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Spectroscopic study of the pH dependence of the optical properties of a water-soluble molecular photo-switch
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117576
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic study of the pH dependence of the optical properties of a water-soluble molecular photo-switch Julie Fitz, Angela Mammana* University of Dayton, Department of Chemistry, 300 College Park, Dayton, OH, 45469, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2019 Received in revised form 25 September 2019 Accepted 28 September 2019 Available online 11 October 2019
In this report we present a UVeVis spectroscopic study of the pH dependent optical properties of 4,40 azobenzene dicarboxylic acid in aqueous solution. A combination of chemical (acid-base) and light stimuli is combined to demonstrate that the system undergoes two types of optical switching cycles. At neutral pH the azobenzene undergoes photo-induced cis-trans isomerisation. Upon pH reduction the UV eVis spectra show changes consistent with aggregation of the azobenzene photo-switch. The photoresponsive behaviour is dependent on the pH and conformation of the azobenzene. The optical properties of the system are dependent on the mode of pH reduction and the isomeric cis/trans composition of the photo-switch, showing hierarchical control of self-assembly. © 2019 Elsevier B.V. All rights reserved.
Keywords: Azobenzene Self-assembly Supramolecular Photochemistry
Exerting dynamic control over chemical systems at the molecular and supramolecular level is a considerable challenge with potential applications in molecular machinery, logic gates, memory storage, liquid crystals, controlling surface interactions, catalysis and optical devices [1e13]. Additionally, studying the effect of environment and molecular structure on self-assembly will shed light on fundamental aspects of weak molecular interactions with the potential to unlock new pathways to the controlled construction and manipulation of increasingly complex chemical architectures [14e17]. Photo-responsive molecules are ideal candidates for manipulating chemical systems because photochemical reactions are specific to a given chromophore within a molecule and do not affect non-absorbing components of the system [18]. A number of photo-active molecules undergo reversible structural changes [19e23]. Azobenzene chromophores have great potential in modulating self-assembly processes because they undergo a reversible photo-isomerisation from a trans isomer to a cis isomer upon irradiation with UV light, and revert back to the trans state upon exposure to visible light or upon heating [24,25]. The bent conformation of the cis isomer is less conducive to dense molecular packing than is the flat trans isomer [26]. In this report, we present a spectroscopic analysis of a simple, water-soluble azobenzene-based supramolecular system that can
* Corresponding author. E-mail address: [email protected] (A. Mammana). https://doi.org/10.1016/j.saa.2019.117576 1386-1425/© 2019 Elsevier B.V. All rights reserved.
be manipulated with acid-base stimuli and photons. The lightabsorbing and pH sensitive system can be cycled between four states. The specific chemical pathway required to access the available states is controlled by the molecular conformation of the photo-switch. 4, 40 -azobenzene dicarboxylic acid (ADA), shown in Fig. 1, was selected as the photo-switch used in this work because of its potential to be water soluble. Water provides an important medium for investigating molecular and supramolecular processes because it is benign and provides an appropriate environment for ultimately exploring biological materials and applications [27,28]. Additionally, an aqueous environment promotes self-assembly in amphiphilic molecules. In particular, aromatic systems containing ionisable functional groups at the periphery have received much attention with heavy emphasis on porphyrin-based structures [29e31]. The paradigms of porphyrin self-assembly and optical responses in aqueous solution provide a blue-print for expanding the scope and variation of self-aggregating systems and exploring non-porphyrin-based structures in water including azobenzene photo-switches as reported herein. ADA is ideal for working in aqueous solution because its carboxylic acids are able to ionise and engender solubility at high pH. At low pH, the hydrogen on the carboxylic acid and the electronegative carbonyl can form hydrogen bonds, providing an impetus for self-assembly. In addition, the two benzene rings in the molecule make it an ideal candidate for an aggregation-oriented system due to the ease with which they can participate in p-p stacking. At low pH some researchers have proposed protonation of the azo bond for specific
2
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Fig. 1. Structure of cis- and trans-ADA and UVeVis spectra in H2O of trans-ADA (blue), cis-ADA (pink) photo-generated after exposure of trans-ADA to UV light, and trans-ADA (dotted light blue) photo-generated after exposure of cis-ADA to visible light.
azobenzene derivatives in specific solvents [32e36]. Azobenzene derivatives for which azo protonation has been proposed in the literature are amino and/or hydroxyl functionalized, but not the dicarboxylic acid functionalized azobenzene in our present work. In order to study the effect of pH on the optical properties of azobenzene, which in turn provides a probe for studying intermolecular interactions, we selected UVeVis absorption spectroscopy since it is a well-established and fundamental spectroscopic technique for characterizing supramolecular interactions. It is wellknown that molecules show shifts in their absorption spectra when they associate in solution or at interfaces [28e30]. A common theme in studies regarding supramolecular chemistry is a hypochromic effect in the absorption spectrum as an indication of aggregation. A shift in absorption bands can also be associated with specific types of aggregates [37]. A blue shift is associated with Htype aggregates in which the chromophores assemble in a parallel plane-to-plane stacking geometry. A red shift is associated with a Jaggregate in which the chromophores assemble in a head-to-tail arrangement. Moreover, a change in the geometry of the azobenzene derivatives can be detected via a change in their absorption spectra as shown in the UVeVis spectra of the two forms (cis and trans) of ADA in H2O (Fig. 1). At pH 7, trans-ADA has two main bands at 228 nm and 330 nm, with the intense peak at 330 nm attributed to the p-p* transition of the trans isomer. Irradiation of the solution with UV light (365 nm) results in a marked decrease in absorbance at both the 228 nm and the 330 nm bands and an increase of two bands at 250 nm and 430 nm corresponding to the cis isomer, with the weak peak at 430 nm attributed to the n-p* transition of the cis isomer [38]. trans-ADA was solubilized in water by increasing the pH to approximately 11 by addition of NaOH. The pH was reduced to 7 prior to experiments performed in this report unless otherwise noted. ADA does not show spectroscopic changes indicative of aggregation at pH 7. An aqueous solution of trans-ADA at pH 7 was prepared and reduced to a pH of 3 by the addition of H2SO4. Comparison of the UVeVis spectra at pH 7 and 3 shows that the spectrum of trans-ADA undergoes a hypochromic effect and blue shift upon reducing the pH to 3, both of which are spectral changes diagnostic of aggregation. The band at 330 nm decreased in intensity as the pH was reduced and continued to decrease with time.
After 60 min, the signal no longer changed (Fig. 2A). The decrease in signal intensity was accompanied by a blue shift from 330 nm to 300 nm, which suggests the formation of an H-aggregate in which the monomers are oriented in a face-to-face or parallel manner [37]. Some preliminary data shown in Figs. S1 and S2 indicate that the structure of the aggregate depends on the modality through which the pH is changed and the isomeric form present in solution when the aggregation takes place. We focused our study on the spectroscopic data obtained upon fast reduction of the pH both for the trans and cis isomers as shown in Fig. 2. A plot of absorbance at 330 nm vs. pH yields a sigmoidal curve with an inflection point around pH 5.3 which corresponds to the pKa of the trans-ADA as shown in Fig. S3. This pKa value corresponds to the carboxyl groups on the ADA and is a higher value compared to benzoic acid (pKa ¼ 4.2) and compared to what is expected for a carboxylic acid attached to monomeric azobenzene [39], suggesting that the protonation process is favoured by the subsequent formation of the supramolecular structure. Upon irradiation, the sample did not exhibit the characteristic spectroscopic changes associated with trans to cis photoisomerisation. There are several possibilities for explaining this loss of photodynamic control, but the most likely reason is that the tight packing of trans-ADA in an H-aggregate geometry provides a hindrance to motion that the energy of excitation cannot overcome. The non-covalent interactions might provide too high of an energy barrier for the photomechanical effect to surpass. Isomerisation also might occur, but could potentially result in a highly strained cis structure that rapidly reverts back to the trans structure too quickly for spectroscopic analysis to detect. Confinement effects on isomerisation and molecular motion are not without precedent. For example, intermolecular crowding in surface-attached monolayers of molecular motors based on overcrowded alkenes has been attributed to intermolecular interactions hindering molecular rotary motion in some tightly packed altitudinal systems [40,41]. Further, isomerisation of azobenzene chromophores covalently bound to polymer side-chains in thin, macromolecular films show isomerisation behaviour that is influenced by a neighbouring effect [42]. Although trans-ADA loses its photo-switching capabilities at low pH, an increase in pH through addition of NaOH disrupts the
J. Fitz, A. Mammana / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117576
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Fig. 2. Effect of decreasing pH on UVeVis spectra of trans-ADA and cis-ADA. (A) trans-ADA at pH ¼ 7 (blue); trans-ADA immediately after lowering the pH to 3 (black); trans-ADA at pH ¼ 3 after 1 h (green). The hypochromic effect and blue-shift is an indication of H-aggregate formation. (B) cis-ADA at pH ¼ 7 (pink); cis-ADA immediately after lowering the pH to 3 (black); cis-ADA at pH ¼ 3 after 1 h (green).)
intermolecular interactions, as shown by the increase of the absorption band at 330 nm (Fig. 3). Upon disassembly of the supramolecular structure, the azobenzene regains its photochromic behaviour in response to UV and visible light. Self-assembly is accordingly a reversible process dependent on pH, and the ability to isomerise from the trans to the cis form of ADA is determined by the degree of aggregation. The cyclical nature of this process is shown in Fig. 3. After exploring the behaviour of the trans form of ADA, the cis form was investigated to see if structural changes in the molecule can modulate its supramolecular behaviour. A solution of transADA was irradiated with UV light to obtain the cis form of the molecule. The pH was reduced with continued UV irradiation to maintain the highest cis-trans ratio possible. After approximately 15 min of irradiation, the ADA reaches a photo-stationary state in which the molecule attains equilibrium between the cis and trans isomers with the ratio of cis to trans depending on the ratio of their respective extinction coefficients at the wavelengths of irradiation. Due to overlap of the absorption bands, we can never assume that our solution is entirely in the cis form. The supramolecular structures obtained could, therefore, contain both the cis and trans isomers in the overall geometry. The pH of the aqueous cis-ADA solution was reduced to 3 by the addition of H2SO4. Immediately following the addition of H2SO4, the 230 nm band in the UVeVis spectrum exhibited a slight red shift and hypochromic effect while the 330 nm and 430 nm bands lost their maxima and were replaced by a broad signal as a result of increased scattering (Fig. 2B). After 60 min, the band at 230 nm disappears and a new weak broad band appears at approximately 360 nm. The absence of a blue shift indicates that the geometry of the cis aggregate is not parallel (H-aggregate), unlike the trans aggregate, showing that there is a photo-controlled interplay between the structure of the self-assembling molecule and the structure of the resulting supramolecular species. The cis-azobenzene assembly at pH 3 was irradiated with visible light in order to isomerise the cis-ADA complex to the trans form. The UVeVis spectrum shows a slight increase in intensity and a blue shift of the peak at 340 nme300 nm. As shown in Fig. 4 the
shape, position and intensity of the absorption spectrum is analogous to the spectrum of the trans-ADA assembly obtained upon reducing the pH of an aqueous solution of trans-ADA from 7 to 3, indicating that irradiation triggered the transformation of the structure of the cis aggregate to the trans aggregated form. When cis-ADA in the aggregated form photo-isomerises to trans-ADA it might either break the existing aggregated species proceeding through a transitional monomeric phase before forming the H-aggregate of trans-ADA, or it might isomerise without disassembling. In the latter case the cis aggregate is expected to have a pre-oriented geometry that favours the formation of the trans H-aggregate. The ability of the cis form of ADA to undergo photo-isomerisation at low pH might be a result of a less tight packing of the chromophores compared to the trans form of ADA at low pH, or possibly strain induced by intermolecular interactions making cis to trans isomerisation energetically favourable relative to the non-covalent intermolecular interactions, in contrast to the trans H-aggregate. Additionally, both cis and trans azobenzene absorb in the 365 nm region with different extinction coefficients [43,44]. After irradiation with UV light, the amount of cis and trans ADA present will depend on the ratio of the extinction coefficients at the irradiation wavelengths employed. The presence of trans isomer in the cis assembly (due to the photo-equilibrium) might disrupt interactions, leading to a less compact structure in which the cis isomer is less hindered, facilitating isomerisation in the pHinduced supramolecular structure. In order to confirm that the cisADA had indeed isomerised to the trans isomer, the pH of the solution was increased to 7 with NaOH. The UVeVis spectrum obtained was superimposable with the initial spectrum of the transADA sample before irradiation with UV light, demonstrating the high efficiency of the photo-isomerisation process for this supramolecular system. The cyclical nature of this process is shown in Fig. 4. We have demonstrated pH and light-induced changes in the UVeVis spectra of trans- and cis-ADA. The observed pH-induced changes in the UVeVis spectra indicate that ADA aggregates at low pH. This pH and photo-responsive supramolecular system is based only on azobenzene monomers functionalized with
4
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Fig. 3. Cycle showing the pH-induced conformational locking of trans-ADA upon reduction of the pH to 3 and de-locking of the photo-switchability upon disassembly via the addition of base to the system. At pH 7, ADA undergoes reversible cis-trans isomerisation regardless of whether or not it was previously aggregated via pH reduction. (See Supporting Information for individual spectra, Figs. S4eS9).
carboxylic acids. The carboxylic acids allow azobenzene to dissolve in water under basic conditions. The photo-responsive properties at low pH are dictated by the structure of the monomeric azobenzene, which can be selected with light. At low pH the cis isomer is able to undergo photo-induced transformation to the trans isomer, however at low pH the molecular structure trans-ADA is locked and unable to undergo photo-isomerisation to the cis form. The photo-
responsive behaviour of trans-ADA can be turned back “on” upon disassembly by adding base to the system. The optical phenomena described herein are reminiscent of spectroscopic changes observed during porphyrin self-assembly and demonstrates that ADA has the potential to be an interesting photo-responsive tool that can be applied to supramolecular systems and interfaced with biological materials. Optical and pH-induced switching between
J. Fitz, A. Mammana / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117576
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Fig. 4. trans-ADA is photo-isomerised to cis-ADA at pH 7. Upon reducing the pH to 3 by the addition of H2SO4, the cis-ADA forms a supramolecular structure. Irradiation with visible light produces the trans form of the aggregate. Increasing the pH to 7 by the addition of NaOH disaggregates the system, producing monomeric trans-ADA. Each step of the cycle has a signature UVeVis spectrum. (See supporting information for individual spectra, Figs. S10eS14).
molecular and supramolecular states and the interplay of molecular and supramolecular structure offer attractive prospects for the development of smart materials and molecular memory devices. Acknowledgements This work was in part supported by University of Dayton (UD) start-up funds to A.M. and by UD Honors program summer funds to A.M. and J.F. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2019.117576.
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[12] Mammana, A. D’Urso, R. Lauceri, R. Purrello, J. Am. Chem. Soc. 129 (2007) 8062. [13] R. Randazzo, A. Mammana, A. D’Urso, R. Lauceri, R. Purrello, Angew. Chem. Int. Ed. 47 (2008) 9879. [14] G.M. Whitesides, M. Boncheva, Proc. Natl. Acad. Sci. U.S.A. 99 (2002) 4769. [15] G.M. Whitesides, B. Grzybowski, Science 295 (2002) 2418. [16] Alberts, A. Johnson, J. Lewis, M. Raff, K. Roberts, P. Walter, Molecular Biology of the Cell, Garland Science, New York, 2003. [17] G.T. Carroll, M.J.M. Jongejan, D. Pijper, B.L. Feringa, Chem. Sci. 1 (2010) 469. [18] N.J. Turro, Modern Molecular Photochemistry, University Science Books, Sausalito, CA, 1991. [19] H. Durr, H. Bouas-Laurent, Photochromism: Molecules and Systems, Elsevier Science, Amsterdam, 2003. [20] R. Klajn, Chem. Soc. Rev. 43 (2014) 148. [21] G. Kumar, D.C. Neckers, Chem. Rev. 89 (1989) 1915. [22] J. Zhang, J.K. Whitesell, M.A. Foxe, Chem. Mater. 13 (2001) 2323. [23] K.-Y. Chen, S.J. Wezenberg, G.T. Carroll, G. London, J.C.M. Kistemaker, T.C. Pijper, B.L. Feringa, J. Org. Chem. 79 (2014) 7032. [24] A. Natansohn, P. Rochon, Chem. Rev. 102 (2002) 4139. [25] K. Ichimura, Chem. Rev. 100 (2000) 1847. [26] S. Yagai, A. Kitamura, Chem. Soc. Rev. 37 (2008) 1520. [27] W. Szymanski, J.M. Beierle, H.A.V. Kistemaker, W.A. Velema, B.L. Feringa, Chem. Rev. 113 (2013) 6114. [28] A. Mammana, G.T. Carroll, J. Areephong, B.L. Feringa, J. Phys. Chem. B 115 (2011) 11581. [29] E. Bellacchio, R. Lauceri, S. Gurrieri, L. Monsu` Scolaro, A. Romeo, R. Purrello,
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic study of the pH dependence of the optical properties of a water-soluble molecular photo-switch Julie Fitz, Angela Mammana* University of Dayton, Department of Chemistry, 300 College Park, Dayton, OH, 45469, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2019 Received in revised form 25 September 2019 Accepted 28 September 2019 Available online 11 October 2019
In this report we present a UVeVis spectroscopic study of the pH dependent optical properties of 4,40 azobenzene dicarboxylic acid in aqueous solution. A combination of chemical (acid-base) and light stimuli is combined to demonstrate that the system undergoes two types of optical switching cycles. At neutral pH the azobenzene undergoes photo-induced cis-trans isomerisation. Upon pH reduction the UV eVis spectra show changes consistent with aggregation of the azobenzene photo-switch. The photoresponsive behaviour is dependent on the pH and conformation of the azobenzene. The optical properties of the system are dependent on the mode of pH reduction and the isomeric cis/trans composition of the photo-switch, showing hierarchical control of self-assembly. © 2019 Elsevier B.V. All rights reserved.
Keywords: Azobenzene Self-assembly Supramolecular Photochemistry
Exerting dynamic control over chemical systems at the molecular and supramolecular level is a considerable challenge with potential applications in molecular machinery, logic gates, memory storage, liquid crystals, controlling surface interactions, catalysis and optical devices [1e13]. Additionally, studying the effect of environment and molecular structure on self-assembly will shed light on fundamental aspects of weak molecular interactions with the potential to unlock new pathways to the controlled construction and manipulation of increasingly complex chemical architectures [14e17]. Photo-responsive molecules are ideal candidates for manipulating chemical systems because photochemical reactions are specific to a given chromophore within a molecule and do not affect non-absorbing components of the system [18]. A number of photo-active molecules undergo reversible structural changes [19e23]. Azobenzene chromophores have great potential in modulating self-assembly processes because they undergo a reversible photo-isomerisation from a trans isomer to a cis isomer upon irradiation with UV light, and revert back to the trans state upon exposure to visible light or upon heating [24,25]. The bent conformation of the cis isomer is less conducive to dense molecular packing than is the flat trans isomer [26]. In this report, we present a spectroscopic analysis of a simple, water-soluble azobenzene-based supramolecular system that can
* Corresponding author. E-mail address: [email protected] (A. Mammana). https://doi.org/10.1016/j.saa.2019.117576 1386-1425/© 2019 Elsevier B.V. All rights reserved.
be manipulated with acid-base stimuli and photons. The lightabsorbing and pH sensitive system can be cycled between four states. The specific chemical pathway required to access the available states is controlled by the molecular conformation of the photo-switch. 4, 40 -azobenzene dicarboxylic acid (ADA), shown in Fig. 1, was selected as the photo-switch used in this work because of its potential to be water soluble. Water provides an important medium for investigating molecular and supramolecular processes because it is benign and provides an appropriate environment for ultimately exploring biological materials and applications [27,28]. Additionally, an aqueous environment promotes self-assembly in amphiphilic molecules. In particular, aromatic systems containing ionisable functional groups at the periphery have received much attention with heavy emphasis on porphyrin-based structures [29e31]. The paradigms of porphyrin self-assembly and optical responses in aqueous solution provide a blue-print for expanding the scope and variation of self-aggregating systems and exploring non-porphyrin-based structures in water including azobenzene photo-switches as reported herein. ADA is ideal for working in aqueous solution because its carboxylic acids are able to ionise and engender solubility at high pH. At low pH, the hydrogen on the carboxylic acid and the electronegative carbonyl can form hydrogen bonds, providing an impetus for self-assembly. In addition, the two benzene rings in the molecule make it an ideal candidate for an aggregation-oriented system due to the ease with which they can participate in p-p stacking. At low pH some researchers have proposed protonation of the azo bond for specific
2
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Fig. 1. Structure of cis- and trans-ADA and UVeVis spectra in H2O of trans-ADA (blue), cis-ADA (pink) photo-generated after exposure of trans-ADA to UV light, and trans-ADA (dotted light blue) photo-generated after exposure of cis-ADA to visible light.
azobenzene derivatives in specific solvents [32e36]. Azobenzene derivatives for which azo protonation has been proposed in the literature are amino and/or hydroxyl functionalized, but not the dicarboxylic acid functionalized azobenzene in our present work. In order to study the effect of pH on the optical properties of azobenzene, which in turn provides a probe for studying intermolecular interactions, we selected UVeVis absorption spectroscopy since it is a well-established and fundamental spectroscopic technique for characterizing supramolecular interactions. It is wellknown that molecules show shifts in their absorption spectra when they associate in solution or at interfaces [28e30]. A common theme in studies regarding supramolecular chemistry is a hypochromic effect in the absorption spectrum as an indication of aggregation. A shift in absorption bands can also be associated with specific types of aggregates [37]. A blue shift is associated with Htype aggregates in which the chromophores assemble in a parallel plane-to-plane stacking geometry. A red shift is associated with a Jaggregate in which the chromophores assemble in a head-to-tail arrangement. Moreover, a change in the geometry of the azobenzene derivatives can be detected via a change in their absorption spectra as shown in the UVeVis spectra of the two forms (cis and trans) of ADA in H2O (Fig. 1). At pH 7, trans-ADA has two main bands at 228 nm and 330 nm, with the intense peak at 330 nm attributed to the p-p* transition of the trans isomer. Irradiation of the solution with UV light (365 nm) results in a marked decrease in absorbance at both the 228 nm and the 330 nm bands and an increase of two bands at 250 nm and 430 nm corresponding to the cis isomer, with the weak peak at 430 nm attributed to the n-p* transition of the cis isomer [38]. trans-ADA was solubilized in water by increasing the pH to approximately 11 by addition of NaOH. The pH was reduced to 7 prior to experiments performed in this report unless otherwise noted. ADA does not show spectroscopic changes indicative of aggregation at pH 7. An aqueous solution of trans-ADA at pH 7 was prepared and reduced to a pH of 3 by the addition of H2SO4. Comparison of the UVeVis spectra at pH 7 and 3 shows that the spectrum of trans-ADA undergoes a hypochromic effect and blue shift upon reducing the pH to 3, both of which are spectral changes diagnostic of aggregation. The band at 330 nm decreased in intensity as the pH was reduced and continued to decrease with time.
After 60 min, the signal no longer changed (Fig. 2A). The decrease in signal intensity was accompanied by a blue shift from 330 nm to 300 nm, which suggests the formation of an H-aggregate in which the monomers are oriented in a face-to-face or parallel manner [37]. Some preliminary data shown in Figs. S1 and S2 indicate that the structure of the aggregate depends on the modality through which the pH is changed and the isomeric form present in solution when the aggregation takes place. We focused our study on the spectroscopic data obtained upon fast reduction of the pH both for the trans and cis isomers as shown in Fig. 2. A plot of absorbance at 330 nm vs. pH yields a sigmoidal curve with an inflection point around pH 5.3 which corresponds to the pKa of the trans-ADA as shown in Fig. S3. This pKa value corresponds to the carboxyl groups on the ADA and is a higher value compared to benzoic acid (pKa ¼ 4.2) and compared to what is expected for a carboxylic acid attached to monomeric azobenzene [39], suggesting that the protonation process is favoured by the subsequent formation of the supramolecular structure. Upon irradiation, the sample did not exhibit the characteristic spectroscopic changes associated with trans to cis photoisomerisation. There are several possibilities for explaining this loss of photodynamic control, but the most likely reason is that the tight packing of trans-ADA in an H-aggregate geometry provides a hindrance to motion that the energy of excitation cannot overcome. The non-covalent interactions might provide too high of an energy barrier for the photomechanical effect to surpass. Isomerisation also might occur, but could potentially result in a highly strained cis structure that rapidly reverts back to the trans structure too quickly for spectroscopic analysis to detect. Confinement effects on isomerisation and molecular motion are not without precedent. For example, intermolecular crowding in surface-attached monolayers of molecular motors based on overcrowded alkenes has been attributed to intermolecular interactions hindering molecular rotary motion in some tightly packed altitudinal systems [40,41]. Further, isomerisation of azobenzene chromophores covalently bound to polymer side-chains in thin, macromolecular films show isomerisation behaviour that is influenced by a neighbouring effect [42]. Although trans-ADA loses its photo-switching capabilities at low pH, an increase in pH through addition of NaOH disrupts the
J. Fitz, A. Mammana / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117576
3
Fig. 2. Effect of decreasing pH on UVeVis spectra of trans-ADA and cis-ADA. (A) trans-ADA at pH ¼ 7 (blue); trans-ADA immediately after lowering the pH to 3 (black); trans-ADA at pH ¼ 3 after 1 h (green). The hypochromic effect and blue-shift is an indication of H-aggregate formation. (B) cis-ADA at pH ¼ 7 (pink); cis-ADA immediately after lowering the pH to 3 (black); cis-ADA at pH ¼ 3 after 1 h (green).)
intermolecular interactions, as shown by the increase of the absorption band at 330 nm (Fig. 3). Upon disassembly of the supramolecular structure, the azobenzene regains its photochromic behaviour in response to UV and visible light. Self-assembly is accordingly a reversible process dependent on pH, and the ability to isomerise from the trans to the cis form of ADA is determined by the degree of aggregation. The cyclical nature of this process is shown in Fig. 3. After exploring the behaviour of the trans form of ADA, the cis form was investigated to see if structural changes in the molecule can modulate its supramolecular behaviour. A solution of transADA was irradiated with UV light to obtain the cis form of the molecule. The pH was reduced with continued UV irradiation to maintain the highest cis-trans ratio possible. After approximately 15 min of irradiation, the ADA reaches a photo-stationary state in which the molecule attains equilibrium between the cis and trans isomers with the ratio of cis to trans depending on the ratio of their respective extinction coefficients at the wavelengths of irradiation. Due to overlap of the absorption bands, we can never assume that our solution is entirely in the cis form. The supramolecular structures obtained could, therefore, contain both the cis and trans isomers in the overall geometry. The pH of the aqueous cis-ADA solution was reduced to 3 by the addition of H2SO4. Immediately following the addition of H2SO4, the 230 nm band in the UVeVis spectrum exhibited a slight red shift and hypochromic effect while the 330 nm and 430 nm bands lost their maxima and were replaced by a broad signal as a result of increased scattering (Fig. 2B). After 60 min, the band at 230 nm disappears and a new weak broad band appears at approximately 360 nm. The absence of a blue shift indicates that the geometry of the cis aggregate is not parallel (H-aggregate), unlike the trans aggregate, showing that there is a photo-controlled interplay between the structure of the self-assembling molecule and the structure of the resulting supramolecular species. The cis-azobenzene assembly at pH 3 was irradiated with visible light in order to isomerise the cis-ADA complex to the trans form. The UVeVis spectrum shows a slight increase in intensity and a blue shift of the peak at 340 nme300 nm. As shown in Fig. 4 the
shape, position and intensity of the absorption spectrum is analogous to the spectrum of the trans-ADA assembly obtained upon reducing the pH of an aqueous solution of trans-ADA from 7 to 3, indicating that irradiation triggered the transformation of the structure of the cis aggregate to the trans aggregated form. When cis-ADA in the aggregated form photo-isomerises to trans-ADA it might either break the existing aggregated species proceeding through a transitional monomeric phase before forming the H-aggregate of trans-ADA, or it might isomerise without disassembling. In the latter case the cis aggregate is expected to have a pre-oriented geometry that favours the formation of the trans H-aggregate. The ability of the cis form of ADA to undergo photo-isomerisation at low pH might be a result of a less tight packing of the chromophores compared to the trans form of ADA at low pH, or possibly strain induced by intermolecular interactions making cis to trans isomerisation energetically favourable relative to the non-covalent intermolecular interactions, in contrast to the trans H-aggregate. Additionally, both cis and trans azobenzene absorb in the 365 nm region with different extinction coefficients [43,44]. After irradiation with UV light, the amount of cis and trans ADA present will depend on the ratio of the extinction coefficients at the irradiation wavelengths employed. The presence of trans isomer in the cis assembly (due to the photo-equilibrium) might disrupt interactions, leading to a less compact structure in which the cis isomer is less hindered, facilitating isomerisation in the pHinduced supramolecular structure. In order to confirm that the cisADA had indeed isomerised to the trans isomer, the pH of the solution was increased to 7 with NaOH. The UVeVis spectrum obtained was superimposable with the initial spectrum of the transADA sample before irradiation with UV light, demonstrating the high efficiency of the photo-isomerisation process for this supramolecular system. The cyclical nature of this process is shown in Fig. 4. We have demonstrated pH and light-induced changes in the UVeVis spectra of trans- and cis-ADA. The observed pH-induced changes in the UVeVis spectra indicate that ADA aggregates at low pH. This pH and photo-responsive supramolecular system is based only on azobenzene monomers functionalized with
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Fig. 3. Cycle showing the pH-induced conformational locking of trans-ADA upon reduction of the pH to 3 and de-locking of the photo-switchability upon disassembly via the addition of base to the system. At pH 7, ADA undergoes reversible cis-trans isomerisation regardless of whether or not it was previously aggregated via pH reduction. (See Supporting Information for individual spectra, Figs. S4eS9).
carboxylic acids. The carboxylic acids allow azobenzene to dissolve in water under basic conditions. The photo-responsive properties at low pH are dictated by the structure of the monomeric azobenzene, which can be selected with light. At low pH the cis isomer is able to undergo photo-induced transformation to the trans isomer, however at low pH the molecular structure trans-ADA is locked and unable to undergo photo-isomerisation to the cis form. The photo-
responsive behaviour of trans-ADA can be turned back “on” upon disassembly by adding base to the system. The optical phenomena described herein are reminiscent of spectroscopic changes observed during porphyrin self-assembly and demonstrates that ADA has the potential to be an interesting photo-responsive tool that can be applied to supramolecular systems and interfaced with biological materials. Optical and pH-induced switching between
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Fig. 4. trans-ADA is photo-isomerised to cis-ADA at pH 7. Upon reducing the pH to 3 by the addition of H2SO4, the cis-ADA forms a supramolecular structure. Irradiation with visible light produces the trans form of the aggregate. Increasing the pH to 7 by the addition of NaOH disaggregates the system, producing monomeric trans-ADA. Each step of the cycle has a signature UVeVis spectrum. (See supporting information for individual spectra, Figs. S10eS14).
molecular and supramolecular states and the interplay of molecular and supramolecular structure offer attractive prospects for the development of smart materials and molecular memory devices. Acknowledgements This work was in part supported by University of Dayton (UD) start-up funds to A.M. and by UD Honors program summer funds to A.M. and J.F. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2019.117576.
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