mercury dithizonates may

however still visibly be seen in more polar solvents like tetrahydrofurane and acetone. In very polar solvents like alcohols the back reaction becomes so fast that it may no longer be seen with the naked eye, however femtosecond laser spectroscopy confirmed the reaction still occuring on exposure to visible light.10 To overcome the problem of fast back reactions in polar solvents 90 % toluene was introduced to THF solutions of the complex, giving a concentration of 2.81 x 10-5 M. On exposure to light from a mercury-halide lamp PhHg(pCOOH-HDz) undergoes a photochromic reaction where the orange form (λmax = 494 nm) changes to bright blue (622 nm), with two isosbestic points at 400 and 552 nm, see Scheme 1 and Figure 11. The carboxy-substituted complex absorbance peaks are redshifted by ca 20 nm compared to the unsubstituted PhHg(HDz) complex, where λmax = 478 nm for the orange solution and 600 nm for the blue solution.

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Absorbance

494 nm

622 nm

Wavelength (nm) Figure 11. UV-visible spectra of photochromic PhHg(p-COOH-HDz) (2.81 x 10-5 M) in a 9:1 Tol:THF solvent mixture, at 20 ºC.

Recently reported femtosecond transient absorption laser measurements showed photoexcitation to occur within 100 fs of exposure to blue-green light, which is immediately followed by radiationless relaxation with a time constant of 1.5 ps.3,10 The excited state orthogonally twisted intermediate undergoes isomerization through a conical intersection, bifurcating towards the new blue trans and orange resting state cis configurations below the conical intersection funnel.

3.7. Chronochromism. It was coincidentally discovered that on standing, solutions of SCH3– HDz (here the sulphur is methylated, with no substituents on the phenyl rings) gradually change colour in certain solvents. This led to the last form of chromism covered by this report, namely chronochromism, i.e. colour change that occurs over time.

At low

concentrations (3.2 x 10-5 M) a pink (λmax = 548 nm) to yellow (λmax = 418 nm) color change in chloroform took place within three hours. UV-visible absorbance readings were recorded at 20 ºC over a period of 5 hours (see Figure 12 and Supporting Information, Figures S5 & S6).

Absorbance

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Wavelength (nm)

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Figure 12. UV-visible spectra of S-methylated dithizone in chloroform, observed over 5 hours. The bent pink form isomerizes to the yellow linear geometry.

The existence of an equilibrium between different isomers of this compound had previously been discussed by Hutton et al,26 with both the pink27 and yellow28 isomers being confirmed by single crystal X-ray crystallography.

On methylation of sulphur (i.e. the

dithizone backbone) in this particular case, only one imine proton remains. This proton is less labile, with the consequence that intramolecular proton transfer no longer occurs, but only isomerization around C−N bonds adjacent to the central sulphur atom.

CONCLUSION Dithizone and its derivatives and complexes are to date singularly unique in exhibiting the forementioned large variety of chromisms. The linear geometry of the π-electron delocalized backbone enables color-changing isomerization, while a very labile imine proton is involved in color-changing intramolecular proton transfer.

One or both of these properties in

concurrence allow a range of external stimuli, varying from typical quanta like protons, electrons and photons, to very subtle intermolecular forces as amongst solvent molecules and ions, and thermal vibrations, to effect distinct color changes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Figure S1. Solvatochromism: Color photographs and UV-visible spectra of 3.2 x 10-5 M solutions of H2Dz in various solvents.

Figure S2. Concentratochromism: UV-visible spectra of solutions of H2Dz (top) and pCOOH–H2Dz in solvents as indicated. Concentrations: 0.8, 1.6, 3.2, 6.5, 9.7 and 13 x 10-5 M (from bottom to top).

Figure S3. Halochromism: UV-vis spectra of the methanolic solutions of p-COOH–H2Dz as is, when sucked up into a new pipette, into a used but cleansed pipette, and passed through new silica gel. The new pipette and new silica gel promote orange thiol formation.

Figure S4. Thermochromism: UV-visible spectra of p-SCH3–H2Dz in ethanol, being heated from 10 ºC to 70 ºC.

Figure S5. Chronochromism: UV-visible spectra of S-methylated dithizone in chloroform, observed over 5 hours. 18


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Figure S6. Isomerization kinetics of S-methylated dithizone measured over a time period of 5 hrs in chloroform.

AUTHOR INFORMATION Corresponding Author *K. G. von Eschwege. E-mail: [email protected]. Telephone: +27-(0)51-4012923.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is based upon research supported by the South African National Research Foundation and the University of the Free State. In memory of the first author of this publication who tragically passed away in a car accident during review of the draft.

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2016, 26, 5271–5278. (15) Tang, J.-H.; Yao, C.-J.; Cui, B.-B.; Zhong, Y.-W. Ruthenium-amine conjugated organometallic materials for multistate near-IR electrochromism and information storage, The Chemical Record, 2016, 16, 754–767. (16) Peng, H.; Leung, F. S. M.; Wu, A. X.; Dong, Y.; Dong, Y. Yu, N.-T.; Feng, X.; Tang, B. Z. Using buckyballs to cut off light! Novel fullerene materials with unique optical Transmission Characteristics, Chemistry of Materials, 2004, 16 (23), 4790-4798. (17) Ghimire, M. M.; Nesterov, V. N.; Omary, M. A. Remarkable aurophilicity and photoluminescence thermochromism in a homoleptic cyclic trinuclear gold(I) imidazolate complex, Inorg Chem. 2017, 56 (20), 12086-12089. (18) Botta, C.; Benedini, S.; Carlucci, L.; Forni, A.; Marinotto, D.; Nitti, A.; Pasini, D.; Righetto, S.; Cariati, E. Polymorphism-dependent aggregation induced emission of a push– pull dye and its multi-stimuli responsive behavior, J. Mater. Chem. C, 2016, 4, 2979-2989. (19) Von Eschwege, K. G. Synthesis and kinetics of electronically altered photochromic dithizonatophenylmercury(II) complexes. J. Photochem & Photobiol. 2013, 252, 159-166.

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Seven Chromisms Associated with Dithizone - ACS Publications

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