Spectroscopic Properties of Anisole at the Air–Ice Interface: A

Article pubs.acs.org/Langmuir

Spectroscopic Properties of Anisole at the Air−Ice Interface: A Combined Experimental−Computational Approach Joseph K’Ekuboni Malongwe,† Dana Nachtigallová,*,§ Pablo Corrochano,† and Petr Klán*,†,‡ †

RECETOX, Faculty of Science, and ‡Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic § Institute of Organic Chemistry and Biochemistry, Flemingovo nam. 2, 166 10 Prague, Czech Republic S Supporting Information *

ABSTRACT: A combined experimental and computational approach was used to investigate the spectroscopic properties of anisole in aqueous solutions and at the ice−air interface in the temperature range of 77−298 K. The absorption, diffuse reflectance, and emission spectra of ice samples containing anisole prepared by different techniques, such as slow freezing (frozen aqueous solutions), shock freezing (ice grains), or anisole vapor deposition on ice grains, were measured to evaluate changes in the contaminated ice matrix that occur at different temperatures. It was found that the position of the lowest absorption band of anisole and its tail shift bathochromically by ∼4 nm in frozen samples compared to liquid aqueous solutions. On the other hand, the emission spectra of aqueous anisole solutions were found to fundamentally change upon freezing. While one emission band (∼290 nm) was observed under all circumstances, the second band at ∼350 nm, assigned to an anisole excimer, appeared only at certain temperatures (150−250 K). Its disappearance at lower temperatures is attributed to the formation of crystalline anisole on the ice surface. DFT and ADC(2) calculations were used to interpret the absorption and emission spectra of anisole monomer and dimer associates. Various stable arrangements of the anisole associates were found at the disordered water−air interface in the ground and excited states, but only those with a substantial overlap of the aromatic rings are manifested by the emission band at ∼350 nm.

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INTRODUCTION The nature of ice and snow at environmentally relevant conditions plays an important role in the air−snow/ice exchange processes of trace organic pollutants.1−5 At relatively high subzero temperatures, the ice crystals are covered by a disordered interface (also called quasi-liquid layer)4 of variable thickness, the magnitude of which is dependent on temperature and ice contamination.4,6−8 Determination of the physicochemical properties, such as spectroscopic characteristics, location, and speciation, of these contaminants at ice−grain boundaries is of crucial importance in the evaluation of their interactions with the ice surface and (photo)chemical reactivities. Many laboratory studies have demonstrated that the degradation products formed at the air−snow/ice interface are different from the corresponding processes occurring in the liquid phase, or they are produced with different efficiencies and reaction rates.2,3,9−16 The spectroscopic characteristics of some ice contaminants have already been determined, despite still vaguely defined ice physical properties and the distribution and local concentrations of heterogeneous multiphase contaminants.17 For example, the absorption and, in some cases, emission spectra of some small aromatic molecules at the ice−air interface have been reported, e.g., benzene,18,19 1-methylnaphthalene,20 naphthalene,21 anthracene,12 pyrene,22 various solvatochromic indicators,23 or methylene blue.24 These data are invaluable in © XXXX American Chemical Society

the elucidation of the concentration, solvation, aggregation, crystal formation, and acid−base behavior of these contaminants as well as their mutual orientations in aggregates or charge-transfer complexes. Various computational simulation methods were used to study the interactions of aromatic hydrocarbons with ice or within their associates.19−21,25,26 At lower concentrations, many of these molecules prefer a parallel alignment on the ice surface due to the interactions of dangling O−H bonds and the delocalized aromatic π-electrons.27 With increasing loading, these interactions compete with those among the adsorbate molecules,19−21 and both parallel and T-shape configurations mediated via π−π stacking and CH−π interactions, respectively, were found to contribute to the formation of selfassociates on the ice surface.25 Because of its methoxy group, anisole is a hydrogen-bond acceptor molecule, whose aromatic ring is more electron-rich than that in benzene. The absorption spectrum of the anisole monomer possesses two absorption bands in the region above 200 nm. In the gas phase, the major bands with λabs max at approximately 215 and 275 nm are assigned to π,π* transitions28 (K- and B-bands, respectively).29 The vibronic Received: April 1, 2016 Revised: May 17, 2016

A

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containing liquid nitrogen using a homemade aspirator nozzle, composed of a pair of 1 mm glass capillaries as described previously (the typical size of ice grains formed ranged from 30 to 250 μm).19,21 Deposition of anisole vapors on pure ice grains (5 g) was carried out at 77 K inside an insulated container equipped with inlets to allow vapor circulation according to the published procedure.9,10 The anisole concentrations in the samples were determined by spectrophotometry. Instrumentation. Absorption and diffuse reflectance spectra were recorded on a Cary 5000i spectrophotometer (Agilent). For liquid samples, standard 1 cm quartz cuvettes were employed. For ice samples, a Praying Mantis (Harrick Scientific) apparatus was used to acquire the diffuse reflectance spectra. This accessory consists of two 6X, 90° off-axis ellipsoid mirrors, arranged to discriminate against the collection of specular reflected radiation; a low-temperature reaction chamber (CHC, Harrick Scientific) was used to control precisely the sample temperature. The sample temperature was always equilibrated; the residence time of the samples in a cryostat was at least 5 min. No apparent change in the reflectance spectra was observed when the samples were kept at the given temperature for more than 20 min. At least three independent spectroscopic measurements were performed in each case, and the results were subsequently averaged. The fluorescence spectra were recorded on a FLS 920 fluorescence spectrometer (Edinburgh Instruments), equipped with a 450 W Xe lamp and a broadband PMT detector with a twin-grating monochromator present in both arms. Handling of frozen samples required specific tools: A round cuvette was used when anisole solutions were frozen in the cryostat because such a shape helped to endure a pressure exerted by increase of the volume during freezing. To avoid possible spectral artifacts arising due the curved surface of the cuvette, the spectra were corrected using a water sample prepared under the same conditions. The fluorescence measurements were performed in the front-face configuration to minimize undesired scattering arising from the ice and snow surfaces. The low fluorescence intensity manifested for anisole samples made us use a combination of 3−8 nm bandpass filters during the emission and excitation spectra acquisition. Computational Methods. The initial structures of the anisole monomer and dimers interacting with ice surface were obtained from the molecular dynamics simulations on the benzene monomer and dimers, employing a TIP5P-Ew model for the water molecules.19 Several snapshots with various mutual orientations within the benzene dimer associates with respect to the ice surface were taken from molecular dynamics trajectories for further optimization of the anisole molecules located on the ice surface performed at the DFT level. In these calculations, the water molecules within the distance of 0.9 nm from the center of mass of each anisole molecule were included in the cluster model. Within this cluster, those water molecules found within 0.6 nm were optimized, whereas those located at larger distances (0.6− 0.9 nm) were fixed. To simulate a water environment in liquid aqueous anisole solutions, the cluster incorporating one anisole and 27 water molecules was used, in which the surrounding molecules located up to 0.6 nm from the center of mass of anisole were considered. Subsequent ab initio calculations included explicit water molecules up to a distance of 0.6 nm from the center of the solute. The equilibrium ground-state geometries were obtained using density functional calculations employing the B97D46 functional and TZVP basis set47 as in our previous investigations.19−21 The reliability of the B97D functional to model aromatic dimers was shown on the calculations on naphthalene dimer complexes by comparing the results with those of the more accurate DFT/CC scheme.48 Calculations of the monomer and dimer absorption spectra in water and/or on the ice surface were performed at the ground-state optimized geometries with the second-order algebraic diagrammatic construction ADC(2),49,50 using a spin-component scaled approach51 with the resolution-ofidentity method and the def2-TZVP basis set. For the calculations of the emission spectra, the structures of the S1 minima were obtained at the same level. The emission energies were calculated as the energy difference between the first singlet-excited (S1) and ground (S0) states at the S1 geometry minimum. Both absorption and emission spectra of anisole monomer and dimer structures were calculated by the cluster

structures of the latter transition have been investigated in detail; normal-mode frequencies of the ground and excited states including those of the methyl and methoxy internal rotations have been determined.30,31 The absorption spectra of anisole in solutions exhibit a weak solvatochromic shift;32 the B-band maximum shifts hypsochromically in polar solvents, such as diethyl ether or ethanol (λmax = 269 nm) or water (λmax = 267 nm)33 compared to hexane (λmax = 271 nm).32 Polar protic solvents obscure the fine structure in the region of 250− 280 nm due to hydrogen bonding.32,34 Anisole displays a characteristic emission with λem max in the region of 290−300 nm in the gas phase as well as in a solution; it is slightly affected by the solvent polarity.35−37 The emission quantum yield in 1,4-dioxane was reported to be 0.36 ± 0.04.36 A characteristic structureless emission band at 340−350 nm, assigned to anisole excimer (Φf = 0.07 ± 0.01 in 1,4-dioxane),36 has been experimentally observed for neat anisole35 and in aprotic solvents.36 Self-association of anisole in the gas phase can occur;38−40 the binding energy between two molecules has been determined to be ∼11 in the ground state and ∼12 kcal mol−1 in the first excited state.40 The ground-state interaction was later reinvestigated by means of a combined experimental (molecular beam-laser spectroscopy) and computational study (a very accurate CCSD(T) method) to get the energy equal to ∼5 kcal mol−1.41 In addition, the interaction of anisole with water molecules in the ground and excited states has been investigated on small gas-phase anisole−(H2O)1−3 clusters to show that the interaction is mediated through a hydrogen bond between the anisole oxygen and water hydrogen atoms and a πinteraction between the water hydrogen and the aromatic system.42−45 The formation of hydrogen bond between the water oxygen and methyl/phenyl hydrogen atoms of anisole has also been discussed.43 In this work, we present combined experimental−computational analyses of anisole in liquid and solid aqueous solutions. The absorption, diffuse reflectance, and emission spectra of anisole in different types of ice samples were used to examine the effects of the temperature, phase, and ice microenvironment on its self-organization and spectroscopic behavior. High level quantum chemical calculations were used to interpret the experimental data based on the calculated spectroscopic properties of the dimer associates with different binding motives hypothetically formed at the ice surface.

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EXPERIMENTAL SECTION

Materials and Methods. Anisole (99.0%) was used after distillation under reduced pressure or without any purification in different sets of experiments (the spectra obtained in both cases were always identical). Milli-Q water was obtained from a Millipore Simplicity 185 (conductivity = 18.5 MΩ). Frozen solutions were prepared by freezing of the liquid aqueous solutions by two different methods:19 (1) An anisole aqueous solution was placed into a cryostat (Optistat, Oxford Instruments or a low-temperature reaction chamber) at the given temperature, allowing for formation of a visually monolithic ice block. (2) An anisole solution was directly placed into a precooled (typically 194 K) mortar, where instantaneous freezing occurs. This sample was subsequently grinded at the same temperature carefully and slowly to minimize heating of the sample. Finally, the samples were covered by aluminum foil and kept at 200 K in a thermally insulated container prior to the spectroscopic measurement at least for 5 min. Ice grains samples were prepared by spraying pressurized aqueous solutions into liquid nitrogen (shock freezing) to produce micrometer-sized ice spheres of a large specific surface area (∼104 cm2 g−1) as described earlier.9,20 This technique relies on spraying water droplets into a top-open insulated tank B

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Langmuir models, which included water molecules to account for the solvation effect. Depending on the dimer structure, 9−18 water molecules were used. The Gaussian program package was used for the DFT calculations.52 The ADC(2) calculations were carried out using the Turbomole program package.53 The methods used for the ground and electronically excited states calculations in this work have already been described before.20,21

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RESULTS Absorption and Diffuse Reflectance Spectra. The absorption spectrum of anisole in aqueous solution at 298 K and the diffuse reflectance spectra of anisole in a frozen solution (prepared by slow f reezing) and on ice grains (artificial snow; prepared by shock f reezing of water droplets in liquid nitrogen) at 253 K are shown in Figure 1. The major band with a Figure 2. Fluorescence excitation (solid lines) and emission spectra (dashed lines; I = normalized intensity) of anisole in aqueous solution (c = 0.3 mmol dm−3, 298 K; λem = 292 nm; λex = 268 nm; black line), frozen solution (c = 0.3 mmol kg−1, 237 K; λem = 336 nm, λex = 268 nm; blue line), and on ice grains (c = 0.3 mmol kg−1, 253 K; λem = 334 nm, λex = 268 nm; red line).

dominated in more concentrated solutions but was still detectable even at c = 0.03 mmol kg−1 (Figure 3). The most

Figure 1. UV spectra of anisole in aqueous solution (c = 4 × 10−4 mol dm−3, 298 K; black line; A = absorbance) and diffuse reflectance spectra of anisole in a frozen solution (c = 3 × 10−3 mol kg−1, 253 K; blue line) and on ice grains (c = 3 × 10−3 mol kg−1, 253 K; red line; the Kubelka−Munk function, f(R)).

maximum at 269 nm as well as the absorption tail observed for frozen samples exhibited a small bathochromic shift of 3−4 nm compared to those of an aqueous solution. Its vibronic structure is more pronounced for solid-phase samples than that for a liquid solution. The second major broad band with λabs max ∼ 220 nm is shown in Figure S1. Neither the temperature nor the anisole concentration had any significant impact on the positions and shapes of the diffuse reflectance spectra (Figure S1). Excitation and Emission Spectra. The normalized steadystate excitation and fluorescence emission spectra of anisole in aqueous solution and ice samples are shown in Figure 2. The excitation of anisole aqueous solutions at 268 nm resulted in a fluorescence signal with λmax = 291 nm. This band corresponds to the anisole monomer emission in water or methanol solutions as reported before.35 For aqueous solid samples at lower temperatures, this emission band was still apparent but its intensity decreased, whereas a new strong emission band at 336 and 334 nm for a frozen or ice grains samples, respectively, appeared in the temperature range of 150−273 K. This band was assigned to emission of the anisole excimer reported for neat anisole.35 No detectable band in this wavelength range was observed even in a highly concentrated aqueous anisole solution (c = 5 mmol L−1) at 298 K (Figure S7). The relative positions of the emission signals changed with the anisole concentrations in the samples; the excimer emission

Figure 3. Concentration dependence (anisole concentrations, c = 0.03 (dotted), 0.30 (dashed), and 3.00 mmol kg−1 (solid)) on the normalized fluorescence emission spectra of frozen solutions (blue lines) and ice grains (red lines) at 237 K (λex = 268 nm).

remarkable difference was found when we compared the relative intensities of anisole monomer/excimer emission in the case of ice samples prepared by different methods. The excimer emission was strong in frozen solutions samples even at very low concentrations (0.03 mmol kg−1), whereas ice grain samples exhibited a weak excimer but strong monomer emission at this concentration. The excitation spectra of both liquid and solid anisole solutions (with λem of the major emission bands at 297 or 334− 336 nm) matched those of the absorption and diffuse reflectance spectra (Figure 1, Figures S1 and S2). Figure S2 shows that their shape and the band positions did not change over a broad temperature range (77−243 K) or with different emission wavelengths (λem = 297−327 nm). We also investigated the effect of temperature on the emission spectra of ice grains and frozen solution samples (Figure 4 and Figure S4, respectively). An aqueous anisole C

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temperature range of 77−263 K (Figure S6). At low temperatures, only emission at ∼291 nm was observed, whereas at higher temperatures, the excimer emission at ∼330 nm was apparent but never reached the relative intensity observed for frozen solutions or ice grains samples. The ratio of the relative intensities of both emission bands at the given temperatures remained comparable during the heating and cooling cycles. Computational Modeling of the Absorption and Emission Spectra of Anisole and Its Dimers. The calculated vertical excitation energies for anisole monomer in the gas phase, solvated in water, and interacting with the ice− surface interface are shown in Table 1. The energies of the first Table 1. Calculated Vertical Excitation Energies for Anisole Monomer phase (interaction type) gas phase aqueous solution ice (π−H-bond) ice (σ−H-bond)

ΔEexc/eV (λabs/nm)a 4.886 4.814 4.809 4.905

(254) (258) (258) (253)

a Determined at the ADC(2) level. λabs is the corresponding absorption band maximum.

(ΔE(S1) = 4.89 eV) and second (ΔE(S2) = 5.97 eV) excited states are in good agreement with the previously reported excitation energies of 5.00 and 6.22 eV, respectively, calculated at the CC2/cc-pVDZ level.55 For the interpretation of absorption/excitation spectra, only the S1 state will be considered in the following text. Analogous to the calculation of benzene excitation energies,19,56,57 the first excited state of anisole is red-shifted upon water solvation by less than 0.1 eV (4 nm). This value can directly be compared to the experimentally observed spectrum, showing only a very small overestimation of the calculated excitation energy (λabs max ∼ 268 nm; Figure 1). The interaction of anisole with the ice surface, via either a π-interaction between the water hydrogens and the aromatic system (π−H-bond) or a hydrogen bond between the anisole lone electron pair on oxygen and water hydrogens (σ− H-bond), discussed by Reimann and co-workers,42 only slightly changes the excitation energy with respect to that in the gas phase. In order to assign the experimental excitation bands of the anisole associates, the calculations of the vertical excitation energies were performed for various structures: the noninteracting monomers (called monomer-like structures here), tilted T-shape (TT-shape), parallel displaced head-to-tail (H-TPD1), and head-to-head mutual orientations (Figure 5). Two anisole molecules in the latter arrangement allow both the interaction of the aromatic rings (H-T-PD2) and the interaction of the methyl group with the π-system of the second anisole molecule (H-H-PD). In this labeling, the benzene ring and the methoxy group are designated as head and tail, respectively. The interaction energies (and the corresponding absorption maxima) were calculated for smaller clusters which include the water molecules that are in direct contact with the solute molecules (Figure 6). The stabilities of the anisole associates on the ice surface were evaluated using the interaction energies corrected for the basis set superposition error (BSSE; Table 2). Here, Erel corresponds to the relative stabilities obtained by comparing the interaction energies of the complexes with that

Figure 4. Normalized fluorescence emission spectra in the excimer region for anisole in ice grains samples: (a) a heating cycle; (b) a cooling cycle (canisole = 0.3 mmol kg−1, λex = 268 nm).

solution (c = 3, 0.3, and 0.03 mmol kg−1) was first slowly (∼3 K min−1) cooled from 298 to 77 K and then heated back to 298 K; the emission spectra were recorded at regular intervals (Figure S4). In addition to the emission band at ∼291 nm, a different emission band appeared at certain temperatures, and its maximum shifted to 320−340 nm. We assign this band to the excimer emission. The relative intensity of excimer emission culminated at temperatures between 200 and 260 K; no excimer signal was detected at the lowest (77 K) and highest (>278 K) temperatures. For ice grains samples (canisole = 0.3 mmol kg−1), the sample was first slowly heated from 77 to 263 K and then cooled back to 77 K (Figures 4a,b); then a heating/ cooling cycle was repeated. Practically the same temperature profile of excimer appearance was obtained even if higher (c = 3 mmol kg−1) or lower (c = 0.03 mmol kg−1) anisole concentrations were used. Figures S5a,b show that the emission spectra exhibited a notable decrease in intensity. This probably cannot be explained solely by an increase in light scattering due to a higher heterogeneity of the ice sample; emission from the S1 state also competes with intersystem crossing which is more efficient at lower temperatures, as it was reported elsewhere.54 When the freezing/heating cycle was repeated under the same experimental conditions, no apparent changes in the emission spectra at the given temperature were observed. A summary of the most relevant spectroscopic data is provided in Table S1. The emission spectra of anisole deposited from vapors on ice grains9,10 (canisole ∼ 0.3 mmol kg−1) were recorded in the D

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Figure 5. Most stable anisole association on the ice surface (H-H-PD2; left) and that with the strongest interaction between the monomers (H-TPD1; right).

Figure 6. Optimized structures of the anisole dimers used for the calculations of the excitation and emission spectra (a: H-H-PD; b: H-T-PD1; c: HT-PD2; d: TT-shape; e: monomer-like).

Erel(add) values indicates that the interaction energies between the subsystems are almost pair-additive. The highest interaction energy between two monomers (Einter(M1−M2) = −4.9 kcal mol−1) was found for the H-T-PD1 structure. Here the monomers possess a mutual orientation that closely resembles the most stable structure of the anisole dimer in the gas phase with binding energies of 5.2 and 5.0 kcal mol−1 estimated experimentally and theoretically, respectively.41 The structures given above certainly do not represent all possible dimer conformations which can be formed at the ice-surface interface. In addition, our calculations did not reflect the effects of temperature on the character of ice surface which, in consequence, may affect the dimer structures. Nevertheless, our results do show that a relatively flexible ice surface allows for various arrangements of the associated molecules possessing a similar energy stabilization, which can then be used in interpreting our spectroscopic observations. The wave functions of the two lowest singlet excited states for anisole associates were obtained from a combination of the electronic configurations of the first singlet excited state of each monomer; their excitation energies (ΔEexc) are given in Table 3. To explore the effects of anisole dimerization, the gas-phase vertical excitation energies of the dimer associates, possessing

Table 2. Interaction Energies and Relative Stabilities of Anisole Dimers on the Ice Surfacea structure

Erel

Einter(M1− ice)

Einter(M2− ice)

Einter(M1− M2 )

Erel(add)b

H-H-PD TT-shape H-T-PD1 H-T-PD2 monomerlike

0 1.9 5.8 9.4 11.1

−12.5 −11.3 −6.9 −8.6 −5.8

−10.8 −11.5 −7.8 −6.9 −9.3

−4.0 −2.0 −4.9 −1.9 −0.1

0 1.4 7.7 9.9 12.0

a All vaules are in kcal mol−1. bErel(add) = Einter(add) − Einter(add)min, where Einter(add) = Einter(M1−ice) + Einter(M2−ice) + Einter(M1−M2). Einter(add)min is the interaction energy of the most stable complex (HH-PD2).

of the most stable structure (H-H-PD). Alternatively, these stabilities are compared using the interaction energies Einter(add), calculated as a sum of the interaction energies of each monomer (M) with the ice surface, Einter(M1−ice) and Einter(M2−ice), and the interaction energies between two monomers, Einter(M1−M2). Erel(add) gives the relative stability of each structure with respect to that of H-H-PD, characterized by Einter(add)min. A good agreement between the Erel and E

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could enhance their absorption of solar radiation.2,3,58 It has been demonstrated that significant shifts in the absorption maxima observed upon freezing of aqueous solutions of some solvatochromic dyes can be interpreted as enhancement of the hydrogen bonding within their associates formed on the ice surface.23 Although some controversial results can be found in the literature,12,59 only insignificant changes in the absorption spectra of some nonsubstituted simple aromatic compounds due to the phase transition of their aqueous solutions have been reported recently.19,21 Thus, the aim of this work was to evaluate effects of an electron-pair-donating methoxy group in anisole on its absorption and emission optical properties in frozen solutions and ice grain samples. The absorption spectrum of anisole (Figure 1 and Figure S1) is characterized by two major absorption bands at ∼220 nm (Kband; S2) and ∼270 nm (B-band; S1),29,60 of which the latter corresponds to the allowed πX̃ 1A1g → π*Ã 1B2u transition.61 The absorption spectra of anisole in the gas phase and solutions are similar.28,35,62 An analysis of the B-band allows the identification of three vibrational transitions: the C−O−CH3 stretching mode can be assigned to the band at ∼274 nm63 and the O−CH3 stretching modes assigned to the ∼264 and 269 nm bands.61,63 At least two of the vibrational transitions remain apparent in the spectrum of anisole in aqueous solutions. For frozen solution and ice grains samples, the major maxima are bathochromically shifted in both the diffuse reflectance (Figure 1) and the excitation (Figure 2) spectra by ∼4 nm (∼70 meV). Acquisition of the excitation spectra was technically challenging for ice samples containing macroscopic cracks and fractures, as their heterogeneity increased light scattering resulting in a loss of the probe signal. On the other hand, the vibronic structure in the spectra is enhanced for the frozen matrix, which can be attributed to a reduction of the vibrational motion by the constraining environment. As noted above, the absorption spectra of anisole are insensitive to changes imposed by solvent properties; only negligible solvatochromic shifts of the B band in the gas phase,62 cyclohexane,32 hexane, or methanol35 have been observed. A small bathochromic shift due to any specific interactions of anisole molecules in frozen matrices thus cannot be reliably interpreted because various solvatochromic,23 concentration,24 and temperature (Figure S2a) effects may play a role. Therefore, neither anisole aggregation, triggered by freezing of its aqueous solutions, nor the solute−solvent hydrogen bonding due to the presence of an electron-donating methoxy group or the π-system of anisole at the air−ice interface caused any significant effect on the positions of the absorption bands. Our calculations showed that the former type of the hydrogen binding (σ−H-bond) accounts for −3.5 kcal mol−1 and a hypsochromic shift of 1 nm, whereas the strength of the π−Hbond is −3.2 kcal mol−1 and the band is shifted to longer wavelengths by 4 nm with respect to that calculated for the gas phase (Table 3). The excitation spectra of frozen solutions were not affected by changing the emission wavelength in the interval of 297−327 nm (Figure S2a), where the emission bands of the monomer and the excimer are found (Figures 2 and 3; see the next paragraph). The emission spectra of anisole in dilute solutions possess a band with λmax ∼ 290 nm, which is only marginally affected by the solvent polarity and is assigned to a monomer emission.35−37,62 Indeed, the maximum at λmax ∼ 291 nm was observed in liquid aqueous solutions (Figure S4 and Table S1). Such small effects of the solvent polarity are reflected also by

Table 3. Calculated Vertical (Two Lowest) Excitation (ΔEexc) and Emission (ΔEem) Energies and the Corresponding Absorption and Emission Band Maxima of Anisole Associates on the Ice Surface structure H-H-PDa TT-shape H-T-PD1 H-T-PD2a monomerlike

ΔEexc/eV (λabs/ nm) gas phase

ΔEexc/eV (λabs/nm) ice−air interface

4.777b, 4.815b (260, 254) 4.794, 4.817 (259, 257) 4.780,b 4.799b (259, 258) 4.761,b 4.827b (260, 257) 4.824, 4.858 (257, 255)

4.795, 4.879 254) 4.787, 4,799 258) 4.734, 4.787 259) 4.779, 4.940 251) 4.796, 4.817 257)

ΔEem/eV (λemis/nm) ice−air interface

(259,

4.391 (282)

(259,

4.389 (283)

(262,

4.323 (287)

(259,

3.718 (333)

(259,

4.411 (281)

Anisole interacts with water molecules via a σ−H-bond. bThe excited state is delocalized over both monomers.

a

mutual orientations taken from the dimer−ice clusters, are compared to the gas-phase excitation energy of the monomer. The dimer excitation energies decrease a little upon complexation. The shift is most prominent for the parallel-displaced structures and accounts for 0.1−0.13 eV, i.e., 5−6 nm. Inspection of the molecular orbitals involved in the excitation reveals that the excited states of these structures results from a transition between molecular orbitals delocalized over both monomers. In the TT-shape and monomer-like structures, whose excitation energies are less affected by complexation (the bathochromic shift is less than 0.1 eV for the S1 states), the excitations are localized on one of the monomers. The water molecules at the ice surface influence the excitation energies in two ways. Analogous to the effects of the H-bond on the anisole monomer excitation spectra (Table 1), the structures H-H-PD and H-T-PD2 (Figure 6 and Figure S9) with the σ− H-bond interaction were characterized by a negligible blue-shift of 1 nm, whereas a small red-shift of 2−3 nm was observed for structures with a π−H-bond interaction, such as H-T-PD1, monomer-like, and TT-shape structures (Figure 6 and Figure S9). For prediction of the emission band maxima, optimization of the S1 state of anisole in the gas phase led to a structure characterized by an emission energy of 4.507 eV (275 nm). In aqueous solution modeled using 15 water molecules, emission from the S1 state was found at 4.391 eV (282 nm). This value is in relatively good agreement with that found experimentally (291 nm; Figure 2, Table S1). The emission energy for the monomer-like structure of 4.411 eV (281 nm) as well as the HH-PD and TT-shape structures, in which the two aromatic rings do not overlap, is close to that of monomer. Only the HT-PD1 orientation was responsible for a bathochromic shift of 6 nm. However, when the two aromatic rings overlap in a faceto-face orientation (H-T-PD2), the emission band is shifted bathochromically by ∼50 nm (λem = 333 nm), that is, to the region where excimer emission was observed experimentally (Figures 2−4).

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DISCUSSION The evaluation and understanding of the optical properties of chromophoric impurities located on the ice surface can be very useful in predictions of their photochemical reactivities in natural snow and ice because a bathochromic shift of their absorption bands due to their specific interactions with ice F

DOI: 10.1021/acs.langmuir.6b01187 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir our calculations, which predict a bathochromic shift of 4 and 7 nm for the excitation and emission spectra, respectively, when the positions of the bands of anisole monomer calculated in the gas phase and an aqueous solution are compared. Besides the emission band at λmax ∼ 290 nm, neat liquid anisole exhibits another band at λmax ∼ 350 nm35 (Figure S3). This band, assigned to an excimer,35,36 is not observed in dilute solutions but can be detected in concentrated solutions of anisole in organic solvents.36,37 Figure S3 shows that the emission intensity markedly increases upon freezing of neat anisole. Thus, upon freezing below ∼200 K, a short-wavelength band becomes dominant, and at 77 K, the band is broader and its vibronic structure is more manifested. At such low temperatures, anisole evidently forms pure crystals, in which the molecules adopt a flat edge-to-face (Cs-symmetric) configuration, similar to the H-T-PD1 structure with no faceto-face π−π stacking64 and the TT-shape structure shown in Figure 6d. Crystalline anisole, analogous to crystalline naphthalene,21 thus does not exhibit excimer emission, and the spectra (Figure S3) obtained at lower temperatures than the melting point of anisole (236 K) must represent emission from the crystalline state. Both emission bands of anisole (λmax ∼ 290 and ∼350 nm) were observed in all types of ice matrices: frozen solutions prepared by a slow freezing method, ice grains samples formed using a shock freezing method, and anisole vapor deposited ice grains, but the excimer emission (λ ∼ 350 nm) was observed only under specific conditions. A broad concentration range of 3−0.03 mmol kg−1 was chosen to prepare different ice surface coverages.9,10 The concentration of anisole on the order of 10−3 mol kg−1 in ice grains samples is close to a monolayer coverage,9 provided that all anisole molecules are ejected to the ice grain surface during freezing.9,10 The lowest concentration used (0.03 mmol kg−1) is well below this limit. On the other hand, anisole aggregation (clustering) in frozen samples is excessive, as the impurities reside in a 3-dimensional cage (micropocket).9 For both types of ice samples, the relative intensity of excimer emission decreased at lower anisole concentrations; however, the effect was more significant for ice grains samples (Figure 3), which must be related to a substantial anisole “dilution” on the surface of small ice grains possessing a significantly larger specific surface area (∼104 cm2 g−1) than that of cavities inside frozen solution matrices.9,20 To ensure that we have samples with a solute evenly distributed on the ice surface, anisole was deposited from vapors on pure ice grains. As expected, only a relatively insignificant (although still apparent) molecular aggregation, and thus excimer emission, was observed at the lowest anisole loadings (c = 0.03 mmol kg−1). Our calculations of the emission spectra performed for various structures indicate that the changes of the emission wavelength are driven mainly by the overlap between the aromatic rings in anisole associates. Among the structures considered in our study, only the structure with a significant face-to-face π−π stacking (H-T-PD2) exhibits a substantial bathochromic shift in the fluorescence spectrum (Table 3). The wave function of this excimer is characterized by the electronic transition between the highest occupied and the lowest unoccupied orbitals which are completely delocalized between the two monomers, allowing for a strong excited state interaction. Similar predictions were also done in our previous studies on naphthalene and 1-methylnaphthalene excimers.20,21

The emission spectra of aqueous anisole solutions fundamentally changed upon freezing; the band at λmax ∼ 350 nm assigned to excimer emission appeared only at certain temperatures. For ice grain samples, this emission was dominant only above ∼150 K during both heating and cooling cycles (Figure 4). Close to this transition temperature, the emission band maximum hypsochromically shifts, and then the band disappears completely at 77 K. For frozen solutions, the excimer emission was absent in the case of liquid and nearly melted (273 K) samples as well as in those frozen to very low temperatures (