Femtosecond Laser Spectroscopy and DFT Studies of Photochromic

Article pubs.acs.org/JPCA

Femtosecond Laser Spectroscopy and DFT Studies of Photochromic Dithizonatomercury Complexes Karel G. von Eschwege,†,* Gurthwin Bosman,‡ Jeanet Conradie,† and Heinrich Schwoerer‡ †

Department of Chemistry, University of the Free State, PO Box 339, Bloemfontein, 9300, South Africa Laser Research Institute, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa

‡

S Supporting Information *

ABSTRACT: The ultrafast dynamics of the photochromic reaction of dithizonatophenylmercury(II) was recently reported. For purpose of investigating the effect of electronically different substituents (X = o-F, m-F, p-F, p-Cl, o-CH3, m-CH3, p-CH3, m,p-diCH3, p-OCH3, o-SCH3, and p-SCH3) on this reaction, a series of phenyl-substituted dithizones were synthesized and complexed with phenylmercury(II). A variation of more than 3 ps in ground state repopulation times was observed, with the o-methyl derivative absorbing both at shortest wavelength and having the fastest repopulation time, while the p-S-methyl derivative lies at the opposite extremity. An increase in both decay times and λmax values is generally reflected by an increase in electron density in the chromophore. Ultrafast rates also proved to be dependent on solvent polarity, while a profound solvatochromic effect was observed in the transition state absorbance. Density functional theory realistically simulated isomer stabilities, electronic spectra and molecular orbitals. Increased electron density enhances stability in the photoexcited blue isomer relative to the orange resting state, as seen from a comparison between orange and blue isomer total bonding energies. A linear trend between computed HOMO energies and experimental λmax of related aliphatic substituted derivatives was found.

1. INTRODUCTION Photochromism in mercury dithizonates was discovered as far back as 1945,1 with the first systematic studies undertaken 20 years later by Meriwether et al.2 Of the 24 metal dithizonates prepared, only 9 were found to be visibly photochromic. Photoexcitation by visible light promotes isomerization to a different color complex, which spontaneously reverts to the original resting state, see Figure 1. The reaction is typically dependent on solvent, temperature, the metal center, alterations on the ligand and the presence of impurities.3 Also, N-deuteration of bis(dithizonato)mercury(II), Hg(HDz)2, was found to result in a 3-fold decrease in the return rate.4 The structures of the orange and photoinduced blue dithizonatophenylmercury(II) (DPM) isomers were determined by X-ray crystallography5 and quantum computational geometry optimization.6 Whereas the above work focused on the photochromic reaction at large, as measured in the seconds time domain, we recently embarked on femtosecond laser spectroscopy research to also establish aspects of the initial photochromic reaction of DPM.7 Ultrafast excitation by 40 fs laser pulse occurred within 100 fs, resulting in a photoreaction with a time constant of 1.5 ps. Results were indicative of an orthogonally twisted excited state that bifurcates below the funnel of the conical intersection toward the orange cis and blue trans configurations. By the use of ultrafast techniques, photochromism was also for the first © 2014 American Chemical Society

Figure 1. Spontaneous radiationless thermal back-reaction of (omethoxy)dithizonatophenylmercury(II) in dichloromethane. The purple-blue photoexcited state (λmax = 610 nm) reverts back to the red ground state (λmax = 505 nm, ε = 28 346 dm3 mol−1 cm−1), with isosbestic points at 405 and 560 nm.3.

Received: October 14, 2013 Revised: January 14, 2014 Published: January 15, 2014 844

dx.doi.org/10.1021/jp410186y | J. Phys. Chem. A 2014, 118, 844−855

The Journal of Physical Chemistry A

Article

Figure 2. Schematic diagram of the femtosecond transient absorption setup.

amounts of water. The nitroformazan solid was dried overnight in an oven at 70 °C (3.71 g, 95%). (o-Methoxy)thiocarbazone. The nitroformazan (1.50 g, 4.7 mmol) was dissolved in a mixture of absolute ethanol (90 mL) and ammonium sulfide solution (10 mL, 20% aq) in a 250 mL stoppered round-bottom flask and stirred for 1 h. After reduction was complete, the solution was added to a water/ice mixture (500 mL). The dirty-white/yellow carbazide was filtered off on a Buchner funnel containing silica gel (for better filtration) and washed with water. The unstable thiocarbazide was immediately oxidized to the dark red thiocarbazone by the addition of cold methanolic potassium hydroxide (2%, 50 mL). Stirring was continued until complete dissolution. (o-Methoxy)dithizone. The dark green dithizone derivative was precipitated by the addition of the thiocarbazone to dilute hydrochloric acid (1%, 100 mL). The product was filtered and again precipitated from an alcoholic alkali solution to which diluted hydrochloric acid was added. The crude product sample was dissolved in dichloromethane and passed through a short silica column using toluene as eluent. A yellow impurity ran ahead of the green product band, while some impurities stayed behind at the origin. Alternatively, instead of running a column, repeating the aforementioned precipitation procedure four times, yielded pure o-methoxydithizone (1.09 g, 76%), as confirmed by thin layer chromatography and NMR spectroscopy. Mp 178−179 °C, UV/vis (dichloromethane): λmax 474 and 641, 1H NMR (300 MHz, CDCl3): δ 4.00 (6 H, s, OCH3), 6.97−7.17 (6 H, m, 2 × C6H4), 7.95−8.02 (2 H, d, 2 × C6H4), 12.92 (2H, s, 2 × NH). (ortho-methoxy)dithizonatophenylmercury(II). Triethylamine (0.05 g, 0.50 mmol) was added to a solution of omethoxydithizone (0.10 g, 0.32 mmol) and phenylmercury(II) chloride (0.11 g, 0.35 mmol) in dichloromethane (30 mL), and stirred for 15 min. The solution was overlaid with absolute ethanol (15 mL) and evaporation of the DCM component liberated 0.16 g (84%) of pure crystalline product. Mp: 212− 213 °C, λmax/nm (dichloromethane): 505. 1H NMR (300 MHz, CDCl3): δ 3.68, 4.03 (6 H, 2 × s, 2 × OCH3), 6.57−7.89 (13 H, m, 2 × C6H4, 1 × C6H5), 9.75 (1H, s, 1 × NH). 2.3. Laser Spectroscopy. Femtosecond transient absorption measurements were performed at the Laser Research Institute at Stellenbosch University. Figure 2 depicts the most essential components in the time-resolved absorption arrangement used for these measurements, of which the details are reported elsewhere.7 However, in view of recent improvements a brief description is presented here. The time-resolved

time observed in strongly polar methanolic mediuma preliminary indication that the photoreaction may indeed occur in many other similar complexes but which have been up to date not visibly observed. In many photochemical studies with potential practical application, chemical tuning of spectral and rate aspects of the photoswitching properties becomes of paramount importance. The chromophore here under consideration lends itself ideally toward chemical modification with regard to these goals. As part of the total project, the second stage here was to synthesize electronically altered dithizonatomercury complexes by symmetrically substituting both phenyl rings on the dithizone ligand with moieties that vary not only in group electronegativity, but also in location on the phenyl groups. These complexes were then studied by means of ultrafast laser spectroscopy, in polar as well as nonpolar solvents, and additionally clarified by appropriate DFT studies.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. General Data. Synthesis reagents (Sigma-Aldrich) and solvents (Merck) were used without additional purification. 1H NMR spectra were recorded on a 300 MHz Bruker Avance DPX 300 NMR spectrometer, at 298 K. Chemical shifts are reported relative to SiMe4 at 0 ppm. Ultraviolet and visible spectra of dilute solutions in quartz cuvettes were recorded, utilizing a Varian Cary 50 Probe UV/visible spectrophotometer. Laser experiments were conducted as described in section 2.3, and quantum computational methods are described in section 2.4. 2.2. Synthesis. The synthesis of o-methoxydithizone and its corresponding phenylmercury complex, as the model compound, is given. (Characterization data of all complexes were reported elsewhere.3) (o-Methoxy)nitroformazan. In a 100 mL beaker, omethoxyaniline (3.00 g, 24.4 mmol) was added to a mixture of concentrated hydrochloric acid (13 mL) and water (25 mL) and cooled to −10 °C on a cold plate while stirring. Sodium nitrite (2.53 g, 36.5 mmol) dissolved in water (5 mL) was slowly added. In a 500 mL beaker, the diazo solution was added to a mixture of sodium acetate trihydrate (40 g), glacial acetic acid (25 mL), and water (10 mL) and stirred at room temperature for 5 min. Nitromethane (7.5 g, 123 mmol) was added and stirred for 1 h, followed by the addition of water (400 mL) and stirring for another 30 min. The red precipitate was filtered through a Buchner funnel and washed with copious 845



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optical density (ΔOD) in each given sample was determined through the following ratio,

spectroscopy system is based on the conventional pump−probe technique, where the sample to be analyzed is activated by a pump pulse and monitored at a later time by a probe pulse. Generally, the pump pulse initiates a photoreaction, where upon the expected absorption changes are detected in the spectrum of the probe pulse. In order to generate a kinetic view of the changes which occur in the sample after pump activation, the temporal delay between the pump and the probe pulse is altered sequentially. Therefore, by monitoring the spectrum of the probe pulse as a function of delay time, a real-time view of the photoreaction is obtained. A commercially available femtosecond amplifier (ClarkMXR: CPA 2101) is used as light source which delivers a bandwidth-limited 150 fs duration 1 mJ pulse at a repetition rate of 1 kHz centered at 775 nm. For the pump pulse, a portion of the light (200−300 μJ) is used to drive a noncollinearly phase-matched parametric amplifier (NOPA). In the NOPA a chirped pulse centered at 480 nm with energy in the micro-Joule range is generated. This chirped pulse is compressed to about 40 fs with a combination of two Brewster prisms and is then reflected off two mirrors mounted in retroreflector geometry on a translation stage. An optical chopper operated at half the repetition rate of the CPA is introduced, which ensures that every second pump pulse is effectively blocked. A reflective type neutral density filter is used in order to fine-tune the pulse energy in accordance with the requirements of the experiment. The reflected pump pulse is sent to one of two simultaneously triggered spectrometers (Andor SR163 spectrograph), where it is used as an on−off signal to determine when the sample is activated or not. The transmitted pump pulse reflects off a mirror mounted on a 2axis piezoelectric adjustable mount which allows for the spatial correction of the pump pulse at the sample position when monitored by the CCD camera. A half-wave plate is used to set the polarization of the pump pulse to the magic angle (54.7°) with respect to the probe pulse. Finally, the pump pulse is focused onto the sample with a spherical mirror (f = 200 mm). A whitelight pulse generated in a 3 mm thick CaF2 crystal was used for the probe pulse. The broad spectrum of this WL allowed for absorption change measurements in the spectral range of 340−650 nm. One drawback of using CaF2 is that it has a very low damage threshold and is therefore continuously moved as to increase its lifetime. Within this regard a circular translation motion is chosen as to ensure that the orientation of the crystal axes relative to the polarization of the fundamental CPA pulse is constant. After whitelight generation and collimation with an off-axis parabolic mirror ( f = 50 mm) the probe pulse is focused with a UV lens (f = 100 mm) onto the liquid sample which is contained in a quartz flow cell (Starna Scientific Ltd.: path length 200 μm). Also, the sample was continually circulated with a reservoir using an inline rotary pump (HNP Mikrosysteme GmbH: MZR 2921) at a speed in excess of 500 mm/s. This guaranteed that a fresh specimen was ready for every laser shot, hence avoiding product buildup. Having passed through the sample the transmitted probe pulse was imaged onto a fiber coupled to the second spectrometer which as previously mentioned is on a common clock with the first spectrometer used to detect the reflected pump light. These spectrometers were fitted with fast line scan cameras (1024 pixel photodiode array, Entwicklungsbüro Stresing) which detected spectra with a 1 kHz repetition rate. Therefore, by using the on−off trigger from the pump spectrometer, the excited versus the nonexcited change in

⎛ T *(τ , λ) ⎞ ΔOD(τ , λ) = −log⎜ ⎟ ⎝ T (λ ) ⎠

Here T* represents the pumped and T the unpumped probe spectrum at a delay time τ and wavelength λ. The sensitivity of the change in optical density is estimated as the standard deviation of the signal before pump−probe overlap which was determined to be less than 10−3 (see Figure 3). The temporal

Figure 3. Kinetic trace of dichloromethane at 600 nm. Inset: Gaussian fit of the coherent artifact.

resolution of the setup was assumed to be equal to the fwhm of the second derivative of a Gaussian function when fitted to the coherent artifact generated with dichloromethane (DCM) solvent as sample (Figure 3, inset). In doing so a temporal resolution of 70 ± 5 fs was obtained. An analogous method to determine the temporal resolution of the setup is to generate stimulated impulsive Raman scattering (SIRS).8 In this method the ultrashort pump pulse activates Raman modes in the solvent. The activation of these modes results in a modification of the refractive index of the solvent which is expressed as oscillations in the kinetic trace. These oscillations are clearly visible in Figure 3 and have a period in the order of 100 fs. Therefore, the temporal resolution is definitely less than this period, because these oscillations are observable. Because of the chirp of the probe white light, the temporal position of the coherent artifact changes with respect to wavelength. This temporal offset is corrected for by recording the maximum position of the coherent artifact for a number of wavelength values, fitting it with a higher order polynomial and subtracting the amount of shift per wavelength position from subsequent transient traces. The dithizone ligand phenyl-substituted mercury complexes were all dissolved in DCM to concentrations in the μM range. These were subsequently introduced into the UTA setup upon which transient spectra were recorded when activated with a 40 fs pump pulse centered at 480 nm. The conditions (temperature, pump fluence, sample acquisitions, etc.) for the various samples were all kept identical. 2.4. Quantum Computational Methods. Calculations were done using the hybrid functional B3LYP9−14 (B3 Becke 3parameter exchange and Lee−Yang−Parr correlation) functional for both exchange and correlation as implemented in the Gaussian program package, version 09.15 Geometries were optimized in gas phase (if no solvent is indicated) with the triple-ζ basis set 6-311G(d,p)16−24 on all atoms except Hg, 846



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Scheme 1. Synthesis of Altered Dithizones from Various Aniline Reagents (X = substituent)

stepping from mono- to difluorinated phenyl rings, resulted in the gradual loss of the typical intense color of dithizone, with consequent loss in photochromic properties as well.31 Mercury complexation reactions were accomplished quantitatively, but with expected losses due to the final recrystallization step. 3.3. Laser Spectroscopy. In this femtosecond spectroscopy study, unsubstituted DPM in dichloromethane was used as reference in order to illustrate the effect that changes in the substitution pattern have on the initial photochromic reaction. A previous study involving photochromic DPM reported the instantaneous rise of excited-state absorption, followed by rapid decay with a time constant of 300 fs.7 The orthogonally twisted intermediate bifurcates through a conical intersection to the two ground states, i.e. the orange and blue states, with a time constant, τ, of 1.5 ps. Bifurcation at the conical intersection implies that repopulation of the orange ground state and formation of the blue photoproduct are inherently coupled. Therefore, only the repopulation time of the orange isomer will be considered as the measurement parameter in this report. This choice was not made arbitrarily, but rather because the absorption band of the blue photoproduct overlaps with that of the intermediate species formed at orthogonal geometry (Figure 4, spectra above 500 nm). Femtosecond photoexcitation of orange DPM results in the immediate (0.5 ps) and decrease in the initial two transition state peaks. 3.3.1. Solvent Effect. Figure 5 shows excited state absorbance changes effected by dissolving unsubstituted DPM

Figure 5. Transient change in optical density ΔOD(λ) of DPM (circles) in dichloromethane, deuterated methanol (squares) and methanol (triangles) at 500 fs after excitation with a short laser pulse (40 fs) at 480 nm. (See Supporting Information, Figure S1 for corresponding 3D plots in all three solvents.)

in dichloromethane, methanol, and deuterated methanol, respectively. The solvatochromic effect is especially pronounced in the molar absorptivities, ε, of the transition state absorption bands at ca 390 and 550 nm, < 1 ps. In DCM the ε ratio at these wavelengths is 3:10, changing to 13:10 in MeOD and 18:10 in MeOH, a condition which partially persists in the blue isomer absorbance, > 1 ps. This change to more intense absorption at higher energy in MeOH is remarkably paralleled by what was observed for the free dithizone ligand in similar solvents.32 Dithizone itself has two visible absorption bands at ca. 450 and 610 nm (both being 60 nm red-shifted with respect to the present corresponding mercury complex), with an ε ratio of 5:10 in DCM, changing to 80:10 in MeOH. By means of TDDFT calculations, this solvent effect was satisfactorally explained in terms of intramolecular transfer of one of the dithizone imine protons to sulfur on dissolution in methanol. Unfortunately, theory and software developments are not advanced enough, as yet, for the successful optimization of excited state complexes containing heavy metals, which necessarily also have to include relativistic effects. While being engaged in ongoing attempts toward finding a computational explanation, we may at this stage at most propose that a similar phenomenon might be responsible for the observed solvatochromism in the photochromic DPM intermediate. Apart from the above spectral differences a change in time constants in the different solvents were also observed, with orange repopulation time constants, τ, in ps

Scheme 2. Fragment of DPM Indicating the Photo-Induced Rotation and Thermal Relaxation Pathways in Non-Polar DCM and Polar Methanola

a

The latter intermediate (bottom) is illustrated as both gaining and losing a proton from/to the protic solvent medium.

(bottom). A polar solvent like methanol will indeed better support polarization of bonds during such a transition than a nonpolar solvent. In further support of this notion is the fact that the visible photochomic reaction is not observed in the presence of trace amounts of acids and bases. The color change reaction in a polar solvent like acetone is nevertheless still clearly visible. Acetone, on the contrary, does not carry labile protons. Additionally, the visible photochromic reaction does take place in very dilute tetrachloromethane or carbon disulfide solutions, i.e., in solvents that are completely devoid of protons and thus not involved in a proton transfer mechanism. This observation is in support of a rotational path in DCM during the reverse reaction, or alternatively, slow inversion that does not include proton transfer. Vibrational energy is required to

1.03 ± 0.08 (DCM) < 1.55 ± 0.15 (MeOH) < 1.61 ± 0.10ps (MeOD)

illustrating a decrease in reaction rate, or increase of τ, of ca. 60% in polar solvents. A very small but detectable deuteration effect (1.61 vs 1.55 ps) is also observed, supporting the notion that only the more labile imine proton on the DPM backbone is replaced by deuterium when dissolved in MeOD. Being part 848



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Table 1. Absorbance Maxima in DCM and Selected Time Constants for the Repopulation of the Orange Isomer and Population of the Blue Isomers of Electronically Altered Dithizonatophenylmercury(II) Complexes, Arranged in Ascending Order of Orange Isomer λmax Values orange ground state

excited state

blue ground state

compound

λmax, nm

τ, ps

λmax, nm

λmax at 1 ps, nm

τ, ps

λmax at 5 s, nm

o-CH3 o-F m-F H p-F m-CH3 p-Cl p-CH3 m,p-diCH3 o-SCH3 o-OCH3 p-SCH3

470 474 476 477 479 483 488 489 493 500 507 519

0.51 1.95 1.42 1.04 1.35 1.36 1.91 1.28 1.53 1.61 1.02 3.79

374 380 384 384 384 387 390 393 394 387 397 426

555 552 555 560 555 559 560 567 570 577 584 613

1.7 2.3 2.5 2.1 1.9 4.4 2.6 2.5 2.5 2.4 1.9 3.6

580 590 591 590 590 591 596 597 602 594 610 617

species, and 37 nm (t = 5 s) in the blue isomer. The observation that introduction of a methyl group at the ortho position greatly accelerates the rate of isomerization is in complete agreement with a similar finding by Herkstroeter 40 years ago when studying a series of substituted azomethine isomerization reactions.39 Experimental data in Figure 6 (squares and triangles) shows the initial depletion and repopulation of the DPM and (p-

overcome the energy barrier that restricts thermal relaxation to the orange resting state. A recent temperature kinetics study clearly illustrates the large effect of temperature on the rate of return, e.g., the blue state half-life may increase to 24 h by decreasing the temperature to ca −50 °C.3 Lastly, linear inversion during the thermal and radiationless back reaction would not require degeneration of the CN double bond axis, which is a prerequisite for rotation to take place.35 A frequent argument that had been used in favor of inversion is that activation energies for aromatic imine isomerizations are approximately 20 kcal/mol36 as opposed to the 40 kcal/mol that is required by aromatic olefins,37,38 the latter which can isomerize only by the torsion mechanism. In fact, some has suggested a mechanistic continuum whereby isomerization in azomethines is neither taking place via rotation alone, nor purely through inversion.39 In conclusion, it may thus only be stated that circumstantial evidence is consistent with the above hypothesis. Exhaustive computational work would however be required to provide additional substantiation, which lies outside the scope of the present study. 3.3.2. Substituent Effect. Selected data obtained by ultrafast transient absorption spectroscopy measurements for the entire series of photochromic complexes in DCM are listed in Table 1 (see Supporting Information Table S1 for an extended data set). The time constants, τ, are indicative of how fast the orange and blue ground state isomers form, following orange resting state excitation to the excited state orthogonal geometry. With the series arranged by increasing λmax values (wavelengths at absorbance maxima of orange resting state in DCM), it becomes clear that this trend is closely followed by λmax values of the excited and blue ground states, with the exception of only a few. This order is not paralleled by corresponding time constants, where little overall tendency is observed. However, as will be seen later, comparison within isolated groups of closely related compounds do indeed reveal correlating trends. As for the entire series, it is nevertheless noticed that the three o-methyl DPM (o-CH3) states absorb light of shortest wavelength while accompanied by smallest time constants. At the opposite extremity the p-S-methyl (p-SCH3) compound in turn absorbs the longest wavelengths while also having largest time constants. By thus employing the methyl and S-methyl substituents in the ortho and para phenyl positions respectively, affects a maximum shift of 49 nm in λmax of the resting state

Figure 6. Transient change in optical density ΔOD(τ) for DPM (squares) and (p-SCH3)DPM (triangles) at λmax of the GSB. The numerical fit model is included (red line).

SCH 3 )DPM ground states. Ground state depletion is accomplished by means of the 40 fs laser pump pulse centered at 480 nm. Because of the manner in which the ground state is depleted, it is expected that the ground state bleach (GSB) signal only shows an exponential increase in the change of optical density. Instead, an initial decrease ( 0.3 eV. Figure 12. Computed HOMO energies correlated with energies of the orange (⧫) and blue isomers (●) of substituted DPM in dichloromethane measured by laser spectroscopy (at central wavelengths of absorption). Halogenated derivatives are encircled, S-methyls are connected by a dashed line and derivatives containing methyls by a solid line.

4. CONCLUSIONS We hereby conclude that substitution on dithizone phenyl rings significantly alter both wavelength of light absorption and ultrafast time dynamics during the photochromic reaction of corresponding phenylmercury complexes. These changes are not mainly the consequence of electronic alterations, but also due to steric effects and inertia. Dithizonatophenylmercury(II) was for the first time observed to be notably solvatochromic in the excited state. Solvent studies revealed a reversed trend between ultrafast dynamics and the overall rate during the photochromic reaction. DFT computational data established favored geometries in both isomers and also indicates electron donating substituents to enhance stabilization of the photoexcited blue state.

molecules traditionally shows good correlation is well illustrated also when comparing computed HOMO energies with experimental data (energies related to λmax values calculated by E = hc/λ). The halogenated compounds represent the lowest HOMO energies, with the meta-F derivative having the overall lowest orange isomer calculated HOMO energy of −5.719 and −5.370 eV in the corresponding blue isomer. An almost linear correlation is seen for the all methylated species, which includes the o-OCH3 derivativethe latter consistently having the highest values. The data pattern of the blue isomer closely resembles that of the orange isomer. Figure 13 illustrates the general trend found for the correlation between the calculated HOMO−LUMO gap

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ASSOCIATED CONTENT

S Supporting Information *

Table S1, arbance maxima, and growth and decay times of the orange, excited and intermediate states, and blue isomers of electronically altered dithizonatophenylmercury complexes; Figure S1, transient change in optical density of DPM in dichloromethane, deuterated methanol, and methanol between 300 and 650 nm, after excitation with a 40 fs short laser pulse at 480 nm; Figure S2, HOMO renderings of selected DPM derivitaves; Figure S3, TDDFT computed oscillators of the orange and blue DPM isomers, in gas phase, DCM, and methanol; and Figure S4, GO9/B3LYP calculated electronic spectra and experimental spectra of the orange and blue isomers of the substituted DPM complexes in dichloromethane. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 13. Computed HOMO−LUMO gap energies, ΔEgap, correlated with energies of the orange isomers of substituted DPM in DCM measured by laser spectroscopy (at central wavelengths of absorption). ortho-Substituted derivatives (o-CH3, o-F, o-OCH3) are indicated by circles (●). The trend among meta and para compounds is indicated by the line through squares (■).

AUTHOR INFORMATION

Corresponding Author

*(K.G.v.E.) Telephone: +27-(0)51-4012923. Fax: +27-(0)514446384. E-mail: [email protected]. Notes

energies with experimental λmax values. A better and consistent overall trend is indeed expected here, as the experimental data also represents energy differences and not relative energies (of HOMO’s) as discussed in the former paragraph. The orthosubstituted species, however, deviates from the otherwise observed trend. The close proximity of an ortho substituent to the backbone has a significant influence on the molecule at large, as also argued with regard to the Hammett series of constants, i.e. Hammett constants for ortho variants do not exist.41,42 DFT calculated HOMO and LUMO energies of dithizone derivatives were previously related to corresponding oxidation (Epa,1) and reduction potentials (Epc,C).31 A similar

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work is based upon research supported by the South African Research Chair Initiative of the Department of Science and Technology and the National Research Foundation. This work additionally received support from the Norwegian Supercomputing Program (NOTUR) through a grant of computer time (Grant No. NN4654K) (J.C.) and the Central Research Fund of the University of the Free State, Bloemfontein. 853



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