Spectroscopic Properties of Naphthalene on the Surface of Ice Grains

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The Journal of Physical Chemistry

Spectroscopic Properties of Naphthalene on the Surface of Ice Grains Revisited: A Combined Experimental–Computational Approach

Ján Krausko,1,3 Joseph K'ekuboni Malongwe,1 Gabriela Bičanová,1,3 Petr Klán,1,3 Dana Nachtigallová,2* Dominik Heger1,3*

1

RECETOX, Faculty of Science, Masaryk University, Kamenice 5, 625 00, Brno, Czech

Republic. 2

Institute of Organic Chemistry and Biochemistry, Flemingovo nam. 2, 166 10 Prague, Czech

Republic. 3

Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00, Brno,

Czech Republic.

[email protected]; [email protected]

Abstract An experimental-computational method is used to investigate the spectroscopic behavior of naphthalene on the surface of ice grains. UV-vis diffuse reflectance and fluorescence spectroscopies of naphthalene combined with DFT and ADC(2) calculations provide evidence for the occurrence of excited-state associates. The measured and calculated bathochromic shifts of the S0→ S1 electronic transitions related to naphthalene dimers or naphthalene–ice interactions do not exceed 3 nm. The bands observed in the emission spectrum of frozen naphthalene

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solutions are assigned to excited dimers of different mutual orientations, naphthalene phosphorescence, and fluorescence of anthracene present as a trace impurity and populated by the energy transfer from excited naphthalene. Photochemical reactivity in/on ice and snow is dependent on the absorption properties and speciation of the compounds present in these media. Hence, within this study, we exploit frozen solutions of naphthalene to demonstrate both the absence of considerable bathochromic shift and a strong tendency to aggregate.

Introduction The cryosphere accommodates many compounds, which include those naturally present or formed by biochemical and (photo)chemical reactions and those imported via long-range transport.1 The release of these compounds upon warming might have environmental consequences.2,3 Their fate depends on the location, speciation, and absorption characteristics.4 Such full characterization is essentially unavailable for most ice impurities.5 For example, their molar absorption coefficients have been obtained only for a few small molecules because the coefficient determination is very difficult due to vaguely defined ice optical properties, geometrical arrangements of ice grains, and heterogeneous multiphase contaminants distribution and their local concentrations.5 Only the relative absorption characteristics of larger molecules on ice surface compared to those of aqueous (aq) solutions are available. The spectroscopy of aromatic hydrocarbons has been reported for various chromophores, namely benzene,6,7 methylnaphthalene,8 naphthalene, anthracene,9 pyrene,10 phenol derivatives,11 cresol red,12 harmine,13 various solvatochromic indicators,14 and methylene blue.15 The following compounds have received considerable attention within recent investigation of cometary ice: pyrene, coronene,16 benzoperylene,17 and fullerene.18


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Ice surface is covered with a rather specific environment, a disordered interface characterized by more or less random orientation of water molecules19-24 whose structure depends on the properties of impurities and temperature.25 Only a few small molecules are dissolved in ice lattice in appreciable amounts (e.g., HF, HCl, HNO3, formaldehyde, NH3).26 Larger molecules, in particular aromatic hydrocarbons, are known (yet indirectly proved by only few examples) to be ejected onto the surface of ice crystals and into the veins between ice grains when their solutions are frozen.1,4,27 The prediction of phenomena related to atmospheric chemistry, including climate changes and the fate of environmental pollutants, requires sound knowledge of the physical-chemical parameters of individual compounds present in cold environments. The photochemistry of pollutants on natural ice surfaces is a major factor that can affect the chemical composition of cold environments;1,28,29 this is exemplified by the halogen explosion preceding an ozone depletion event in the Arctic observed at every sunrise.2 One of the aspects influencing the mechanism and kinetics of relevant photochemical reactions depends on the compartmental location of solutes in ice.1,30,31 The results of the rates of photodegradation of impurities on ice were found inconsistent by different groups,6,9,32,33 and the reason for the observed discrepancies has not been understood yet. Changes in ice structure, including sintering,34 ageing, metamorphism,35 and the crystallization of amorphous ice10, were shown to lead to the formation of aggregated impurities; they were observed even for vapor deposition of sub-monolayer concentrations.7,8,36,37 Various computational simulation methods8,38-43 were used to characterize the interactions of aromatic hydrocarbons with ice or within their associates.8,9,25,36,40,41,43-46 According to the current view, the adsorption of aromatic hydrocarbons at air/ice or air/water



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interfaces is followed by dissolution in the disordered interface or bulk water.44 Computer simulations performed for aromatic hydrocarbons42 revealed that the adsorption follows the Langmuir isotherm up to a certain pressure. Under these conditions, the adsorbed molecules align parallel to the surface, where the dangling OH bonds form OH…π hydrogen bonds. The additional molecules interact strongly with those previously adsorbed. At higher pressures, the importance of lateral interactions increases between the adsorbates.8 Naphthalene is a sensitive probe for the detection of excited state dimers. The vibrationally resolved electronic absorption spectrum of the gas phase naphthalene monomer has been comprehensively studied.47-49 Three absorption systems were resolved in the region of 30000–53000 cm–1 (333–189 nm): (i) system I (S0→S1 transition, 320–290 nm, 0–0 band: 312.3 nm); (ii) system II (S0→S2 transition, 290–250 nm); and (iii) system III (S0→S3 transition, 222– 200 nm). For crystalline naphthalene, the 0–0 band of the lowest singlet transition is shifted to 317.7 nm and 316.2 nm for the a (long axis) and b (short axis) polarized components, respectively.50-53 Vibrationally resolved spectra of naphthalene solutions were reported for isooctane (0–0 band: 314.8 nm),50 hexane (0–0 band: 314.9 nm),48 and ethanol (0–0 band: 314.9 nm).54 The observed molar absorption coefficients and oscillator strengths show that the first excited state is very weak (the molar absorption coefficients of naphthalene for the S0→S1 transition in the crystalline50 and gas47 phases are on the order of 100–200 M–1 cm–1; the total oscillator strengths are f = 0.00196 (octane)50 and f = 0.002 (gas phase).47 Naphthalene is known to form excimers at high concentrations in nonpolar solvents,55 solid solutions in silicate glass,56 and in the adsorbed state on silica57 or inside cyclodextrins,58 in contrast to aqueous solutions where only monomer emission is observed.58 The mutual orientation of naphthalene molecules in crystals does not allow for intermolecular electronic


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interactions; thus, no excimers are observed in pure naphthalene crystals at atmospheric pressure.59 Only naphthalene crystals exposed to increased pressure allow for the formation of excimers, possibly preferentially at lattice defects.60,61 The observed maxima of naphthalene excimers range from 375 to 420 nm.57,58 The fluorescence emission spectrum of naphthalene in ice showed monomer emission around 320 nm and emission with maxima at 387, 408 and 434 nm, which were assigned to excimeric emission of naphthalene.9 In a subsequent study, numerous bands assigned to the excimeric emission of naphthalene were observed along with strong scattering.62 The multiple emission bands were not discussed in detail previously. A broad unresolved excimer emission was observed for 1-methylnaphthalene in frozen solutions and vapor-deposited on ice grains under various conditions.8 The character of naphthalene associates has been investigated computationally using various methods.8,63-68 The most stable arrangement in the ground state is a parallel-displaced structure, which is energetically preferred over a T-shape structure by 1.6 kcal mol–1. The naphthalene excimer resembles a sandwich face-to-face structure with intermolecular distances in the range of 0.325–0.340 nm.69 A smaller value of 0.300 nm was found with CC269 and the multi-configurational quasi-degenerate perturbation theory methods.70 All values fall into the experimentally observed range of 0.300–0.360 nm.55. Ardura et al. performed dynamics studies to explore possible orientations of the naphthalene associates43 at the air-ice interface. Both parallel and T-shape structures stabilized by π-π stacking and CH–π interactions, respectively, were found. In our previous study on 1-methylnaphthalene, various structures of dimers, including parallel-displaced stacking and T-shape structures, were predicted to be formed on ice surface.8



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The former species were responsible for the fluorescence-band red shift with respect to that of the monomer, whereas the latter ones could not be distinguished from the monomer fluorescence band. Despite the relatively extensive research, the character of the interactions at the airice/water interfaces and many other related phenomena are not well understood yet. Spectroscopic techniques and high-level ab initio calculations have recently helped to provide more insight into the characterization of adsorbed molecules at these interfaces.7 To improve the understanding of the physical chemistry of adsorbates in/on ice, we present a spectroscopic study of naphthalene self-organization and aggregate formation. The character of the contaminant–ice and contaminant–contaminant interactions in the ground and excited states was modeled by DFT and ADC(2) calculations. Weak interactions on the surface simulated by high-level quantum chemical calculations matched our experimental observations remarkably well. In particular, this work provides answers to two previously opened questions, namely those of whether the absorption spectrum of naphthalene on ice is bathochromically shifted and what the origin of the emission spectrum observed from frozen aqueous solutions is.

Experimental Section Materials and Methods. Naphthalene of analytical purity obtained from Fluka (anal. standard, 99.75%+) and Lachema (pure, sublimed) was used as purchased. The water was purified on a Millipore Simplicity 185. The naphthalene frozen solutions for fluorescence measurements were prepared by gradual immersion of the aqueous naphthalene solutions into liquid nitrogen (7.8 × 10-5 mol dm-3; 10 mg l–1, or 1 × 10-4 mol dm-3) in 2.5 mm quartz tubes. Both the excitation and the emission spectra were obtained at 77 K in a Dewar vessel. The


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reference spectra of pure ice were recorded under the same conditions. The naphthalene frozen solutions for diffuse reflectance were prepared by fine grinding of the frozen saturated aqueous solution in a mortar cooled by dry ice. The samples were wrapped in an aluminum foil and kept on dry ice in a thermally insulated box prior to the measurement. The same procedure was used for the preparation of reference ice samples from pure water. Instrumental Techniques. The UV-Vis absorption spectra were recorded on a Cary 5000 spectrometer (Agilent). The liquid samples were measured in 10-mm quartz cuvettes at ambient temperature (295 ± 2) K. The diffuse reflectance spectra were recorded using a Harrick Praying Mantis Diffuse Reflectance (PM-DR) accessory equipped with a temperature-controlled powder holder with quartz windows. The frozen solution sample temperature was kept constant at (253 ± 2) K. The holder was cooled by a mixture of dry ice and acetone. Moisture condensation on the cell windows was prevented by purging the PM-DR accessory with dry nitrogen. Each sample was equilibrated for 2 minutes prior to the measurements. No apparent change in the reflectance spectra was observed when the samples were kept at a constant temperature for 20 minutes. The average reflectance spectra obtained from at least two independent measurements were used for the calculation of the Kubelka-Munk f(R) function with ground pure ice sample spectra as the reference. The fluorescence measurements were performed at a perpendicular-geometry FLS 920 fluorescence spectrometer (Edinburgh Instruments) equipped with a 450 W Xe lamp and a PMT detector with a double grating monochromator for both the excitation and the emission. The measurements were carried out in standard 10-mm fluorescence cells (solution measurements) and in custom-made quartz tubes (3 mm in diameter, frozen samples). For measurements at 77



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K, the sample cell was placed in an Optistat DN2 (Oxford Instruments) cryostat kept at (77 ± 0.5) K. Ab Initio Calculations. The equilibrium geometries of the ground-state naphthalene monomer and dimers in the gas phase, interacting with water molecules in the liquid phase, and on ice surface were optimized using density functional calculations employing the B97D64 functional and TZVP basis set.71 This approach was shown to provide a reliable description of naphthalene dimer complexes via comparison with a more accurate DFT/CC scheme.68 Initial structures of the naphthalene monomer and dimers interacting with water and icesurfaces were taken from the previous studies on 1-methylnaphthalene.8 The cluster models used for subsequent ab initio calculations included all water molecules within the distance of 9 Å from the center of the solute complexes. These complexes and all water molecules within the distance of 6.5 Å from the center of naphthalene were optimized, while the remaining water molecules were kept frozen. The calculations of the monomer and dimer absorption spectra were performed at the ground-state optimized geometries, using the second-order algebraic diagrammatic construction ADC(2)72-74 with the resolution-of-identity method and the TZVP basis set. This approach was successfully used in our previous study on the spectroscopy of benzene and its associates on ice surfaces.7 For the simulations of the excited state behavior of naphthalene monomer and dimer associates in water and/or on ice surface, all water molecules within the distance of 0.65 nm from the solute center were considered. The vibrationally resolved absorption spectrum of naphthalene in the gas phase was calculated by means of the TD-DFT method75,76 employing the B3-LYP functional77 and TZVP basis set.


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The emission spectra were calculated as the energy difference between the first singletexcited (S1) and ground (S0) states at the S1 geometry minima. These minima were optimized at the ADC(2) level using a spin-component scaled approach78 and the TZVP basis set. A Gaussian program package was used for the DFT calculations.79 The ADC(2) calculations were carried out using a Turbomole program package.80

Results Absorption. The vibrationally resolved absorption and fluorescence excitation spectra of aqueous naphthalene (at 253.15 K) in the region of 240–330 nm and the emission spectra in the region of 300–400 nm are shown in Figure 1. The maxima of the absorption spectrum closely matched those of the excitation spectrum (Table 1). The absorption spectrum comprises several vibrational levels of the two lowest singlet transitions of naphthalene, S0→S1 (1A→1Lb) and S0→S2 (1A→1La).58,81



250

275

300

325

350

375

0.7 0.6 0.5

400

normalized fluorescence intensity / a u

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A

0.4 0.3 0.2 0.1

0.8

0.4

λ / nm 0.0 250

275

300

325

350

375

0.0 400

Figure 1. The UV-vis absorption (green), fluorescence excitation (black, λem = 330 nm), and emission (blue, λexc = 287 nm; red, λexc = 310 nm) spectra of naphthalene in the aqueous solutions (Fluka, 7.8 × 10-5 mol dm-3; 10 mg l-1). The excitation of the naphthalene aqueous solutions at both the S0→S2 transition (λexc = 287 nm) and the S0→S1 transition (λexc = 310 nm) resulted in fluorescence emission in the region of 300–400 nm possessing a complex vibrational pattern consistent with the previous reports.9,8285

The 0–0 transition of the S0→S1 transition band is close to 314.3 nm (derived from the absorption spectrum (Figure 1) and also estimated from the intersection point of the fluorescence excitation and emission spectra, Figure S2)). This value differs only slightly from that of 314.9 nm reported for a naphthalene solution in hexane47,48 and ethanol.54 The weak band observed in 10 ACS Paragon Plus Environment

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the excitation spectrum of the naphthalene solutions in water at 319.2 nm (Figure S1) is identified as a hot band.48,54 The measured excitation spectrum of naphthalene in water showed good agreement with the computed vibrationally resolved absorption spectrum of naphthalene in the gas phase (Figure 2) in terms of the peak positions. The hot band at 319.4 nm is absent in the calculated spectrum.

40000 0.10

38000

36000

34000

32000

-1 ν~ / cm

0.08 6000

0.06

0.04

intensity / a u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3000

0.02

λ / nm 0.00

0 260

280

300

320

Figure 2. The fluorescence excitation spectrum of the naphthalene solution in water (λem = 330 nm, red solid line), and the calculated vibrational progression for the S1 state of naphthalene (black dashed line). The lowest-energy peak of the calculated spectrum was placed at 314.3 nm to match the 0–0 band of the first singlet transition in the excitation spectrum.



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Figure 3. a) The Kubelka-Munk f(R) spectra of crushed naphthalene mixed with silica obtained by diffuse reflectance measurements at various sample loadings: 1.70 mg g-1 (black), 2.34 mg g-1 (red), 2.91 mg g-1 (blue), 3.87 mg g-1 (magenta), 6.86 mg g-1 (green); b) The Kubelka-Munk f(R) spectra of crushed naphthalene mixed with silica (a detail of the S0→S1 transition region) at various sample loadings: 1.70 mg g-1 (black), 2.34 mg g-1 (red), 2.91 mg g-1 (blue), 3.87 mg g-1 (magenta), 6.86 mg g-1 (green), and the calculated absorption spectrum of the S1 state of naphthalene (black dashed line). The lowest-energy peak of the calculated spectrum was placed at 314.1 nm to match the 0–0 band of the first singlet transition in the Kubelka-Munk f(R) spectra.

The diffuse reflectance spectra of crushed naphthalene crystals mixed with silica (Figure 3a) show the strong transitions of S0→S3 (200–230 nm) and S0→S2 (240–290 nm), and they also exhibit a relatively weak S0→S1 band (290–320 nm). The assignments of their maxima to the vibrational progressions are listed in Table 1. The first singlet transition is shown in detail in Figure 3b. The spectra demonstrate good agreement with the calculated vibrationally-resolved S0→S1 absorption spectrum in terms of the peak positions.



50000

45000

40000

35000

30000

0.16 -1 ν~/ cm

0.14

Kubelka-Munk f(R)/a.u

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0.12

0.10

λ / nm 0.08 200

220

240

260

280

300

320

340

Figure 4. The diffuse reflectance spectra of the frozen saturated solution of naphthalene in water at 253.15 K. The diffuse reflectance spectra of the frozen saturated solutions of naphthalene (Figure 4) exhibit, despite a low signal-to-noise ratio, the same features as the corresponding spectra of naphthalene in different environments (silica; aqueous solution), namely a strong S0→S3 and S0→S2 absorption and a rather weak S0→S1 absorption spanning no further than 320 nm. The band maxima are given in Table 1. Fluorescence. The fluorescence excitation spectra of the naphthalene frozen aqueous solutions (Fluka, 7.8×10-5 mol×dm-3; 10 mg l–1) at λem = 333 nm and λem = 412 nm are presented in Figure 5a. While we observed pronounced and well-resolved S0→S1 absorption peaks, the structure of the S0→S2 absorption bands became obscured, and the S0→S3 absorption maximum 14 ACS Paragon Plus Environment

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around 220 nm is partially missing (possibly because of light reabsorption); instead, a broad structureless band with the maximum around 235 nm is evident. This band was more intense at the emission detection wavelength of 412 nm as compared to that of 333 nm. The fluorescence excitation spectra of the naphthalene frozen aqueous solutions (Lachema, 1.0×10-4 mol dm-3) were measured at various emission wavelengths, as indicated in Figure 5c; these spectra show features very similar to those indicated in Figure 5a, Figure S4 (Fluka). A derivative analysis of the spectra revealed that two series of bands were present in the S1 region (see Table 1). These series can be reproduced by the calculated absorption spectrum of the S1 state depending on the wavelength set to the 0–0 band energy. Setting this value to 314.2 nm provides the series found also in the absorption and excitation spectra of the aqueous naphthalene solutions and in the diffuse reflectance spectrum of crystalline naphthalene mixed with silica (vide supra). The second series (Table 1) can be reproduced well by placing the 0–0 band of the calculated absorption spectrum of the S1 state of naphthalene to 312.3 nm. In both series derived from the spectra of the liquid aqueous solutions, an intense band observed in the frozen aqueous solutions at 315.7 nm is missing.



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Figure 5. a) The fluorescence excitation spectra of frozen naphthalene solutions (blue: λem = 333 nm; black: λem = 412 nm, 77 K, Fluka, 7.8×10-5 mol dm-3;10 mg l-1); b) Details of the S0→S1 transition region in the fluorescence excitation spectrum of the frozen naphthalene solution (blue: Fluka, 7.8×10-5 mol dm-3; λem = 333 nm), and the calculated absorption spectrum of the S1 state of naphthalene (black line); c) The fluorescence excitation spectra of the frozen naphthalene solutions (Lachema, 1.0×10-4 mol dm-3) at various wavelengths of detection (325 nm – orange; 333 nm – green; 350 nm – cyan; 367 nm – magenta; 388 nm – blue). The steep rise in intensity above 320 nm in the case of λem= 333 nm (green) is caused by scattered excitation light.



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Table 1. The positions of the band maxima in the absorption/excitation/diffuse reflectance spectra of naphthalene under different conditions. λabsa

λexcb

λexc c

K-M f(R)d

K-M f(R)e

214.2

211

222.4

218.4

Ecalcf

Ecalcg

236.2 262.4

261.8

266

265.8

272.4

272

276.4

276.4

283.3

283.2

287.9

287.6

264.4 272.4

273.2 275.2

283.6

284

289.1 290.9 293.3

292.8

293.9 295.9

297

296.6

296.6

295.6 296.4

296

298.7 300.4

300.2

301.2

298.7 300.2

300.1

303 305.7

305

305.5

302.6 305

307.6

304.2 307.5 18

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S0→S1

310.4

309.8

309.5

309.8

309.1

0–1 312.3 S0→S1

314.4

314.3

314.2

312.3 313.6

314.2

314.2

0–0 315.7 S0→S1

320.2

319.2

319.4

1–0

a

From the absorption spectrum of a frozen aqueous solution of naphthalene (Fluka, 7.8×10-5 mol

dm-3; 10 mg l-1). bFrom the excitation spectrum (λem = 333 nm) of an aqueous solution of naphthalene (Fluka, 7.8×10-5 mol dm-3; 10 mg l-1). cFrom the excitation spectrum (λem = 333 nm) of an aqueous solution of naphthalene at 77 K (Fluka, 7.8×10-5 mol dm-3; 10 mg l-1). dThe Kubelka-Munk remission function of a frozen saturated solution of naphthalene in water, eThe Kubelka-Munk remission function of a naphthalene powder mixed with silica. f The calculated S1 absorption band maxima (0–0 band centered at 314.2 nm).

g

The calculated S1 absorption

band maxima (0–0 band centered at 312.3 nm)



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Figure 6. a) The fluorescence emission spectra of naphthalene (Lachema): the aqueous solutions at room temperature (red: λexc = 274 nm) and frozen at 77 K (blue: λexc = 274 nm), both of c = 1.0×10-4 mol dm-3, and of crystalline naphthalene at 77 K (black: λexc = 274 nm); b) The fluorescence emission spectra of naphthalene (Fluka): the solution in water at 77 K (blue and cyan: λexc = 274 nm) and the crystals at 296 K (black: λexc = 274 nm) The emission spectrum from the frozen naphthalene solution digitized from ref. 9 (magenta) is given for comparison. The fluorescence emission spectra of the frozen and liquid aqueous solutions of naphthalene are shown in Figure 6 and described in Table S1. In addition to the bands which exhibit the same maxima as those obtained for the liquid solutions and assigned to the monomer, new spectral features appeared after the solution had been frozen. The emission intensity, especially in the spectral region of 340–500 nm (Figure 6), increased with respect to that of the liquid phase. These features can be attributed to various naphthalene excimers and anthracene impurity

86,87

(Figure 6b), as will be shown in the Discussion. By contrast, only monomeric

emission and phosphorescence were observed when the solution of naphthalene in methylcyclohexane was frozen at 77 K (Figure S3). In addition, the phosphorescence spectrum of naphthalene in the frozen aqueous solution was measured with the delay of 1 ms at 77 K (Figure S5). Computational Modeling of the Absorption and Emission Spectra. The absorption energies of the three lowest excited states of naphthalene in the gas phase (calculated in the C1 symmetry) are listed in Table 2 along with the corresponding oscillator strengths. As already shown in our previous study of benzene,7 the ADC(2) and CC2 methods systematically overestimate the excitation energies compared to the CC3 and CASPT2 methods (see Table 2 for comparison with the previously reported data on the excitation energies of the S1 (11B3u), S2(11B2u) and S3 (21Ag) states. For the interpretation of the absorption and excitation spectra, the vibrational progression of the S0→S1 electronic transition was calculated in the gas phase at the B3LYP/TZVP level. The difference between the S0→S1 vertical excitation energy (35101 cm–1) 21 ACS Paragon Plus Environment


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and the corresponding 0–0 transition energy (32304 cm-1) calculated at the B3LYP/TZVP level equaled 797 cm-1. This value was subtracted from the ADC(2) absorption energies of the S0→S1 transition to obtain the 0–0 band position at the ADC(2) level. A comparison with the experimental value of 32030 cm–1 (312.3 nm) acquired in the gas phase

47

gives the correction

factor of 897 cm–1 (0.111 eV), which was used to correct the calculated absorption spectra of the monomer in the gas phase, water, and ice (see Table 2). The interactions with the liquid and ice-surface water molecules caused small bathochromic shifts of the absorption energies with respect to those of isolated naphthalene. Only negligible shifts of 1–2 nm were obtained for the absorption wavelengths of the S1water and S1ice states. Note that the positions of the relevant 0–0 bands are in excellent agreement with the experiment (Table 3). Although the shifts observed for the S2 and S3 states are more pronounced, they do not exceed the value of 6 nm.

Table 2. The calculated vertical excitation energies (Ecalc) and oscillator strengths (f) of the naphthalene monomer; the corrected excitation energies Ecorr; and the corresponding wavelengths of the absorption peaks (λcorr) for the given excited states. Ecalc/eVa f

S1vacuum 4.428

3×10–5

Ecorr/eV (λcorr/nm)b

literature data 4.03,88

4.45,89

4.41,89 4.317 (287.2)

4.27,90 4.24,91 4.26 92

S2vacuum 4.931

0.107

4.56,88

4.96,89

5.21,89 4.820 (257.2)

5.03,90 4.77,91 4.6292

S3vacuum 6.205

1.636

5.39,88

6.22,89

6.23,89 6.094 (203.5)

5.98,90 5.90,91 6.0592


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S1water

4.407

8×10–4

4.296 (288.6)

S2water

4.828

0.100

4.717 (262.9)

S3water

6.006

1.010

5.895 (210.3)

S1ice

4.413

4×10–3

4.302 (288.2)

Sice 2

4.885

0.104

4.774 (259.7)

Sice 3

6.092

1.057

5.981 (207.3)

a

Calculated with the ADC(2)/TZVP level with a geometry optimized at the B97D/TZVP level.

b

Corrected by subtraction of 0.111eV (897 cm-1).

Table 3. The calculated S0→S1 0–0 transition energies corrected by subtraction of 0.111eV (897 cm-1). The experimental values are given in parentheses. λcorr/nm S1vacuum

312.30 (312.3)a

S1water

313.96 (314.3)b

S1ice a

313.49 (312.3, 314.2)b

This value47 was used to obtain the correction factor. bA value based on this study.

To assign the experimental absorption/excitation bands of naphthalene associates, the calculations of the vertical excitation energies were performed for dimers in the gas phase and for naphthalene interacting with the given ice surface. In the latter calculations, the mutual orientation of the two monomers was obtained via ground-state optimization on water-molecule 23 ACS Paragon Plus Environment


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clusters as described before.7 For the gas phase calculations, two different parallel displaced structures, namely (N(S0)-PD1 and N(S0)-PD2), and a T-shape structure (N(S0)-TS) were considered (Figure 7). Similar structures for naphthalene dimers were found in the molecular dynamics simulations of naphthalene at the ice/air surface performed by Ardura.43

Figure 7. The cluster models of naphthalene dimer associates in the gas phase: the parallel displaced (Np(S0)-PD1), (Np(S0)-PD2) and a T-shape (Np(S0)-TS) structures.


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Figure 8. The cluster models of naphthalene dimer associates on ice surface: the parallel displaced (Np(S0)-PD3) and tilted T-shape (Np(S0)-TST) structures used for the calculations of the vertical excitation energies. Two representative orientations of naphthalene monomers on ice surface were also considered, namely the parallel displaced Np(S0)-PD3 and tilted T-shape Np(S0)-TST structures (Figure 8). The vertical excitation energies of the eight lowest excited states corrected by subtraction of 0.111eV (897 cm–1) and the corresponding wavelengths and oscillator strengths are all listed in Table 4. Table 4. The corrected vertical excitation energies (Ecorr)a,b, absorption band maxima (λcorr), and oscillator strengths (f) of naphthalene dimers optimized in the gas phase and on ice surface for the given excited states. Ecorr /eV ( λcorr /nm) f

Np-(S0)PD1c

S1d

S2d

S3d

S4d

S5d

S6d

S7d

S8d

4.268

4.281

4.708

4.753

5.340

5.373

5.687

5.887

( 290.5)

(289.6)

(218.0)

(210.6)

(263.3) (260.8) (232.2) (230.7)



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Np(S0)-PD2c

Np(S0)-TSc

Np(S0)-PD3d

Np(S0)-TSTd

a

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5 × 10–4

5 × 10–4

0.138

0.031

0.080

0.007

0.230

5 × 10–4

4.262

4.273

4.688

4.73

5.223

5.268

5.294

5.673

(290.9)

(290.1)

(234.2)

(218.5)

0.001

4 × 10–5

0.008

0.140

0.006

0.018

0.035

0.018

4.297

4.319

4.726

4.776

5.188

5.715

5.945

5.971

(288.5)

(287.1)

(208.5)

(207.6)

4 × 10–5

1 × 10–6

0.082

0.137

0.007

0.003)

1 × 10–4

0.005)

4.296

4.681

4.756

4.914

5.437

5.745

5.926

5.979

(288.6)

(264.9)

(209.2)

(213.9)

0.002

0.131

0.081

0.011

0.055

0.014

1.068

0.913

4.287

4.311

4.709

4.755

5.730

5.876

5.971

6.003

(289.2)

(287.6)

(207.6)

(206.5)

5 × 10–4

9 × 10–4

0.083

0.034

(264.5) (262.5) (237.4) (235.3)

(262.3) (259.6) (239.0) (216.9)

(260.7) (252.3) (228.0) (215.8)

(263.3) (260.7) (216.4) (211.0) 0.062

0.115

0.875

0.797

Corrected by subtraction of 0.111eV (897 cm-1). bThe charge transfer states are indicated in

bold. cOptimized in the gas phase. dOptimized on ice surface.

In Table 4, the S1d and S2d states of the dimers resulted from a linear combination of the S1 states of the two monomers. Their transition energies were in the interval of 286–290 nm. The next pair of the S3d and S4d states of the dimers was a result of a linear combination of the S2 state of the monomer. The energies of these states were slightly shifted to a smaller value from the monomer S2 state, with the largest value of 7 nm. The calculations predicted new bands in the region of approximately 220–240 nm in the spectra of the dimers. The wavefunctions of all eight


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states of both parallel-displaced structures in the gas phase, (Np(S0)-PD1 and Np(S0)-PD2), were characterized by electronic excitations between completely delocalized molecular π orbitals. In the first four excited states of a T-shape structure, the wavefunctions were localized on one of the monomers. Contribution of the charge transfer between the subunits in higher states appeared in addition to a localized character of the excitation. The overlap of monomers in the Np(S0)-PD3 complex was much smaller compared to that in the gas-phase optimized structures, which was reflected by a different character of the excited states wavefunctions. In particular, the first four states were localized on one monomer. The higher states had also a significant charge transfer character. The same trends were found for the Np(S0)-TST structure, which exhibits a localized character of the S1d–S4d states and a charge transfer character of the higher excited states. The optimization of the S1 state of naphthalene led to the structure characterized by the emission energy of 4.136 eV (300 nm). For the emission spectra interpretation, a uniform shift of 0.187 eV was applied to correct the calculated energy of the fluorescence band at 314.6 nm observed experimentally. Three S1 minima (see Figure 9) were found during the optimization in the gas phase, in particular, the parallel displaced (Np(S1)-PD1 and Np(S1)-PD2) and face-toface (Np(S1)-FF) structures. The resulting emission energies are given in Table 5. The corrected S1→S0 transitions of naphthalene dimers spanned a broad energy region of 2.7–3.5 eV corresponding to 355–458 nm. The results show that the emission wavelength is directly proportional to the extent of stacking (Figure 9 and Table 5). The structures with stronger stacking, and consequently a larger orbital overlap, were found to emit at longer wavelengths.



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Figure 9. The excited state minima of naphthalene dimer optimized at the ADC(2)/TZVP: the parallel displaced (Np(S1)-PD1 and Np(S1)-PD2) and face-to-face (Np(S1)-FF) structures.


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Table 5. The gas-phase energies of the S1→S0 transitions and emission bands of the naphthalene monomer and dimers. species

∆E(S1→ S0)a/eV (∆λem/nm)

monomer

3.949 (314)

Np(S1)-PD1

2.895 (428)

Np(S1)-PD2

3.489 (355)

Np(S1)-FF

2.708 (458)

a

Corrected by subtraction of 0.187 eV.

Discussion The Absorption Spectrum of Naphthalene on Ice. The absorption spectrum of aqueous naphthalene is characterized by three electronic transition systems (Figure 1 and Figure S1). The transition to the lowest excited state is parity-forbidden, with the molar absorption coefficient of approximately 200 M–1 cm–1 at 314.3 nm (Figure S1). However, this absorption band is the most relevant to terrestrial environmental conditions as the sunlight intensity ratio at 315/280 nm typically approaches 1000:193 at Earth’s surface. Our spectroscopic and computational analyses of naphthalene in frozen aqueous solutions show a negligible shift of the excitation/absorption energies with respect to those of the liquid and gas phases (Table 3). The results of the experiments are discussed in the following text to justify this finding. The naphthalene 0–0 transition was reported to be very insensitive to solvents (314.9 nm in hexane47,48 and ethanol54) and phase change (312.3 nm in the gas phase;47-49 317.7 and 316.2 nm in crystals50,51,53,94). No substantial shift in the excitation spectrum of 1-methylnaphalene was



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observed in/on ice surface,8 and therefore it would be surprising to see any larger band shift after freezing naphthalene solutions. The vibrational resolution of the electronic transitions can serve as a guideline for the band assignments. However, the vibrational progression of naphthalene is more complex than that of, for example, benzene.7 By comparison to the gas phase vibrationally resolved excitation spectra,95 we were able to assign the lowest vibrational bands of naphthalene in condensed media as the hot (319.2 nm, 0–0 – 506 cm-1), 0–0 (314.3 nm), and 0–I bands (309.8 nm) to absorption to the 8'1 (0–0 + 438 cm-1, b1g) or 91 (0–0 + 501 cm-1, ag) vibronic levels of S1 (Figure S2). The assignment of the other absorption bands is rather ambiguous; the band at 305 nm may correspond to absorption to the 8'1 91 (0–0 + 938 cm-1, b1g) vibronic levels. The calculated vibrational structure of the S0→S1 electronic state reproduces the experimental data with excellent accuracy. Our comparison of the naphthalene aqueous solution fluorescence excitation spectrum with the calculated vibrationally-resolved electronic S0→S1 spectrum (Figure 2) shows a nearly exact match in the peak positions. The observed hot band (at 319.2 nm) is not shown in the simulation, because only the vibrationally cooled ground state was considered in the calculation. The calculated relative band intensities are not expected to match the experimental data perfectly. The gradual intensity increase towards lower wavelengths can be caused by the S1→S2 mixing or intensity borrowing.54 The assignment discussed above leaves no doubt that the observed transition belongs to the S0→S1 electronic transition of naphthalene monomer and will help to interpret the spectra obtained in the frozen aqueous solutions. To our knowledge, a detailed analysis of the S0→S1 transition of aqueous naphthalene has not been reported yet; in reality, however, the energy difference is quite close to that found in isooctane50, hexane,48 and ethanol.54 30 ACS Paragon Plus Environment

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Our UV diffuse reflectance measurement of naphthalene crystals mixed with silica showed the lowest energy hot absorption band at 319.4 nm. The next five vibrational bands closely matched those calculated for the gas phase (Figure 3). Such agreement suggests that the interactions between naphthalene entities in a crystal do not influence the spectra. This has been already discussed in the literature59 and will be mentioned below. A low signal-to-noise ratio of the diffuse reflectance absorption of naphthalene in the frozen aqueous solution (Figure 4) did not allow for a vibrational resolution of the S0→S1 electronic transition. The most prominent band in this region is located at 313.6 nm and corresponds to the 0–0 vibration of the S0→S1 electronic transition. The positions of the S0→S2 vibrational transitions in the case of liquid and frozen aq naphthalene are comparable within the experimental error. A much better vibrational resolution of the naphthalene S0–S1 electronic transition in frozen aqueous solutions was brought in by the fluorescence excitation spectrum (Figure 5). There are bands corresponding to those described in aqueous solutions, and a new series of peaks not observed in the solutions is apparent (Table 1). This can be explained by a hypsochromic band shift of approximately 200 cm–1 found in liquid aqueous solutions. Whether these new peaks result from a better vibrational resolution in water matrix at a lower temperature or whether they are related to the new microenvironment in the frozen aq solution remains an open question. We did not observe any eminent signals in the excitation spectra of frozen aq naphthalene solution above 320 nm (Figure 5 and Figure S4) for any emission wavelengths examined. Note that the steep rise in intensity above 320 nm (Figure 5c, λem= 333 nm, green) is caused by scattered excitation light. However, trace impurities in the naphthalene samples were reported to



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exhibit absorption in this region.53,86 Importantly, the significant, lowest-energy band in a frozen solution at 315.7 nm is not red shifted compared to that occurring in an aqueous solution. Another argument on the position of the 0–0 band of the S0→S1 transition can be obtained from the emission spectra for various conditions. The emission bands corresponding to the 0–0 transitions for the liquid and frozen aqueous solutions exhibit the difference of only 1nm. In the case of the frozen aqueous solutions, this emission can arise either from separated naphthalene monomers interacting with ice or from naphthalene microcrystals formed upon freezing. The absence of a significant shift of the 0–0 emission bands gives a limit to the position of the 0–0 absorption band in a frozen environment, as also observed in the corresponding excitation spectra. As a result, none of the discussed interactions - neither the freezing reinforced naphthalene-water ones nor those within microcrystals - can lead to an absorption shift. The effect of interactions between monomers in dimers is discussed in the following paragraphs. In agreement with our experimental observations, the calculated S0→S1 vertical excitation energies of naphthalene in water and ice are close to the value obtained in the gas phase (Table 4). Our calculations predict that the S0→S1 transition is very weak in all environments; the oscillator strengths were found in the range of 3 × 10–5 – 4 × 10–3. Such low values cannot be considered to predict the changes in relative intensities discussed above. The oscillator strength change of the discussed transitions is difficult to evaluate. The main problem of the experimental approach lies in the heterogeneous composition of the ice sample and its scattering. The Kubelka-Munk remission function of diffuse reflectance is related to molar absorbance but also to an unknown wavelength-dependent sample scattering factor, and it is therefore not directly comparable to the absorbance of solutions. Further, the intensity of emitted light in the excitation spectrum of ice samples cannot be compared to that of aqueous


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solutions due to undefined ice scattering and, therefore, unknown probed volume by the excitation source. Bree et al.50 reported that the oscillator strengths are essentially the same in crystals and aqueous solutions. Since no change in the oscillator strength was observed upon the gas phase-to-solution transition,47,50 we do not expect it to be significantly influenced by freezing. The oscillator strength, however, decreases upon the formation of dimers. For example, the absorption spectra of anthracene dimer derivatives showed a small bathochromic shift and reduction of the oscillator strength by the factor of 2 in comparison to that of the monomer.96 Note that this behavior was proposed to be a general trend for dimers.97 At this moment, we can conclude that the relevant literature, together with our experiments and calculations, agrees that the shift of the 0–0 band of the S0→S1 transition in different environments of frozen aqueous solutions is not larger than 3 nm. An evaluation of the oscillator strength change is rather more difficult since we cannot provide its direct determination. Nevertheless, the literature suggests that the oscillator strength remains constant for monomers and is reduced if dimers are formed. 96,97 Naphthalene Samples Contaminated by Anthracene. The luminescence of the naphthalene crystals obtained from Lachema (resublimed) and from Fluka (analytical standard) was examined: the former crystals showed typical naphthalene fluorescence85,98 (black line in Figure 6a), whereas the latter crystals exhibited - besides the naphthalene emission - significant emission bands at 387, 412, and 437 nm (black line in Figure 6b) assigned to the anthracene emission. The GC-MS analysis of our samples (Fluka) provided no evidence of anthracene impurity. However, anthracene is a common impurity of naphthalene, and energy transfer was observed in mixed crystals of the two compounds; it was found that a ratio as low as 1: 10–7 is sufficient to observe an efficient anthracene fluorescence.86 The above-mentioned vibrationally



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resolved bands of anthracene show the very regular solvatochromic spectral shifts of (575 ± 77) and (411 ± 52) cm-1 against corresponding bands in water and methylcyclohexane, respectively.99 The excimer emission in the solutions is typically characterized by a structureless band;59 therefore, it was surprising to observe structured, regularly spaced luminescence bands above 380 nm in the naphthalene frozen aqueous solutions. The contamination of the crystals naturally affects also the prepared frozen aqueous solutions, which is obvious from the indicated emission of anthracene (Figure 6b). In the frozen aqueous solution of sublimed naphthalene (Lachema), the anthracene emission is very weak (Figure 6a). A good match between the excitation spectra measured at naphthalene and anthracene specific emission wavelengths (Figure 5 and Figure S4) confirmed that the higher electronic state of anthracene was populated via the energy transfer from the photoexcited naphthalene donor. Previously, these emission bands were erroneously interpreted as naphthalene excimers by Donaldson and Kahan 9,43,62 The anthracene emission bands (at 387, 412, and 437 nm) were superimposed on a broad naphthalene-origin shoulder, and they were quite variable – at times clearly observable, at other times vanishing in the noise (two blue lines in Figure 6b demonstrate the variability). We did not find a cause-and-effect relationship between the sample preparation and intensities of these bands. The emission bands cannot be assigned to phosphorescence either, as it was observed only in the spectra above 469 nm (Figure S5).100 We measured the phosphorescence spectra of frozen aqueous solutions with a 1-ms time delay obtaining the peaks at 469.5, 480, 503, 518, 543 and 588 nm and, therefore, the bands at 472 and 482 nm reported in the previous work of Malley and Kahan62 are also probably of phosphorescence origin. Naphthalene Excimers. The emission spectra of naphthalene of the highest available purity (Lachema, sublimed) with minimized anthracene fluorescence are shown in Figure 6a. The 34 ACS Paragon Plus Environment

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spectra in solid phases (the crystals and frozen aqueous solution) show diminished intensities of the highest energy bands at wavelengths lower than 325 nm, probably caused by self-absorption compared to the aqueous solutions. The frozen aqueous solution shows an increased emission intensity in the range of 340–500 nm compared to that of the liquid aqueous solution or crystals. We interpret this shoulder as an indication for the existence of naphthalene excimers. A similar observation was assigned to naphthalene emission in silica glasses56 with λmax = 375 nm and on silica gel57 with λmax = 390 nm. The positions of the observed excimer emissions fall well within both the range of the values previously observed (λmax = 375–440 nm)

101

and our calculations

(355–458 nm). The ADC(2) calculations were performed to support the interpretation of the observed emission; they, for example, indicate how the emission energies of the excimers strongly depend on their structures (see Figure 9 and Table 5). A small overlap between the two monomers can result in an emission whose wavelength is very close to that of monomers. The calculations also show that these interactions influence the electronic states of naphthalene significantly as excimer stabilizations are observed. In our previously reported studies on 1methylnaphthalene,8 the emission peak of the T-shape structures overlapped with that of the monomer. On the contrary, the stacking stabilizes the excimers substantially, and the emission is predicted to be at as far as 473 nm. The experimentally observed elevated baseline (from 340 to 500 nm) described herein indicates that the structure and consequently the interaction energies of the excimers gradually change. The excimer emission observed as a shoulder was also reported for benzene,7,102 in contrast to the separate band characterizing excimers of pyrene and other aromatic compounds.55 The excimer emission on surfaces is often manifested by a broad shoulder extending bathochromically from the monomer emission, as opposed to the excimers in solutions most 35 ACS Paragon Plus Environment


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often exhibiting a distinct well-spaced band, red-shifted from the monomer.55 A probable explanation can be deduced from our calculations: the liquid phase enables the excimers to adopt energetically more stable orientations, whereas the constraints caused by the solid phase do not allow for reorientation, and therefore less stabilized excimers, emitting at wavelengths closer to that of the monomer, are observed. A detailed examination of the emission spectrum of the frozen aqueous solution (Lachema) revealed very small but discernible bands at 413, 438, 470, 480, 503 nm. The 413 and 438 nm bands most likely belong to the anthracene (vide supra), while those above 470 nm can be attributed to the phosphorescence of naphthalene. The appearance of anthracene bands in the frozen aqueous solution compared to their invisibility in the crystals can lead to the question of why the energy transfer from naphthalene to anthracene is more efficient in frozen aqueous solutions than in naphthalene crystals. Viable explanations can be sought in a more efficient naphthalene-to-anthracene energy transfer caused by either a more homogeneous distribution of the compounds in microcrystals formed upon freezing or longer range energy transfer within the veins in ice. We cannot give a definitive answer based on our current observations, and the matter will be of concern in our further research. We attempted a simple quantification procedure, but the results were inconclusive as the mechanism of energy transfer in solid media is complex. 86,103 Similarly, luminescence (very likely ascribable to the anthracene emission) was observed only after annealing and crystallization of amorphous films prepared on a cold gold coated substrate.104 No anthracene luminescence was observed from either the original crystalline naphthalene (with monomer emission) or the 40 K deposited amorphous films.


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Interpretation. The strong similarity of the fluorescence emission spectra of the frozen aqueous solutions to those of the crystals (Figure 6a) at wavelengths below 330 nm and the enhanced propensity for naphthalene-anthracene energy transfer are indicative of the presence of microcrystals in the frozen aqueous solutions. By cooling a nearly saturated solution (at room temperature), crystallization is expected to occur. The emission spectrum of a frozen aqueous solution (blue line, Figure 6a) above 330 nm shows clear signs of crystals, namely resolved bands, which are superimposed on the broad emission that corresponds to the excimers. Due to the lattice arrangement not allowing for any stacking interactions, naphthalene single crystals are known to exhibit solely monomer emission.59 At high pressures or at the crystal lattice defect locations, however, the molecular orientations allowing electronic interactions that lead to energy delocalization are possible; the resulting fluorescence was observed for the naphthalene crystals exposed to high pressure and UV light simultaneously.105 Our calculations predicted an increased absorbance in the frozen solution within the energy region of approximately 235 nm. This is due to the stacked and also the weakly interacting T-shape dimers. The excitation spectrum recorded at 412 nm reveals an increased absorbance at approximately 235 nm (Figure 5a); however, anthracene has a strong absorption at this wavelength range, and its direct excitation cannot be excluded. The presence of ground state complexes is thus not clearly derivable from our experimental data. We reassign the bands previously interpreted as naphthalene excimer emission9,62 to those of anthracene fluorescence, provide experimental and theoretical evidence for the formation of naphthalene excimers (whose signal was hidden below anthracene fluorescence in the previous studies), and bring strong support for a negligible red shift in the absorption spectrum of naphthalene on ice in contrast to the work of Donaldson,9 who proposed that the 37 ACS Paragon Plus Environment


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bathochromic absorption shift at least partially explains a higher naphthalene reactivity on ice upon irradiation at longer wavelengths. The question of whether crystals, rather than separated monomers, are formed during the freezing of aqueous solutions cannot be unambiguously answered by the experiments reported in this study, because both monomers with perfect crystals and constrained aggregates with defective crystals are expected to emit in the same spectral regions centered at 330 and 400 nm, respectively. Below we propose a comparison of the observed excimer emission of naphthalene and 1-methylnaphthalene to provide tentative answers. While the excimeric emission of naphthalene is manifested by a shoulder in the range of between 340 and 500 nm, broad unresolved bands at λmax ~350 and 390 nm were observed for 1methylnaphthalene.8 It is likely that these differences result from compound-specific behavior upon freezing, namely, the growth of microcrystals in the case of naphthalene and the formation of frozen amorphous solid in the case of 1-methylnaphthalene (the melting point of 1methylnaphthalene106 is 251 K). Both emission bands correspond to those of the individual phases at the given temperatures. In the case of the naphthalene-naphthalene or 1methylnaphthalene-1-methylnaphthalene interactions in dimers solvated by water molecules, the spectral behavior should be similar for both compounds. Hence, we are inclined to explain the observed emission as a result of the formed microcrystals (for naphthalene) or micro droplets (for 1-methylnaphthalene) frozen in/on the ice grains, with minor contribution of dimers formed at the disordered interface of ice crystals. Natural snow was shown to undergo metamorphism accompanied by sintering.4,34,35,107 During this process, small snowflakes are gradually changed into a sintered ice material with large crystals. Dry or wet sintering, both common in nature, moves the masses of water present 38 ACS Paragon Plus Environment

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in ice, and therefore redistribution and aggregations of the impurities are expected.4,34,108 The veins among the ice crystals containing highly concentrated mixtures of solutes can lead to solute crystallization, as demonstrated via scanning electron microscopy for several natural ices.

109-112

As a result, our experiment can, in some cases, model naturally occurring behavior when solutes are aggregated in frozen aqueous matrices.

Conclusion Our spectroscopic analyses employing the absorption and fluorescence of naphthalene in frozen aqueous solutions, interpreted through the use of high-level quantum chemical calculations (ADC2), revealed that naphthalene is aggregated upon freezing. It is shown that the position of the lowest absorption band of naphthalene does not change upon interaction with ice. The interactions of naphthalene with ice did not exhibit any change in the absorption spectrum, and the naphthalene–naphthalene interactions do not shift the absorption maxima by more than 3 nm. The excimer emission signals are well distinguished from the phosphorescence signals and explained by the existence of stacked structures of various geometries. When naphthalene is contaminated by anthracene, we observe the naphthalene-to-anthracene energy transfer in frozen aqueous solutions of naphthalene but not in the source solution measured at room temperature. The results agree with the overall picture of frozen aqueous solutions of contaminants that form aggregates on the internal or external ice surfaces and in the veins or pockets within the frozen matrix, where increased solute concentrations may facilitate crystallization.



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Supporting Information The absorption and fluorescence excitation spectra of naphthalene in, the normalized fluorescence excitation and spectra of a naphthalene solution in water, the fluorescence emission spectrum of a frozen solution of naphthalene in methylcyclohexane, the fluorescence excitation spectra of a frozen aq solution of naphthalene and he phosphorescence emission spectrum of a frozen aq solution of naphthalene in water. This material is available free of charge via the Internet at http://pubs.acs.org

Acknowledgement

The authors would like to thank to Joggi Wirz and Rafał Kania for their beneficial comments and related discussions, Jaromír Literák for GC-MS analysis. This project was supported by the National Sustainability Programme of the Czech Ministry of Education, Youth and Sports (LO1214) and the RECETOX research infrastructure (LM2011028) and the grand of Czech Science Foundation (15-12386S). This work was part of the research project RVO:61388963 of the Institute of Organic Chemistry and Biochemistry AS CR, v.v.i.

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287x401mm (300 x 300 DPI)



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Spectroscopic Properties of Naphthalene on the Surface of Ice Grains

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