Tuning Solvatochromism of Azo Dyes with Intramolecular Hydrogen

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Tuning Solvatochromism of Azo Dyes with Intramolecular Hydrogen Bonding in Solution and on Titanium Dioxide Nanoparticles Lei Zhang,† Jacqueline M. Cole,*,†,‡ and Xiaogang Liu† †

Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, U.K. Argonne National Laboratory, 9700 S Cass Avenue, Argonne, IL 60439, U.S.A.

‡

S Supporting Information *

ABSTRACT: “Smart tuning” of optical properties in three azo dyes containing intramolecular hydrogen bonding is realized by the judicious control of solvents, when the dyes are in solution or adsorbed onto titanium dioxide nanoparticles. In solution, certain solvents destabilizing intramolecular hydrogen bonding induce a distinctive ≈70 nm “blue-shifted” absorption peak, compared with other solvents. In parallel, the optical properties of azo dye/TiO2 nanocomposites can be tuned using solvents with different hydrogen-bond accepting/donating abilities, giving insights into smart materials and dyesensitized solar cell device design. It is proposed that intramolecular hydrogen bonding alone plays the leading role in such phenomena, which is fundamentally different to other mechanisms, such as tautomerism and cis−trans isomerization, that explain the optical control of azo dyes. Hybrid density functional theory (DFT) is employed in order to trace the origin of this optical control, and these calculations support the mechanism involving intramolecular hydrogen bonding. Two complementary studies are also reported: 1H NMR spectroscopy is conducted in order to further understand the solvent effects on intramolecular hydrogen bonding; crystal structure analysis from associated research indicates the importance of intramolecular hydrogen bonding on intramolecular charge transfer.

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INTRODUCTION Azo dyes, with their −NN− group connected to arene rings, form the largest category of industrial dyes.1 This family of dyes enjoys vivid color, high molar extinction coefficients, and light fastness.2,3 In recent years, azo dyes have shown potential in dye-sensitized solar cells (DSSCs),4 devices that mimic natural photosynthesis by utilizing a monolayer of dyes on a semiconductor oxide (usually TiO2) to stimulate an electrical current.5,6 It has been demonstrated that intramolecular hydrogen bonding is crucial to the chemical and optical properties of azo dyes.2,7−9 In this study, the optical properties of three azo dyes are tuned by exploiting the opening or forming of intramolecular hydrogen bonds. The subject dyes are: (1) 2-[2-[4-(dimethylamino)phenyl]diazenyl]-benzoic acid; (2) 2-[2-[4-(diethylamino)phenyl]diazenyl]-benzoic acid; and (3) 2-[2-[4-(dipropylamino)phenyl]diazenyl]-benzoic acid (see Scheme 1). First, UV−vis absorption spectra of dyes, both in solution and on TiO2 nanoparticles, are examined. A mechanism involving intramolecular hydrogen bonding is proposed to explain the observed solvatochromism. Second, 1H NMR spectroscopic analysis and density functional theory (DFT) are performed in order to further study the effect of intramolecular hydrogen bonding. Finally, the resonance forms of related azo dyes are studied using harmonic−oscillation stabilization energy calculations in order to understand the associated intramolecular charge transfer processes in this series of compounds; in particular, the influence of intramolecular hydrogen bonding on such processes is considered. © XXXX American Chemical Society

Scheme 1. Molecular Structures of 1 (R = Methyl), 2 (R = Ethyl), and 3 (R = Propyl). The Hydrogen Atom in the −COOH Group Is Either Interacting with a Nearby Nitrogen Atom in the Azo Group To Form an Intramolecular Hydrogen Bond or Interacting with Solvent Molecules by Forming an Intermolecular Hydrogen Bond

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EXPERIMENTAL AND COMPUTATIONAL METHODS Materials, 1H NMR, and UV−vis Absorption Spectroscopy. Compounds 1, 2, and 3 were supplied by Sigma-Aldrich and used without further purification. An Agilent8453 Diode Array spectrophotometer was used to obtain UV−vis Received: September 4, 2013 Revised: November 14, 2013

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dx.doi.org/10.1021/jp4088783 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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absorption spectra of these dyes, both in solution and on TiO2 nanoparticles. Dilute solution was used, and the concentrations were estimated to be ≈20 μM. The associated absorption was normalized to 1. The uncertainty in the UV−vis measurement is 2 nm. A Bruker Avance 500 CryoUltrashield NMR spectrometer was employed in order to acquire 1H NMR spectra on compounds dissolved in deuterated acetonitrile and dimethylsulfoxide (DMSO). Fabrication of Dye/TiO2 Nanocomposites. The dye/ TiO2 nanocomposites were fabricated according to a similar route to make DSSCs.10 TiO2 paste (DSL 18NR-T) and FTOcoated glass (TEC15) were purchased from Dyesol. A single TiO2 layer was deposited onto the FTO glass via the doctorblade method. These electrodes supporting the TiO2 layer were heated at 500 °C for 30 min for sintering purposes and sensitized in a 0.5 mM solution of the subject azo dye in acetonitrile overnight. The resulting electrodes were characterized via UV−vis absorption spectroscopy after alternately rinsing in ethanol, and then DMSO, in order to demonstrate the manipulation of optical properties of dyes via solvent control, both in solution and on nanoparticles. The rinsing process was very gentle to avoid over-removal of dyes adsorbed on nanoparticles. All of the samples were carefully dried in nitrogen in order to make sure they were solvent free, prior to UV−vis absorption spectroscopy. Quantum Chemical Calculations. Gaussian 0911 was carried out for all calculations. Structural optimizations of the ground state were performed by a hybrid DFT method using a PBE1PBE functional12 and 6-31++g(d,p) basis set.13 The frequencies were checked, ensuring that they were positive. Solvent effects were included using the popular integral equation formalism polarizable continuum model (IEFPCM)14 in both acetonitrile and DMSO.

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RESULTS AND DISCUSSION Solvatochromism in Solution. The solvent interaction with solute, in terms of hydrogen bonding, can be described by the hydrogen-bond accepting parameter, β, for which reference values are available in the literature for a given solvent.15,16 In order to favor the formation of an intermolecular hydrogen bond between solvent and solute, β should be large to compete with intramolecular hydrogen bonding. In addition, the complementary hydrogen-bond donating parameter, α, should be kept small to retain favor in this competition. Figure 1 depicts normalized UV−vis absorption spectra of 1, 2, and 3 in different solvents. There are two bands in the visible regime, centered at ∼425 (band 1) and ∼500 nm (band 2). Band 2 is found to dominate where 1, 2, and 3 are solvated in acetonitrile, acetone, chloroform, cyclohexane, DCM, or ethylacetate; all of these solvents have a small β value (0.00− 0.45), which is therefore indicative of UV−vis absorption induced by intramolecular hydrogen bonding (vide infra). There is just a small hint of band 1 present in these cases, manifest by a light shoulder on the low wavelength side of the primary absorption profile. When alcohol solvents, such as methanol, ethanol, and hexanol, which have larger β (0.66− 0.84) but also large α (0.80−0.98), are used instead, the band 1 shoulder becomes more obvious. Dimethyl formamide (DMF), which has a comparable hydrogen-bond accepting parameter, β, (0.69) to the alcohols but a much lower hydrogen-bond donating parameter, α (0.00), presents comparably intense peaks for band 1 and 2 when employed as a solvent; due to the superposition of these two similarly intense peaks, a plateau

Figure 1. Normalized UV−vis absorption spectra of 1 (a), 2 (b), and 3 (c) in acetonitrile, acetone, chloroform, cyclohexane, DCM, ethylacetate, ethanol, methanol, hexanol, DMF, and DMSO.

region exists. Finally, dimethylsulfoxide (DMSO), which has the smallest α (0.00) and largest β (0.76) among all the solvents under investigation, was selected as the solvent; in this extreme limit of α and β, band 1 is now the dominant absorption peak, with band 2 appearing as a small shoulder. In summary, the solvation of 1, 2, and 3 produces two UV−vis B



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vestigated (see Figure S1); no similar changes in absorption peaks are observed. This leaves the third possibility, that a model involving intra/ intermolecular hydrogen bonding is responsible for the herein observed solvatochromism. In order to assess this option, DFT was employed in order to calculate the electronic-charge distribution of frontier molecular orbitals, which can identify the key molecular constituents that are responsible for the optical absorption. Previous studies have ascribed the two absorption bands located in the visible region of azo dyes to mainly π → π* (HOMO → LUMO) and n → π* (HOMO-1 → LUMO) transitions.17,25,26 Figure 3 shows both HOMO-1 orbitals for ‘intra’ and ‘inter1’, as defined in Figure S2, where intra represents the intramolecular hydrogen-bond configuration, while inter1 is the configuration without an intramolecular hydrogen bond. The HOMO-1 orbitals are more delocalized in intra compared with inter1 (see the extra charge highlighted in the red circle of Figure 2), thanks to the

absorption bands, where band 1 is stabilized by using solvents with larger β and smaller α values, and band 2 by those with smaller β values. The band 1/band 2 ratios follow the order of DMSO > DMF > alcohol > the others. These three sets of spectra also show differences between the three azo dyes, which chemically differ by only the number of alkyl −[CH2]− carbon units in the donor group. For example, 1 (containing the dimethyl group) exhibits a greater extent of band 1 relative to band 2 than in 2 and 3 (containing diethyl and dipropyl groups, respectively), presenting with an order of band 1/band 2 ratios, 1 > 3 > 2. Specifically, the absorption peaks of band 1 for 1 are more dominant than those for 2 and 3, where the solvent is methanol, ethanol, or hexanol. When DMF is employed, band 1 for 1 is more distinct and red-shifted relative to the analogous peaks for 2 and 3, where band 1 forms part of a “flat” plateau from a comparable band 1/band 2 ratio. This trend appears to be associated with the relative stability of the intramolecular hydrogen bond present in 1, 2, and 3, as observed in their corresponding crystal structure, cf. sequentially increasing (decreasing) hydrogen-bond X···Y strengths (lengths) for 1 (2.588 Å), 3 (2.581 Å), and 2 (2.575 Å); note that this is a fair comparison given that these hydrogen bonds exist both in the solution and the solid state, as shown previously.17 In other words, the stronger the intramolecular hydrogen bonding, the weaker band 1 compared with band 2, which is consistent with the differences in α and β in the UV− vis absorption band 1 solvent trends mentioned above. There is controversy over the mechanistic origin of solvatochromism in these azo dyes. Three causal hypotheses exist that are centered around tautomerism, cis−trans photoisomerization, and intramolecular hydrogen-bonding. Considering these options in turn: Tautomerism is a phenomenon frequently observed in azo compounds.9,18−20 The associated hypothesis specifies that azo compounds could have two tautomers: an azo form with motif “−N=N−” that is stabilized in certain solvents, and a hydrazone form with motif “−N−NH−”, that exists when joined to a quinoidal ring structure, that is stabilized in other solvents. A large amount of previous research has employed tautomerism to explain large UV−vis peak shifts in different solvents.20−22 It is argued that the azo form manifests a blue-shifted absorption peak, while the hydrazone form, associated with a quinoidal structure, has a red-shifted absorption peak under UV−vis irradiation. However, tautomerism does not appear to be the origin of solvatochromism observed in this study: for if tautomerism were to be its mechanistic origin, the more polar solvent, such as DMSO, should stabilize the more polar hydrazone form, and lead to a “red shift”. However, the opposite is observed in this study: the solution in DMSO exhibits a “blue-shifted” peak. Cis−trans (trans−cis) isomerization is the second hypothesis. It specifies that azobenzene derivatives can exist in either the cis or trans conformation under light irradiation, and the quantum yields are affected by solvent polarity and viscosity.23 Such a mechanism would manifest similar large changes in the UV−vis absorption spectra of the dyes; however, in-house photoisomerization experiments have been attempted, but the three dyes exhibited no photochromism after 420 nm light irradiation. In addition, thermal isomerization (cis → trans) occurs in the dark after some time (seconds to days),23,24 which is also not observed in this study. In order to ensure a thorough assessment, solvatochromism of a similar azo dye that lacks intramolecular hydrogen bonding (i.e., (4-[2-[4(dimethylamino)phenyl]diazenyl]-benzoic acid) was also in-

Figure 2. HOMO-1 molecular orbital for inter1 and intra.

“bridging effect” of intramolecular hydrogen bonding that “diffuses” the orbital to the carboxylic acid moiety. Previous studies17,26 have confirmed that the π*-orbital is localized in the area around the acceptor (the carboxylic group) for these aminoazobenzoic compounds due to “push−pull” effects. This means that there is a better overlap between HOMO-1 and LUMO orbitals for intra relative to inter1, leading to a larger oscillation strength for the n-to-π* transition; in other words, intramolecular hydrogen bonding promotes the n-to-π* transition contribution. In consideration of the fact that the n-to-π* optical band is known to be generally red-shifted, or retain the same wavelength, relative to the π-to-π* band in similar azo dyes,23 the peak at ∼500 nm (band 2) is unambiguously assigned to the n-to-π* transition and that at ∼425 nm (band 1) to the π-to-π* transition. Tuning Optical Properties of Azo Dyes Adsorbed onto TiO2 Nanoparticles. In recent years, semiconductor− organic nanocomposites have received great interest due to their novel properties and potential applications in many molecular devices.27−31 It was therefore deemed pertinent to compare the solvent-based optical absorption behavior of the azo dyes with the same adsorbed onto TiO2 nanoparticles. Figure 3 shows the optical absorption spectra of nanocomposites of 1, 2, and 3 on TiO2 nanoparticles, dye-sensitized in acetonitrile, followed by an alternate rinsing in ethanol (red curve) and DMSO (blue curve). The first cycle of rinsing is shown in the figure, and the following cycles are almost the same as this cycle (see Figure S2 for the cyclical pattern), except that the absorption intensity decreases gradually across all wavelengths due to dye−aggregate removal during successive rinsing processes. Ethanol is used to represent small β solvents for cyclical rinsing for cleaning purposes. In common with solution-based absorption spectra, there are two bands in the visible region for these nanocomposites, one C



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at ∼550 nm (band 2) and the other located at 400−424 nm (band 1). It is observed that band 2 emerges for ethanol and disappears for DMSO, which agrees with the solution-based absorption spectra. This indicates that on TiO2 nanoparticles, the intramolecular hydrogen bonds open up in DMSO but form in ethanol/acetonitrile. In addition to the changes in band 2 triggered by the solvents, band 1 is also modified accordingly: band 1 is blueshifted to ∼400 nm after DMSO rinsing and red-shifted to ∼425 nm after ethanol rinsing. Band 1 corresponds to the π-toπ* transition and is closely related to π...π staggering.32 When the organics are self-assembled onto a substrate, they aggregate in such a way that either leads to a red- or blue-shifted UV−vis absorption spectrum compared with the original solution-based one. The cause of the blue-shifted absorption spectra is termed as “H-aggregation”, while the one for red-shifted absorption spectra is termed as “J-aggregation”. Figure 3 reveals that DMSO induces H-aggregation33−35 of the azo dyes in the film compared with solution. There are changes of intensity of the absorption spectra between the dyes in solution and on TiO2 nanoparticles, probably due to dye aggregation (π...π staggering, dimer, and trimer, etc.), which impacts on the absorption spectra. It is evident that, on the TiO2 surface, the dyes are closely packed in a monolayer/multilayer without solvation and have a tendency to form aggregates, which have the absorption spectra coinciding with band 1. As mentioned above, a slight intensity decrease is observed after each rinsing cycle across all wavelengths (see Figure S2), as some dye aggregates are washed away in the rinsing process. This indicates that the dye aggregates, in addition to the monolayer adsorbed on nanoparticles, play a significant role in the solvatochromism. Figure 4 depicts a visual inspection of the acetonitrile-based dye-sensitized azo−TiO2 nanocomposites on glass electrode substrates, after rinsing first in ethanol and then in DMSO. There are corresponding color changes when the film is rinsed in DMSO, implying that the solvents affect the n-to-π* or π-toπ* transitions. The color changes can be induced and suppressed cyclically by repeatedly rinsing in DMSO and ethanol, if the rinsing process is gentle enough. Given that these dye-sensitized TiO2 nanocomposites represent one of the electrodes in a DSSC, the demonstration of its solvatochromism-based tunability could have significant implications for DSSC device design. Evidently, the limited cycling ability of the optoelectronic tuning demonstrated here, owing to the sensitivity of dye-adsorbates to the rinsing process, would need attention. However, various practical means could circumvent these issues, such as (i) partial dye dissolution in the rinsing solvents so that an equilibrium level of TiO2...dye sensitization is established and maintained; or (ii) physical nanoporous meshes could be judiciously placed onto the electrode to provide a protective cover to the rinsing process. To summarize, the solvent rinsing selectively opens or reforms intramolecular hydrogen bonding, leading to the respective disappearance or appearance of band 2 associated with the n-to-π* transition. In the meantime, the solvents also affect band 1, possibly by influencing the π...π staggering. Such a solid-state nanocomposite system offers more freedom to repetitively tune the optical properties through solvent interactions. Trends in 1H NMR Chemical Shifts Due to Intramolecular Hydrogen Bonding. 1H NMR spectroscopy is a

Figure 3. UV−vis absorption spectra of 1 (a), 2 (b), and 3 (c) adsorbed onto TiO2 nanoparticles. Ethanol induces an appearance of band 2 and a red-shifted peak in band 1, while DMSO rinsing leads to the disappearance of band 2 and a blue-shift of band 1. Only the first cycle of rinsing is shown; this pattern of disappearing/emerging band 2 and red-/blue-shifting of band 1 is preserved in subsequent cycles. D



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possess intermolecular but no intramolecular hydrogen bonding. The relative energy of 1 against different solvents was initially modeled within an implicit solvent model (i.e., ignoring intermolecular interactions between azo dye and solvent molecule by simply emulating the solvent via a dielectric medium according to the associated constants, ε: vacuum 1, chloroform 4.71, ethanol 24.85, acetonitrile 35.69, and water 78.36) (Figure S4). It is observed that as the dielectric constant of the solvents increases, the compound becomes more stable. This implicit model seems to indicate that the configuration with intramolecular hydrogen bonding is always the most stable one across all solvents. However, it is not correct, as the implicit model neglects the solute−solvent local intermolecular interactions. In order to create a more sophisticated model that incorporates solute−solvent local intermolecular interactions, DFT calculations were performed on 1, using an explicit solvent molecule model. Table 1 lists the resultant energies Table 1. Relative Energy (in Terms of Both HF and eV Units) of Intra, Inter1, and Inter2 Configurations of 1, in Acetonitrile and DMSO. An Explicit Solvent Molecule Model Is Used To Embrace Local Solvent−Solute Interactions for All the Configurations

Figure 4. Photos of three azo dyes adsorbed on TiO2 nanoparticles sensitized in acetonitrile. They are rinsed in ethanol (left column), followed by rinsing in DMSO (right column). The color changes are repeatable. The color changes from orange (yellow) to yellow (orange) after rinsing in DMSO (ethanol). These nanocomposites are solvent free. After several cycles of rinsing, the color gradually becomes lighter due to the successive removal of dye aggregates; nevertheless, the visual difference between those rinsed by ethanol and DMSO is maintained.

acetonitrile

DMSO

useful technique to probe changes in intramolecular hydrogen bonding in compounds due to solvents.36−39 The hydrogen involved in intramolecular hydrogen bonding usually exhibits a larger chemical shift in 1H NMR, relative to intermolecular hydrogen bonding40,41 (i.e., azo dyes should display larger chemical shifts when they are dissolved in acetonitrile than in DMSO). Indeed, this is observed: chemicals shifts in DMSO and acetonitrile are, respectively, 13.183, 14.112 ppm (1); 13.019, − ppm (2); 12.206, 14.253 ppm (3). Interestingly, the peak corresponding to the carboxylic-acid hydrogen is missing where 2 is dissolved in acetonitrile; although the origin of this anomaly is inconclusive, it is possibly due to gelation42 or solidification of DMSO43 as the solution in the NMR test tube becomes abnormally viscous. Apart from that, the change in alkyl group (methyl → ethyl → propyl) does not have a large impact on the chemical shift of the −COOH hydrogen. It should also be noted that a change of chemical shift could also be due to solute−solvent dipole interactions, rather than purely the effects of intramolecular hydrogen bonding. However, the good consistency between 1H NMR data and UV−vis absorption spectra results provides good assurance that solvatochromism of these azo dyes does in fact owe its origins to intramolecular hydrogen bonding. Determining the Stability of Intramolecular Hydrogen Bonding via Density Functional Theory. One of the subject azo−dye compounds (1) was used as an exemplar for quantum chemical calculations, affording its total energy (sum of electronic and zero point energy) with different configurations due to the presence of intramolecular hydrogen bonding: Intra, Inter1, and Inter2 (Scheme S2). As described earlier, Intra corresponds to the configuration with intramolecular hydrogen bonding, while both Inter1 and Inter2

intra inter1 inter2 intra inter1 inter2

E/HF

ΔE/eV

−1026.640707 −1026.634852 −1026.635077 −1446.882376 −1446.883935 −1446.871250

0 0.16 0.15 0.04 0 0.35

more stable Intra

Inter1

(sum of electronic and zero point energy, calculated energy, and relative energy) which show that Intra is no longer always the most stable configuration, when considering local solvent− solute interactions. In acetonitrile, the Intra configuration has an energy 0.15−0.16 eV lower than that of Inter1 and Inter2; however, in DMSO, Inter1 exhibits the lowest energy. This indicates that DMSO damages the intramolecular hydrogen bonding, while acetonitrile stabilizes the intramolecular hydrogen-bonding configuration (i.e., this corroborates the experimental UV−vis absorption spectra findings). Impact of Intramolecular Hydrogen Bonding on Intramolecular Charge Transfer. In order to further understand the effects of intramolecular hydrogen bonding on the optical properties of these azo dyes, the intramolecular charge transfer (ICT) in such compounds, with and without intramolecular hydrogen bonding, was evaluated using crystal structure data. While the optical properties in question are solvent-based, the use of solid-state structural information was deemed suitable because recent work has demonstrated the retention of intramolecular hydrogen bonding between solvent and solid-state media17 in these types of azo dyes. To this end, a statistical survey of hydrogen bonding was performed on all known crystal structures that contain the aminoazobenzene backbone, and a −COOH side group, which is common to the subject dyes; data were sourced from a combination of the Cambridge Structural Database44 and our internal structural database.17 Seventeen crystal structures matched the structural search criteria, with nine of them displaying intramolecular hydrogen bonding. E



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The relative extents of ICT in these crystal structures was quantified using the Harmonic Oscillation Stabilization Energy (HOSE) model, which calculates the relative contributions of the resonance structures in the benzenoid part of an organic molecule, namely quinoidal (Q), and the two Kekulé forms (K1 and K2), according to:45 n1

HOSE = 301.15[∑ (R r′ − 1.467)2 (44.39 − 26.02R r′) r=1 n2

+

∑ (R r″ − 1.349)2 (44.39 − 26.02R r″)] r=1

(1)

where the relative contributions of different resonance states can be extracted from: Ci =

(HOSEi)−1 N

∑ j = 1 (HOSEj)−1

(2)

It should be pointed out that these resonance forms do not actually exist in nature, but it has been acknowledged that the understanding of structure−property relationships can be greatly facilitated by calculating the relative contributions of the resonance structure.46−50 For each of the 17 crystal structures in the survey, a HOSE calculation was performed on the benzenoid ring connecting the electron rich nitrogen atom and azo group; the relative contributions of Q, K1, and K2 manifest different abilities to facilitate the transfer of electrons from the nitrogen atom with an electron lone pair (Figure S1).51 In the context of this study, the HOSE calculations of the quinoidal contribution were most useful since this indicates the extent of ICT ensuing in a push− pull system.52 Figure 5a clearly shows that the extent of Q contribution is larger for compounds containing intramolecular hydrogen bonds (IntraHB) than without (No IntraHB). In other words, intramolecular hydrogen bonds stabilize the quinoidal structure, thus facilitating ICT. This can be rationalized intuitively: the hydrogen atom involved in intramolecular hydrogen bonding is electrophilic due to its neighboring electronegative oxygen atom. By forming intramolecular hydrogen bonds, the carboxylic acid group attracts electrons from the azo group, and the azo group further attracts electrons from the electron-rich nitrogen atom through the conjugated benzenoid ring; in this way, ICT is more efficient and the quinoidal structure is manifested. The azo (−NN−) bond length is another important indicator of ICT for azo compounds; a larger azo group bond length is often related to more efficient charge transfer.53 Figure 5b compares azo bond lengths from the 17 surveyed crystal structures; compounds without intramolecular hydrogen bonds have bond lengths from 1.23 to 1.28 Å, while compounds with intramolecular hydrogen bonds have longer bond lengths from 1.275 to 1.285 Å, indicating better ICT caused by the formation of intramolecular hydrogen bonds. The data collection temperature range of the 17 crystal structures is from 120 to 298 K. Comparing crystal structures at this range of temperatures might lead to some extent of bias; however, any such bias is minimized by using a similar data range of IntraHB and No IntraHB, as each IntraHB and No IntraHB has three crystal structures determined at 120 K, while most others are collected at room temperature or near room temperature. Furthermore, the focus of this study involves

Figure 5. Q contribution and −NN− bond length for both compounds with intramolecular (IntraHB) and without intramolecular hydrogen-bonds (No IntraHB).

comparing Ci values determined from the crystal structures rather than determining absolute values, so any bias is mitigated via its dilution.

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CONCLUSIONS The tuning of optical properties of azo dyes, both in solution and as adsorbed onto TiO2 nanoparticles, is realized by exploiting the link between their intramolecular hydrogen bonding and solvatochromism. In solution, intramolecular hydrogen bonds in these dyes are stabilized in solvents with smaller hydrogen-bond accepting abilities, affording a dominant peak at ∼500 nm; these hydrogen bonds partially open in solvents with larger hydrogen-bond accepting abilities, leading to the emergence of an absorption peak at ∼450 nm. For azo dyes adsorbed onto TiO2 nanoparticles, the cyclical rinsing of selective solvents is found to trigger the repeated appearance and disappearance of the band at ∼500 nm and a corresponding red- and blue-shift of the band at ∼400 nm; this provides the basis for smart optical control of these nanocomposites. As such, the results could have implications for DSSC device design. DFT calculations confirm the effect of intramolecular hydrogen bonding on frontier molecular orbitals, with part of the n-orbital diffusing to the carboxylic acid group. DFT calculations also predict the stability of intramolecular hydrogen bonding in different solvents, considering local solvent−solute interactions explicitly. 1H NMR spectroscopy corroborates the intramolecular hydrogenbonding influences, while crystal structure analyses yield associated information on the impact of intramolecular F



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hydrogen bonding on ICT in these dyes and, thence, the effect on their optical properties.

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ASSOCIATED CONTENT

* Supporting Information S

Solvatochromism of (4-[2-[4- (dimethylamino)phenyl]diazenyl]-benzoic acid in different solvents (tolune, acetone, chloroform, DMF, ethanol, IPA, methanol, acetonitrile, and DMSO); cyclic patterns of absorbance at 550 nm and peak wavelength of band 1 versus rinsing number for 1 on nanoparticles; HOMO-1, HOMO, LUMO, and LUMO+1 molecular orbital distributions for Inter1 and Intra of 1; three configurations of 1; and relative energy of Intra, Inter1, and Inter2 configurations of 1 in different solvents without considering local solute−solvent interactions are deposited in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Fax: +44 (0)1223 373536. Tel.: +44 (0)1223 337470. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.M.C. thanks the Fulbright Commission for a UK−US Fulbright Scholar Award and Argonne National Laboratory where work done was supported by DOE Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC0206CH11357. X.L. is indebted to the Singapore Economic Development Board for a Clean Energy Scholarship. The authors acknowledge support from the EPSRC U.K. National Service for Computational Chemistry Software (NSCCS), based at Imperial College London, and contributions from its staff in assisting with this work. The authors also thank Chi Hu for her technical assistance in NMR experiments.

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REFERENCES

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Tuning Solvatochromism of Azo Dyes with Intramolecular Hydrogen

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