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. 2024 Jan 18;128(4):785–791. doi: 10.1021/acs.jpca.3c07128

Acid Violet 3: A Base-Activated Water-Soluble Photoswitch

Ing-Angsara Thongchai 1, Zachary J Knepp 1, Domenica R Fertal 1, Helen Flynn 1, Elizabeth R Young 1,*, Lisa A Fredin 1,*
PMCID: PMC10839829  PMID: 38236752

Abstract

graphic file with name jp3c07128_0010.jpg

Acidic azo dyes are widely used for their vibrant colors. However, if their photophysics were better understood and controllable, they could be integrated into many more applications such as photosensing, photomedicine, and nonlinear optics. Here, the proton-controlled photophysics of a widely used acid, hydrazo dye, acid violet 3 (AV3) is explored. Density functional theory is used to predict the ground- and excited-state potential energy surfaces, and the proposed photoisomerization mechanism is confirmed with spectroscopic experiments. The ground-state and first two excited-state surfaces of the three readily accessible protonation states, AV3–H, AV3, and AV3+H, are investigated along both the dihedral rotation and inversion coordinates. The deprotonated AV3–H undergoes photoisomerization with blue light (λex = 453 nm) through a dihedral rotation mechanism. Upon the formation of the cis-isomer, the reversion of AV3–H is predicted to occur through a mixed rotational and inversion mechanism. In contrast, AV3 and its protonated form, AV3+H, do not undergo photoisomerization because there is no driving force for either the rotation or inversion of the azo bond in the excited state. In addition, when the azo bond is acidic, the ground-state dihedral rotation reversion mechanism barrier is lower. The mechanistic insights gained here through the combination of theory and experiment provide a roadmap to control the reactivity of AV3 across 11 orders of magnitude of proton concentration, making them interesting candidates for a range of pharmaceuticals.

Introduction

Acid dyes, with one or more SO3H or COOH1 functional groups, are a diverse class of water-soluble anionic dyes that are widely recognized for their vibrant colors. The water solubility of these dyes has led to their wide adoption, as approximately 30–40% of dyes used in industry are acid dyes.2 In particular, acid violet dyes are extensively used across multiple industries including textiles, printing, and cosmetics3,4 because of their vivid purple colors. Acid violet 3 (AV3) is particularly interesting because it is an example of a water-soluble azo dye (including an N=N bond). Thus, it should combine both the beneficial properties of acid and azo dyes: water solubility and photoswitching capabilities.5−9 These properties can open doors to new potential uses of AV3 in optical devices,10 photopharmacology, and drug delivery11 if its photophysics can be controlled.

Like many azo dyes, the fundamental photophysics of AV3 is understudied.12 The impact of the pH, substituents, and heteroaryl groups13−16 on the physical and chemical properties of AV3 has not been explored systematically. In particular, AV3 has multiple protons that act as excited-state control handles. Early spectroscopic studies of azo dyes show that they undergo a trans-to-cis isomerization of the azo bond under blue or UV light exposure.12−14,17,18 A significant amount of work on these dyes has focused on the detailed mechanism of photoisomerization and reversion of model dyes.19−26 Most of these dyes, such as symmetric azobenzene, undergo photoisomerization through a rotation mechanism, either a pure dihedral rotation or some combination of rotational and inversion coordinates.24 In contrast, for many azo dyes, the thermal reversion on the ground-state surface is dominated by an inversion transition state.15,16,19−23

Efforts to control these processes using environmental factors, including solvent and pH,8,15 have provided handles to control photoswitching.5−9 For example, the nitrogens of the azo bond provide natural protonation sites that can geometrically hinder photochemical or thermal isomerization.27−31 The isomerization dynamics of azo dyes that have additional (E/Z) rotational isomers can even be controlled via long-range proton transfer.32,33 This control has been used to stabilize azo polymers34 or materials35 by building hydrogen bonding to the azo bond. In particular, hydrazone dyes,36 like AV3, have an oxygen near the azo bond, which offers a readily available protonation site that has been shown to hydrogen bond with the nitrogens of the azo bond, potentially locking the molecule in the trans-configuration.37,38 Consequently, we hypothesized that the photoisomerization process of AV3 can be selectively modulated by manipulating the pH of the dye solution.

AV3 possesses two hydroxyl groups that have been proposed to create a hydrogen-bonding network with the azo bond. The reported structure of AV3 varies based on the purchasing company, with most listing the structure by the IUPAC name: 3-(4-aminophenylazo)-4,5-dihydroxy-2,7-naphthalenedisulfonic acid disodium salt (Scheme 1 right). Transferring the hydrogen formally to the azo bond (Scheme 1 left) would result in a similar protonation of the bond that has been shown to shut down photoisomerization.15 With this in mind, we hypothesize that electronic modulation on the phenyl side could be used to disrupt the hydrogen bonding by changing the electron density on the hydroxyl and the azo bond.

Scheme 1. Two Resonance Structures of Hydrazo AV3.

Scheme 1

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The right is based on the IUPAC name, with the hydroxyl groups drawn to emphasize possible hydrogen bonding interactions.

Here, an in-depth investigation into the photoisomerization mechanisms of AV3 in its three protonation states is conducted by exploring the potential energy surfaces of the dyes with density functional theory (DFT). The electronic modulation is not predicted to be strong enough to disrupt the formation of an N–H covalent bond on the azo. In addition, it is found that the neutral state of AV3 and its acidic form, AV3+H, do not undergo photoisomerization because there is no driving force to either rotate or invert the azo bond in the low-lying singlet excited states. Spectroscopic experiments confirm that AV3 photoisomerizes only when the azo bond is unprotonated.

Methods

Quantum Mechanical Calculations

AV3 and all possible protonated and deprotonated states, with sodium counterions omitted, were optimized at CAM-B3LYP/6-311++G(d,p)/PCM(water) level of theory in Gaussian 1639 and confirmed as true minima with no imaginary frequencies. The molecular orbitals show typical azo dye ordering (Figure S5). Linear response time-dependent DFT (LR-TDDFT) was used to predict the first 30 singlet excitation energies and their corresponding oscillator strengths of AV3, AV3–H, and AV3+H in both the trans- and cis-geometries. Potential energy curves (PECs) along the azo-bond dihedral rotation (CNNC) and the two inversion (CNN of phenyl and naphthyl) were constructed from relaxed scans along the ground state (S0). At each geometry, TDDFT was used to predict the (S1) and (S2) energies. While the minima on the S1 and S2 surfaces do not portray true minima, our previous work optimizing excited-state minima showed very similar geometries to those along the PECs.15 Ground-state transition states were optimized from the highest energy point along each scan and were confirmed as transition states by a single imaginary frequency.

Experimental Section

Materials

AV3 or 3-[2-(4-aminophenyl)diazenyl]-4,5-dihydroxy-disodium salt (CAS 1681-60-3) was purchased from Tokyo Chemical Industry (TCI) and used without further purification. Sodium hydroxide (NaOH) and sulfuric acid (H2SO4), purchased from Sigma-Aldrich and EMD Millipore Corporation, were used as received to deprotonate and protonate AV3. Distilled water used to prepare solutions was collected on the day of the experiments from the distillation system maintained by the Lehigh University Chemistry Department.

Photometric pH Titration

Photometric pH titrations were performed with an aqueous stock solution (0.455 mM) of AV3. The acidic, basic, and neutral solutions (45.5 μM) were prepared by diluting 1 mL of the stock solution in a 10 mL volumetric flask with distilled water, so the same concentration of AV3 was maintained throughout the course of the titration. The basic solution contained 0.2 mL of 0.126 M NaOH to deprotonate the dye; the acidic solution contained 0.1 mL of 0.5 M H2SO4 to protonate the dye; and no addition was made to the neutral solution. The pH titration started with 2 mL of neutral solution in a 1 cm quartz cuvette and began with small additions of either basic (10 μL) or acidic (5 μL) solutions. The volume of additions gradually increased as more basic or acidic conditions were required to produce a change in the dye absorption spectra (Figure S6). The total volume of basic and acidic solution added to the starting neutral solution was 4.8 and 0.5 mL, respectively. The pH and UV–visible spectra were recorded after each addition with a VWR sympHony benchtop pH meter and Ocean Insight diode array spectrometer. The titration was complete when the characteristic absorption spectrum of the basic or acidic solution was reached and did not change with further additions (Figure S7).

Photoisomerization

The photoisomerization of AV3 (Figures S8 and S9) was accomplished by illumination with a blue LED (λmax = 453 nm, fwhm = 18 nm) using an LEDi-RGB photolysis lamp head and a Luzchem LED Illuminator. The absorption spectra were recorded with the Ocean Insight diode array spectrometer. A 45.5 μM solution of AV3 (∼3 mL) was placed in a 1 cm quartz cuvette, and the UV–visible spectrum was recorded prior to any illumination. The LEDi-RGB photolysis lamp head was centered above the cuvette (∼1 cm) and the lamp power was increased incrementally from 0 to 411 mW. The UV–visible spectra were recorded after each power increase. Change in illumination intensity was used to track the photoisomerization of AV3 because of the fast isomerization and reversion rates between the trans- and cis-isomers that occurred within our instrument response time.

Results and Discussion

Impact of Electronic Effects on AV3 Resonance Structure

Optimization of trans-AV3 shows an N–H covalent bond (1.03 Å) and an arylketone (Scheme 1 left) rather than two hydroxyl groups. Bond analysis shows that protonation of the azo bond results in an N–N single bond rather than a double bond. The N–N bond has a stabilized n-orbital compared to the azo N=N double bond (Figure S5), effectively removing the unallowed n–π* transition from the lowest energy excitations. The hydroxyl isomer is 0.21 eV higher in energy than the optimized structure with an N–H bond, indicating that it might be a metastable form of AV3. However, the PEC along the hydrogen bonding coordinate (Figure 1) shows that the barrier to transfer the hydrogen between the N and the O is 0.3 eV. The fact that this barrier is similar in energy to the O–H isomer indicating that there should be fast reformation of the N–H bond even if the O–H is formed transiently.

Figure 1.

Figure 1

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Optimized constrained PEC along the H bonding coordinate between N and H (left) and O–H (right). The fully optimized structures including the transition state are shown (stars) with the transition state vector in Figure S10. CAM-B3LYP/6-311++G(d,p)/PCM(water).

We speculated that introducing other functional groups in place of the amine on the phenyl moiety could allow the O–H covalent bond to be maintained and permit the azo bond to rotate in the neutral AV3 form. However, each of the substituents (Figure 2), with para Hammett parameters ranging from −0.83 to 0.78, shows a clear N–H covalent bond over an O–H bond. The electron donating groups red shift the absorption, while the electron withdrawing groups blue shift the absorption (Figure 3), allowing the absorption maximum to be tuned by ∼100 nm.

Figure 2.

Figure 2

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Optimized structures of various AV3 derivatives. Substituents are changed from the NH2 functional group in AV3 to fluorine (F), morpholine (Morph), N(CH3)2 (NMe2), NO2, OCH3 (OMe), and OH groups. Fully optimized structures are shown in Figure S1.

Figure 3.

Figure 3

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Predicted absorption spectra of AV3 (R = NH2) and tested substituents at the CAM-B3LYP/6-311++G(d,p)/PCM(water) level of theory. Full transitions of each protonation state for each substituent are shown in Figure S2.

Impact of Protonation State of AV3

Three distinct protonation states of AV3 were observed experimentally (Figure 4) with clean isosbestic points between them (Figure S7). Each possible protonation state of trans-AV3 was computed (Figure S3 and Table S1), and the lowest energy structures showed that the azo bond is unprotonated to form AV3–H and the amine is protonated to form AV3+H (Scheme 2).

Figure 4.

Figure 4

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UV–vis absorption at pH 1.85 (red), pH 5.80 (purple), and pH 12.25 (yellow) and TDDFT-predicted transitions for AV3–H (yellow), AV3 (purple), and AV3+H (red) at the CAM-B3LYP/6-311++G(d,p)/PCM(water) level of theory. Computational absorption and transitions for 200–700 nm are shown in Figure S2.

Scheme 2. Protonation States of AV3.

Scheme 2

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Matching basic (pH = 12.23) AV3–H, near neutral (pH = 5.80) AV3, and acidic conditions (pH = 1.85) AV3+H, where the changing hydrogens are highlighted in red.

Experimentally, deprotonation of AV3 to form AV3–H (pH 12.25) results in a blue shift (∼80 nm) and broadening of the lowest energy absorption feature. Protonation of AV3 to AV3+H (pH 1.85) results in a smaller blue shift (∼40 nm) of the same feature with comparable full width at half-maximum. These trends are well captured by the predicted transitions (Figure 4, full spectra in Figure S2) of each protonation state with an overestimation of the transition energies expected from the long-range-corrected CAM-B3LYP functional.

The cis-isomer of each protonation state (Figure S4 and Table S2) is a metastable state 0.3–0.7 eV higher in energy than the ground-state trans. While the trans-geometries are nearly identical in each protonation state, with a 180° azo dihedral (CNNC) and 121° phenyl angle (CNN), the cis-isomers show more variation in their dihedrals from 13 to 40°. The N–H bond in both AV3 and AV3+H is 0.7 Å shorter than the O to H distance in the trans-structures, confirming an N–H covalent bond and an O···H hydrogen-bonding interaction that must be broken to form the cis-isomer.

Photoisomerization

Based on the protonation of other azo dyes,15,32,37,38 photoisomerization of AV3 and AV3+H is not expected as the H–N bond seems to restrict rotation around the azo bond to form the cis-isomer (Figure S8). In contrast, AV3–H isomerizes when exposed to blue light. While the decrease in the absorption at 460 nm is not large, the difference spectrum (final-initial) matches well with the cis–trans computational difference spectrum (Figure 5), after accounting for the expected CAM-B3LYP blue shift. The relatively small change in the absorption is likely due to the overlapping trans- and cis-absorption spectra (Figure S6). The feature at ∼450 nm in the experimental difference spectra for AV3 and AV3+H is caused by light scattering from the photolysis lamp and is not representative of photoisomerization or degradation(Figure 5).

Figure 5.

Figure 5

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Difference spectra of the initial and maximum light intensity (411 mW, λex = 453 nm) and calculated (dotted lines) cis–trans difference spectra for each AV3 protonation state. Sharp negative peaks at λ ∼ 450 nm are due to scattered light from the blue LED used in the photoisomerization experimental setup and thus are shaded out.

PECs along both the azo dihedral rotation (CNNC) and the phenyl inversion (CNN) reaction coordinates (Figure 6) show that AV3 and AV3+H are energetically locked into the trans-isomer (CNNC 180°) in both of the lowest-energy singlet excited states, S1 and S2. Interestingly, the inversion barriers are higher on all singlet surfaces for AV3 and AV3+H, and the dihedral rotation on the S1 in both protonation states are slightly uphill, indicating no driving force for isomerization in the excited state. While the PECs indicate that the decay occurs at the Franck–Condon geometry, which could possibly lead to emission, no convincing emission was seen for either AV3 or AV3+H experimentally. In addition, the oscillator strength between S1 and S0 is low in AV3 and AV3+H, indicating nonideal overlap. This result is consistent with other azo and acid dyes that have very low quantum yields,40−43 due to all their many degrees of freedom, which deactivate via nonradiative processes.

Figure 6.

Figure 6

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Lowest three singlet PECs along the CNNC dihedral (blue) and the phenyl CNN angle (red) of each AV3 protonation state. CAM-B3LYP/6-311++G(d,p)/PCM(water). Notably, the inversion PECs (red) are only shown for the first 60° due to the linearization of the bond at this point, and the higher barriers are shown on these excited-state surfaces.

In contrast, the AV3–H S1 has a minimum at ∼90° dihedral angle. This minimum indicates that a fast internal conversion would happen at a conical intersection between the S1 and S0 surfaces near the transition state between the isomers, allowing for the population of the metastable cis-isomer along with some repopulation of the trans-isomer. The energy of the minimal crossing point between S1 and S0 is similar to those calculated for other azo dyes,44−48 for which molecular dynamics trajectories have been used to predict that 15% of the photons absorbed result in the formation of the cis-isomer. Thus, these PECs predict that prolonged illumination can result in a buildup of cis-isomer, as seen experimentally. The phenyl inversion of AV3–H also has a minimum on S1 that differs from the initial trans-geometry. However, the minimum is far from the trans–cis-transition state on the S0 surface, indicating that the majority of photoisomerization of AV3–H occurs through a rotational mechanism. Importantly, as the excited-state surfaces are based on the underlying ground-state geometries, the S1 of each molecule was optimized for two geometry steps to confirm the curvature of the PECs. No coordinates other than the CNNC dihedral angle in AV3–H changed significantly in any of the TDDFT optimizations, supporting the PEC picture of the Franck–Condon region of the surfaces.

Reversion

Like many azo dyes, AV3–H reverts back to the trans-isomer thermally after removal of the applied light source. This thermal process occurs on the ground-state surface (S0), and thus the transition states between cis and trans provide insights into the thermal reversion process. From the initial scans along the ground-state PECs, the barriers for rotation and inversion between the cis and trans of AV3–H are similar (Figure 6). The fully optimized transition states (Figure 7) are 0.11 eV different in energy for AV3–H (Table S2), indicating a mixture of reversion paths. In contrast, the rotational transition states are 0.27 and 0.55 eV more stable for AV3 and AV3+H, respectively. Thus, the protonated forms of AV3 would be expected to undergo cis-to-trans isomerization through a purely rotational mechanism, in contrast to the many azo dyes that undergo thermal reversion through at least an inversion-assisted mechanism.

Figure 7.

Figure 7

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Optimized dihedral rotation (rot TS) and phenyl inversion (inv TS) transition states on the ground-state surface. CAM-B3LYP/6-311++G(d,p)/PCM(water). Optimized transition states and vectors are shown in Figure S11.

Conclusions

AV3 is an acidic, water-soluble hydrazone-based dye that possesses three easily accessible protonation states, namely, AV3–H, AV3, and AV3+H. AV3 and its protonated form (AV3+H) do not photoisomerize because there is no driving force to rotate or invert the azo bond in the excited state. Due to the removal of the hydrazone proton, AV3–H is able to undergo photoisomerization with blue light through a dihedral rotation of the freed azo bond. The reversion of AV3–H should then occur through a mixed rotational and inversion mechanism, as the transition states are nearly isoenergetic. Interestingly, the AV3 and AV3+H ground states have a lower barrier for a dihedral reversion from cis and trans, which is uncommon in azo dyes. Electronic tuning on the phenyl side of the azo does not free up the azo bond for rotation in the neutral form for substituents with Hammett parameters of ±1. The promising ability to control the photophysics of these water-soluble dyes through pH modulation provides new avenues for their use as photoswitches.

Acknowledgments

E.R.Y. and L.A.F. thank The Pittsburgh Foundation (UN2020-114823) for funding. Acknowledgement is made to the donors of the American Chemical Society Petroleum Research Fund for their partial support of this research. Research computing resources were provided by Lehigh University, partially supported by the NSF CC* Compute program through grant number OAC-2019035 and the TG-CHE190011 allocation from the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation grant number ACI-1548562. This work made use of the Lehigh University NMR Facility, partially supported by NSF MRI-1725883.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.3c07128.

  • Additional computational details including optimized structures and coordinates, TDDFT absorption spectra and predicted transitions of all protonation states of all molecules, MO diagrams, transition-state geometries and normal mode vectors, and experimental UV–vis titration and photoisomerization of AV3 (PDF).

The authors declare no competing financial interest.

Supplementary Material

jp3c07128_si_001.pdf (10.3MB, pdf)

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