Solid State Photochemistry of Hydroxylated Naphthalenes on Minerals

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Solid State Photochemistry of Hydroxylated Naphthalenes on Minerals: Probing Polycyclic Aromatic Hydrocarbon Transformation Pathways under Astrochemically-Relevant Conditions Simone Potenti, Paola Manini, Teresa Fornaro, Giovanni Poggiali, Orlando Crescenzi, Alessandra Napolitano, John Robert Brucato, Vincenzo Barone, and Marco d'Ischia ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00060 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Earth and Space Chemistry

Solid State Photochemistry of Hydroxylated Naphthalenes on Minerals: Probing Polycyclic Aromatic Hydrocarbon Transformation Pathways under Astrochemically-Relevant Conditions Simone Potenti,† Paola Manini,§ Teresa Fornaro,~ Giovanni Poggiali,‡,# Orlando Crescenzi,§ Alessandra Napolitano,§ John R. Brucato,# Vincenzo Barone,† Marco d’Ischia§*

†

Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy

§

Department of Chemical Sciences, University of Naples Federico II, Complesso Universitario

Monte S. Angelo, Via Cintia 4, 80126 Napoli, Italy ~

Geophysical Laboratory of the Carnegie Institution for Science, 5251 Broad Branch Rd. NW,

Washington, DC 20015, USA ‡

University of Florence, Department of Physics and Astronomy, Via Sansone 1, 50019 Sesto

Fiorentino, Italy #

INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

ACS Paragon Plus Environment

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KEYWORDS: oxygenated polycyclic aromatic hydrocarbons, astrochemistry, oxidative polymerization, mineral coating, Mars relevant irradiation conditions

ABSTRACT. Oxygenated derivatives of polycyclic aromatic hydrocarbons (PAHs), or oxyPAHs, recently captured the interest of the scientific community for their photochemical reactivity in a water ice matrix mimicking the interstellar medium. Furthermore, oxyPAHs are interesting molecules for the study of the origin of life for their prebiotic potential. However, their stability and transformation pathways under astrophysically relevant conditions have remained largely unexplored. Herein we report the photochemical behavior of 1-naphthol (1HN), 1,6- and 1,8-dihydroxynaphthalene (DHN) either as pure powdered solids or adsorbed on forsterite or anatase surface. All the compounds showed an extensive decrease of main vibrational bands, accompanied in the case of DHNs by the formation of new molecular species. Irradiation of 1,8-DHN at 80 K resulted in the IR-detectable generation of CO2 (2340 cm–1), a process reported by other authors following irradiation of PAHs in water ice analogues at 14 K. These results, when compared to model autoxidation experiments, indicated a high susceptibility of hydroxylated naphthalene derivatives to UV radiation leading to free radical and carbonylcontaining extended quinone intermediates (preliminary DFT calculations) with partial degradation and decarboxylation. Based on these results, oxyPAH formation and photoprocessing on minerals is proposed as a plausible pathway of PAHs transformation under astrochemical conditions of prebiotic relevance.


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TABLE OF CONTENTS GRAPHIC


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INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) and related organic systems are among the major carbon-bearing components present in astrophysical environments, e.g. diffuse and dense interstellar clouds, and it is generally accepted that they carry up to 20 % of the total elemental carbon.1 PAHs can be produced in circumstellar carbon-rich envelopes as well as in cold interstellar environments mainly through collisional reactions occurring in the gas-phase via very fast route reactions that can take place at temperatures as low as 10 K.1–6 Evidence of PAHs in the interstellar medium (ISM) is based on infrared emission bands observed in the range of 3–14 µm (3300–700 cm–1) and on the spectral signatures of the socalled diffuse interstellar bands.7–10 In cold molecular clouds, PAHs are supposed to be present as agglomerates, charged clusters or very small grains11–15 which can provide nucleation sites for atomic species and small molecules forming small dust grains covered with ice mantles. Laboratory experiments showed that isolated PAHs can also be embodied in icy grain mantles.16 However, all attempts made to detect PAHs on cold interstellar ice grains have failed so far.17,18 Whereas the identification of PAHs in the interstellar medium is of relevance to the origin of IR emissions from Earth- or space-based spectroscopic observations, their occurrence on planetary surfaces and environments, especially Mars surface, has been implicated in current theories on abiogenesis in astrobiology. PAHs were originally supposed to be present on Mars based on their identification in carbonaceous chondritic meteorites19 – naphthalene, anthracene, phenanthrene, pyrene, fluoranthene and their methylated derivatives were amongst detected PAHs20 – and in Martian meteorites,21–23 including the Zag and Monahans (1998) meteorites24 and the shergottite meteorites Elephant Moraine 79001 (EETA79001) and Allan Hills 84001


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(ALH84001).25 Definitive detection of organic compounds in the Martian regolith however awaited data from the Sample Analysis at Mars (SAM) instrument on board the Curiosity rover in Gale Crater.26–28 Very recently, evolved gas analysis performed by SAM on pyrolyzed (> 500°C) drilled samples revealed notably thiophene derivatives and a peak near detection limits compatible with the presence of naphthalene.29 One possible source of PAHs and other organic compounds on Mars may be provided by interplanetary dust particles (IDPs) and micrometeorites.30,31 Micrometeorites are small extraterrestrial dust particles typically in the range 50 µm to 2 mm, while IDPs are small grains with size generally less of hundred micrometers. Micrometeorites may also contain exceptionally high carbon content in the form of organic matter, such as the ultracarbonaceous Antarctic micrometeorites (UCAMMs)32 and IDPs were found with up to 10% of organic carbon by mass.33,34 Laboratory analyses demonstrated that they both contain extra-terrestrial molecules including PAHs and their alkylated derivatives, aliphatic hydrocarbons and ketones.35–37 Furthermore, carbonaceous chondrites (CC) are dominated by two components: a solvent insoluble macromolecular fraction (insoluble organic matter, IOM), usually representing more than 75% (w/w) of the organic matter, and the residual fraction, which is composed by solventsoluble matter.19,38–40 The structure of the IOM of some carbonaceous meteorites is clearly different from terrestrial coals and kerogens,41 as it consists of small aromatic units linked by short and branched aliphatic chains,42–48 suggesting an abiotic process that could be related to chemistry in ice.47 Interest in the astrochemistry of PAHs relates mainly to the so-called PAH world hypothesis dominating astrobiological theories in the past decades.5 According to this theory, stacked PAH layers composing aggregate structures may have served as templates for the


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assembly of nucleic acid bases. PAH deposits within ALH 8400149 have been proposed to arise from decay products of Martian organisms.21,22,50 Alternatively, PAHs might have been synthesized on Mars from hydrogen- and carbon monoxide-rich volcanic gases or hydrothermal fluids.51 Given the widespread occurrence of PAHs in astronomically-relevant environments, a crucial question is whether and to what extent PAHs may be susceptible to degradation by UV and cosmic radiation.52–54 Irradiation of naphthalene and other PAHs in a water ice matrix has been shown to lead to oxygenated products, including mainly phenolic and quinone derivatives, e.g. 1-naphthol.55–58 Although hydrogenation and oxygenation reactions may occur on PAHs,59 little is known on the kinetics of the reactions between PAHs and H, O, or OH radicals, and whether such reactions can occur at low temperatures, e.g. 10 K, or if they are active at higher temperatures, which are more representative of planetary environments. Unfortunately, the dominant spectral features of the water ice matrix overshadow those of PAHs or of simple hydrocarbons, whereby elucidation of radiation-induced processing in astrophysical ices remains a challenging task. In this context, a critical role in PAHs reprocessing may be played by dust grain chemistry. Minerals, especially silicates, are ubiquitous in space including comets, circumstellar envelopes around young stars, evolved stars and planetary nebulae60 and have been suggested to play a role as catalysts.61,62 On Mars, silicates such as olivine, pyroxene and plagioclase are very abundant63–65 and are able to trap several astrochemically relevant CHON-bearing molecules on their surface. However, apart from studies on the conversion of naphthalene and dihydroxynaphthalenes to oxygenated derivatives,66 the transformation of oxyPAHs on mineral


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surface under Martian relevant irradiation conditions67 and the possible role of these processes in prebiotic chemistry have remained virtually unaddressed. Herein, we report the results of a proof-of-concept study directed to probe the photochemical susceptibility of some representative components of the putative astrochemical pool of oxyPAHs,

namely

1-naphthol

(1-hydroxynaphthalene,

1-HN),

1,6-

and

1,8-

dihydroxynaphthalene (DHN) under conditions of relevance to prebiotic processes on Martian surface. OH

OH

OH OH

HO 1-HN

1,6-DHN

1,8-DHN

The aim of the study was to investigate the stability and photochemical behavior of representative oxyPAHs interacting with Martian-type minerals, i.e. forsterite and anatase under irradiation conditions that mimic those found on the planet surface. Although forsterite is of great relevance in a plethora of astrophysical environments,64,68–70 anatase has no particular astrochemical importance and was included for comparison because of the presence of OH-type surface groups71,72 similar to those found on forsterite and for its well-known photocatalytic properties.73,74 According to its spectral features, anatase surface is supposed to display hydroxyl groups like forsterite. More specifically, the study was directed to assess whether PAH structure affects photochemical pathways, whether these latter are subject to mineral-specific effects and whether photodegradation products of oxyPAHs adsorbed on minerals share features in common with melanin-type polymers generated by autoxidative processes on mineral surface. Substrate


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selection was guided by the reported generation of 1-HN under astrophysically relevant conditions, suggesting its presence in the PAHs pool delivered onto planetary surfaces. The two structurally-related DHNs share with 1-HN an OH group on the 1-position of the naphthalene system and exhibit a meta-like substitution pattern, which imparts relative resistance to oxidation and polymerization compared to ortho/para isomers because of the lack of easily accessible routes to stable semiquinone/quinone products. Studies of the photochemistry of 1,8-DHN were expected to benefit moreover from the recent elucidation of the oxidation chemistry and mode of polymerization of this peculiar system.75,76

EXPERIMENTAL SECTION FT-IR spectra of 1-HN, 1,8-DHN, 1,6-DHN, either pure or adsorbed on forsterite and anatase, were obtained between 8000 and 400 cm–1, with 4 cm–1 resolution, using a Bruker Vertex 70v instrument equipped with a Praying MantisTM Diffuse Reflection Accessory (Harrick DRIFT). UV irradiation was performed using a Newport Hg-Xe 300W lamp equipped with an EOP140 UV fiber and samples were irradiated under a flux of nitrogen to limit the role of O2 in photochemical processes to better reproduce non-terrestrial conditions. Mineral powders were obtained from raw mineral samples after milling and sieving procedures. Organic contaminants were removed by 30-minute sonication of mineral dust in water and methanol, alternating and repeating them until the complete disappearance of characteristic IR signals of organics was achieved. Then the mineral powder was separated by centrifugation and dried in oven.


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A methanol solution of the selected HN was prepared and added dropwise to the mineral powder in a test tube. As long as it filled the porosity, no separate liquid phase was observed. The addition of the solution was stopped at “incipient wetness” (the porosity is completely filled). The test tube was vortexed and sonicated for few seconds to homogenize, and the solvent was dried in mild conditions (50 °C in oven). The mud formed during the drying process was accurately broken into pieces in order to ease further drying. No aliphatic C–H features were observed in the IR spectra of the dried samples, confirming the complete removal of methanol. Despite the well-known volatility of HNs, their IR features were unequivocally observed after drying phase. Therefore, no particular caution was used to prevent their eventual partial loss. The absorbed volume at “incipient wetness” was first estimated in a separate experiment, adding pure solvent dropwise to a known amount of dry mineral powder until a separate liquid phase was observed. Spiking solutions were prepared in order to achieve a 1:20 molecule/mineral ratio (on a mass/mass basis). Spiked mineral powder sample was accurately placed in the properly cleaned and dried DRIFTS sample vessel. The latter was put in its housing inside the spectrometer sample compartment and the UV fiber was put in front of the open window of the sample compartment for UV irradiation. The system was isolated and UV irradiation was conducted under a gaseous nitrogen flow. The infrared reflectance spectra were then recorded in situ at known intervals of time, with the UV lamp off during spectrum acquisition (see Figure 1 in the Results and Discussion section).

For low-temperature analyses, the spiked mineral powder was put in a sample vessel inside a cold finger, equipped with a sapphire window (transparent to UV-Vis and IR, up to 1500


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cm–1), cooled by liquid nitrogen. Before cooling down the cold finger, it was slowly evacuated (final P = 10−5−10−6 mbar) by means of a vacuum pump, and the system was kept under vacuum at 300 K (dynamical vacuum) overnight to remove residual water. No significant changes in the DRIFTS spectra were observed, hence, significant loss of HNs was ruled out. Then, the temperature was lowered up to 80 K. The temperature of the apparatus was maintained at 80 K during UV irradiation. A schematic representation of the experimental setup is shown in Figure 2 in the Results and Discussion section.

Autoxidative polymerization processes on HNs adsorbed on forsterite and anatase were carried out by exposing samples overnight to gaseous ammonia according to the reported ammonia induced solid state polymerization (AISSP) procedure.77 Computational methods. Quantum chemistry calculations were performed with the Gaussian package of programs.78 Structures were geometry-optimized in vacuo at the DFT level, with a hybrid functional (PBE0)79 and a reasonably large basis set [6-31+G(d,p)]. For the simulation of infrared spectra, harmonic frequencies were scaled by a factor of 0.0954780 and a Gaussian line shape with 20 cm–1 half-width at half-maximum was imposed. RESULTS AND DISCUSSION Experimental setup. Infrared reflectance spectra were recorded in situ using the experimental setup in Figure 1 in the case of room temperature experiments. Low temperature experiments were run using the experimental setup in Figure 2 (a detailed description is reported in the experimental section).


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Figure 1. Schematic representation of experimental setup during standard experiments (T = 300 K, P = 1 atm). Black arrows denote N2 fluxes. 1: Mirrors. 2: Sample holder.

To vacuum pump Liquid N2 Penning gauge 10−5−10−6 mbar Thermostat

Sample chamber

Infrared spectrometer

N2 flux Resistor cables T probe cable

N2 outflow

2

1

IR source 1

4 3 1

1 Detector

Figure 2. Schematic representation of experimental setup during low-temperature experiments (T = 80 K, P = 10−5−10−6 mbar). 1: Mirrors. 2: Resistor. 3: Sapphire window. 4: Sample holder.


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UV irradiation of HNs. Peak assignment of 1-HN, 1,8-DHN and 1,6-DHN was made accordingly to van Gemert81 and Ram82 and good agreement was found between the observed peak positions and those reported in the literature. The DRIFTS spectra of mineral powders and HNs, both in pure form and after adsorption, are shown in Figures 3-5. Some IR bands displayed non-negligible shifts (i.e. > 10 cm–1) as a consequence of adsorption on minerals. This behavior was particularly evident in the case of OH stretching bands, and might be related to a strong involvement of the phenolic functionalities in the adsorption process. Dramatic changes in relative intensities – often sought as clues for chemisorption phenomena in addition to physisorption – were also observed, but could be due to signal saturation phenomena ascribable to minerals. For the purposes of this study reflectance units were preferred over Kubelka-Munk (KM) units, despite their common use for reporting DRIFTS spectra, to avoid artifacts, since the KM theory of reflectance works optimally under assumptions that hardly apply to the present conditions, e.g. core particles smaller than total thickness.


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1-HN on anatase 120

Pure anatase Pure 1-HN

100

80

60

40

Reflectance (%)

Adsorbed 1-HN

20

0 7400

6400

5400

4400

3400

2400

1400

400

Wavenumber (cm−1)

1-HN on forsterite 90

Pure forsterite

80

Pure 1-HN

70

Adsorbed 1-HN 60 50 40 30

Reflectance (%)

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


20 10 0 4900

4400

3900

3400

2900

2400

1900

1400

900

400

Wavenumber (cm−1)

Figure 3. DRIFTS spectra of mineral powders and 1-HN, both in pure form and after adsorption.


13


1,8-DHN on anatase 120

Pure anatase

Adsorbed 1,8-DHN

80

60

40

Reflectance (%)

100

Pure 1,8-DHN

20

0 7400

6400

5400

4400

3400

2400

1400

400

Wavenumber (cm−1)

1,8-DHN on forsterite Pure forsterite

80

Pure 1,8-DHN

70

Adsorbed 1,8-DHN

60 50 40 30

Reflectance (%)

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

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20 10 0 4900

4400

3900

3400

2900

2400

1900

1400

900

400

Wavenumber (cm−1)

Figure 4. DRIFTS spectra of mineral powders and 1,8-DHN, both in pure form and after adsorption.


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Pure anatase 100

Pure 1,6-DHN

80

60

40

Reflectance (%)

Adsorbed 1,6-DHN

20

0 7400

6400

5400

4400

3400

2400

1400

400

Wavenumber (cm−1)

1,6-DHN on forsterite 80

Pure forsterite Pure 1,6-DHN

70

Adsorbed 1,6-DHN

60 50 40 30

Reflectance (%)

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


20 10 0 4900

4400

3900

3400

2900

2400

1900

1400

900

400

Wavenumber (cm−1)

Figure 5. DRIFTS spectra of mineral powders and 1,6-DHN, both in pure form and after adsorption. All HNs showed limited reactivity when irradiated as pure powders with UV light, but much faster degradation rates when adsorbed on minerals (Figures 6-8).


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1-HN on anatase

Pure 1-HN Before irradiation

Before irradiation 20 s 1 min 2 min 3 min 5 min 8 min 15 min 30 min

75

56 min 65

2h 6h

1-HN on forsterite

55

8h 45

85

Before irradiation 3s 10 s 20 s 30 s 60 s 2 min 10 min 60 min

95

85

75

80 75 70 65

65 60

35

55

25

45

55 50 45

15

4000

3500

3000

5 2000 4000

2500

Wavenumber (cm−1)

35

3500

3000

2500

Wavenumber (cm−1)

25 2000 4000

40

3500

3000

35 2000

2500

Wavenumber (cm−1)

Figure 6. DRIFTS analysis (OH stretching spectral region) of the effects of UV irradiation on samples of 1-HN, in pure form (left) and adsorbed on anatase (mid) and forsterite (right). Reflectance (%) values are displayed on the y axis. Pure 1,8-DHN

1,8-DHN on anatase

1,8-DHN on forsterite

Before irradiation

Before irradiation 2 min

85

90

65

144 s

55

Before irradiation 140 s

80

11 min

6.6 min 58.2 min

2h

70

40 min

45

75

10 min 65

5h

4.5 h

60

5.5 h

35

55 50 45

25 40 15

4000

3500

3000

2500

Wavenumber (cm−1)

5 2000 4000

35

30

3500

3000

2500

Wavenumber (cm−1)

20 2000 4000

3500

3000

25 2000

2500

Wavenumber (cm−1)

Figure 7. DRIFTS analysis (OH stretching spectral region) of the effects of UV irradiation on samples of 1,8-DHN, in pure form (left) and adsorbed on anatase (mid) and forsterite (right). Reflectance (%) values are displayed on the y axis.


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Pure 1,6-DHN

1,6-DHN on anatase


50

Before irradiation

45

⎯⎯ Before irradiation

80

⎯⎯ 1 h

28.1 min

40

2.7 h 35

5.7 h 30

⎯⎯ Before irradiation

80

⎯⎯ 1 h 70

⎯⎯ 2 h

⎯⎯ 2 h 70

⎯⎯ 3 h

60

⎯⎯ 5 h

⎯⎯ 4 h ⎯⎯ 6.25 h 60

50

25 20

40 50

15 30 10

40 20

5

4000

3500

3000

2500

Wavenumber (cm−1)

0 2000 4000

3500

3000

2500

10 2000 4000

Wavenumber (cm−1)

3500

3000

2500

30 2000

Wavenumber (cm−1)

Figure 8. DRIFTS analysis (OH stretching spectral region) of the effects of UV irradiation on samples of 1,6-DHN, in pure form (left) and adsorbed on anatase (mid) and forsterite (right). Reflectance (%) values are displayed on the y axis. Data in Figures 6-8 show the rapid loss of the OH stretching band (3500–3000 cm–1) for all the HNs examined when adsorbed on minerals. The fastest abatement of pure organics was observed in the case of 1-HN. Moreover, adsorption on forsterite accelerated the kinetics of the photodegradation process with all substrates, apparently with higher rates with respect to anatase. Interestingly, marked changes in the spectral region above 4000 cm–1 were detected specifically in the case of 1,8-DHN with both minerals, suggesting the formation of species with high absorptivity for NIR radiation up to visible radiation (Figure 9). Pure mineral photolysis as a consequence of UV irradiation was preliminarily ruled out.


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1,8-DHN on anatase

85 90 75 80

65

70

60 55

Before irradiation

50

Before irradiation

144 s 11 min

10 min

40 min

5h

5.5 h 6500

5500

35

2h

30

4.5 h

7500

45

140 s

40

4500

3500

20 2500

7500

6500

Wavenumber (cm−1)

5500

4500

3500

25 2500

Wavenumber (cm−1) –1

Figure 9. DRIFTS analysis (8000−2500 cm ) of the effects of UV irradiation on samples of 1,8DHN adsorbed on anatase (left) and forsterite (right). Reflectance (%) values are displayed on the y axis. In the 1700–1000 cm–1 region of the spectrum 1-HN showed almost complete abatement of all bands on forsterite and a less pronounced decrease on anatase (Figure 10). In the latter case, broad and weak bands were found to develop after irradiation, one of which around 1500 cm–1. In contrast with the case of 1-HN, irradiation of 1,8-DHN led to the generation of detectable bands at 1666 (C=C), 1517, 1456, 1363 cm–1 on forsterite, and at 1515, 1365 cm–1 on anatase (Figure 11). A change of the profile of bands around 1650 and 1600 cm–1 was also visible. Ill-defined spectral changes were conversely observed during irradiation of 1,6-DHN on forsterite, with the noticeable exception of a minor band at 1554 cm–1 (Figure 12).


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1-HN on anatase 50

Before irradiation

45

2 min

40

6 min

1634 ν ring

1518 ν ring 1458 δ CH

1581 1599 ν ring ν ring

20 min

30

90 min

25 20

1207 δ CH

1387 ν ring

1364 ν ring

1312 δ OH

1271 δ CH

15

1240 δ CH

1084 δ CH 1148 δ CH

1700

1600

1500

1400 1300 Wavenumber (cm−1)

Reflectance (%)

35

10 min

1200

1016 ν ring

10 5

1045 ν ring 1100

0 1000

1-HN on forsterite 50

Before irradiation 1634 ν ring

45

20 s 1518 ν ring


40

60 s 1458 δ CH

1387 ν ring

1364 1310 ν ring δ OH 1271 δ CH

2.5 min

35

10 min

30

60 min

1240 δ CH

25

791 δ CH ?

768 δ CH

20 15 10 5

1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

Wavenumber (cm−1)

Figure 10. DRIFTS analysis (fingerprint spectral region) of the effects of UV irradiation on samples of 1-HN adsorbed on anatase (top) and forsterite (bottom). The red arrow denotes a new band appearing as a consequence of UV irradiation.


19

Reflectance (%)

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



1,8-DHN on anatase

25

20

1205 ν ring

1462 ν ring 1410 ν ring

1600

30

1500

1285 δ OH ?

1400

15

1236 δ CH

1300

Reflectance (%)

1377 ν ring

1524 ν ring

35

Before irradiation 144 s 11 min 40 min 64 min 125 min 3.25 h 4.5 h 5.5 h

1033 ν ring

1200

10

5 1000

1100

Wavenumber (cm−1)


1524 ν ring

Before irradiation 140 s 10 min 1h 2h 3h 5h

1377 ν ring

1462 ν ring

40

35

30

25

1236 δ CH

20

1408 ν ring 1283 δ OH ? 1700

1600

1500

1400

1300

1205 ν ring 1200

15

746 δ CH ?

10 1100

Wavenumber (cm−1)

1000

900

800

700

Figure 11. DRIFTS analysis (fingerprint spectral region) of the effects of UV irradiation on samples of 1,8-DHN adsorbed on anatase (top) and forsterite (bottom). Red arrows denote new bands appearing as a consequence of UV irradiation.


20

Reflectance (%)

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

Page 20 of 56

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Before irradiation 20 min 1h 2h 3h 3.75 h 5h

23 21

Reflectance (%)

19 17 15 13

1462 ν ring

1522 ν ring

1412 ν ring

11

1380 ν ring

1600

1500

1049 ν ring

1286 δ OH ?

1400

1163 δ CH

1225 δ CH

1300

Wavenumber (cm−1)

1200

1082 ν ring

9 7 5 1000

1100



Before irradiation 20 min 1h 2h 3h 4h 5h 6.25 h

1522 ν ring 1464 ν ring 1412 ν ring

1584 ν ring

40 35 30 25

746 δ CH ?

1379 ν ring 781 δ CH ?

1283 δ OH ? 1223 δ CH

20 15

Reflectance (%)

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


10

1157 δ CH

5 1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

Wavenumber (cm−1)

Figure 12. DRIFTS analysis (fingerprint spectral region) of the effects of UV irradiation on samples of 1,6-DHN adsorbed on anatase (top) and forsterite (bottom). The red arrow denotes a new band appearing as a consequence of UV irradiation.


21


Page 22 of 56

The analyses of the peak areas were conducted by choosing the baselines separately for each peak (manual insertion of baseline anchor points) as long as baseline shifts were negligible. Further quantitative analyses were carried out for IR peaks which featured stable baselines during UV irradiation, and did not overlap with other signals, including the ones appearing de novo during irradiation. Since the peak areas are proportional to the number of molecules, the kinetics of photodegradation was studied through evaluation of peak areas reduction with irradiation. Specifically, each peak area was plotted as a function of time and fitted with an exponential function, in which the fraction of molecules at time t, N(t)/N0, follows an exponential law with irradiation time N(t)/N0= Be−βt + C, where N0 is the initial number of molecules in the sample, B is the fraction of molecules that interact with UV radiation, and C is the fraction of molecules that do not interact with UV. In Figure 13 peak areas for more intense bands measured at various irradiation time are fitted with the exponential law.


22

Page 23 of 56 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


Figure 13. Analyses of peak areas. From top to bottom: 1,8-DHN, 1,6-DHN and 1-HN, on anatase (left) and forsterite (right).


23


The half-lifetimes were obtained as t1/2 = (ln2)/β and the UV destruction cross section σ, that represents the probability of interaction between UV radiation and molecule, was measured, with β = σΦtot/A0 and Φtot/A0 is the total incident flux of UV photons (in the range 200–350 nm, since HNs mainly absorb in this spectral region, as shown in Figure 14). The best-fit values for β, B, C, the destruction cross sections σ and the half-lifetimes t1/2 for the investigated HNs in our laboratory conditions are reported in Table 1.

2 1.8 1.6 1.4

Absorbance (AU)

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

Page 24 of 56

1.2 1 0.8 0.6 0.4 0.2 0 202

222

242

262

282

302

322

342

362

382

λ (nm) Figure 14. UV absorption spectra (methanol) of 1-HN (green trace), 1-8-DHN (black trace) and 1,6-DHN (red trace). Table 1. Decaying parameters for the main IR bands. Peak (cm–1)

β (min–1)

1033 1205 1236 1285 1377

0.048 ± 0.004 0.10 ± 0.02 0.042 ± 0.007 0.026 ± 0.006 0.15 ± 0.09

B

C 1,8-DHN on anatasea,c 0.383 ± 0.008 0.412 ± 0.009 0.46 ± 0.04 0.53 ± 0.03 0.58 ± 0.03 0.10 ± 0.02 0.49 ± 0.04 0.12 ± 0.03 0.8 ± 0.2 0.1 ± 0.2


σ (cm2)a,b

t1/2lab(min)

(9.9 ± 0.8)⋅10–21 (2.1 ± 0.4)⋅10–20 (9 ± 1)⋅10–21 (5 ± 1)⋅10–21 (3 ± 2)⋅10–20

14 ± 1 7±1 17 ± 3 27 ± 6 5±3

24

Page 25 of 56 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


1410 1462 1524

0.024 ± 0.006 0.11 ± 0.03 0.039 ± 0.008

1236 1283 1377 1408 1462 1524

0.12 ± 0.02 0.11 ± 0.02 0.11 ± 0.02 0.08 ± 0.03 0.07 ± 0.02 0.012 ± 0.003

1049 1082 1163 1225 1286 1380 1412 1462 1522

0.011 ± 0.002 0.0053 ± 0.0005 0.011 ± 0.003 0.011 ± 0.003 0.0112 ± 0.0006 0.007 ± 0.002 0.027 ± 0.005 0.0093 ± 0.0004 0.0060 ± 0.0004

781 1157 1223 1283 1379 1464 1522 1584 1609 1641

0.008 ± 0.001 0.0063 ± 0.0005 0.0046 ± 0.0007 0.0055 ± 0.0008 0.0054 ± 0.0005 0.009 ± 0.002 0.0040 ± 0.0006 0.0047 ± 0.0006 0.0053± 0.0009 0.0053 ± 0.0003

1240 1271 1312 1364+1387e 1458 1518 1581 1599 1634

0.24 ± 0.03 0.17 ± 0.03 0.24 ± 0.02 0.17 ± 0.02 0.22 ± 0.03 0.13 ± 0.01 0.17 ± 0.02 0.24 ± 0.03 0.17 ± 0.01

768 791 1240 1271 1310 1364+1387e 1458 1518 1581+1599e 1634

2.0 ± 0.3 3.8 ± 0.4 4±1 0.7 ± 0.1 0.8 ± 0.1 1.3 ± 0.2 0.7 ± 0.2 0.9 ± 0.1 0.7 ± 0.1 2.7 ± 0.6

0.27 ± 0.02 0.66 ± 0.02 0.22 ± 0.02 0.77 ± 0.01 0.74 ± 0.01 0.26 ± 0.02 1,8-DHN on forsteritea,c 0.78 ± 0.04 0.21 ± 0.02 0.82 ± 0.05 0.16 ± 0.04 0.92 ± 0.05 0.05 ± 0.03 0.52 ± 0.06 0.46 ± 0.03 0.49 ± 0.04 0.49 ± 0.02 0.83 ± 0.06 0.14 ± 0.06 1,6-DHN on anataseb,d 0.54 ± 0.03 0.33 ± 0.03 0.74 ± 0.03 0.17 ± 0.03 0.56 ± 0.04 0.48 ± 0.04 0.45 ± 0.05 0.57 ± 0.06 0.449 ± 0.006 0.612 ± 0.007 0.46 ± 0.04 0.56 ± 0.05 0.59 ± 0.03 0.41 ± 0.02 0.427 ± 0.007 0.574 ± 0.007 0.54 ± 0.02 0.46 ± 0.02 1,6-DHN on forsteritea,c 0.83 ± 0.04 0.13 ± 0.04 0.71 ± 0.02 0.27 ± 0.02 0.75 ± 0.06 0.21 ± 0.06 0.74 ± 0.04 0.27 ± 0.04 0.74 ± 0.03 0.24 ± 0.03 0.54 ± 0.05 0.35 ± 0.05 0.78 ± 0.06 0.20 ± 0.07 0.61 ± 0.04 0.37 ± 0.04 0.75 ± 0.06 0.23 ± 0.06 0.74 ± 0.02 0.24 ± 0.02 1-HN on anataseb,d 0.69 ± 0.03 0.25 ± 0.02 0.47 ± 0.03 0.47 ± 0.03 0.86 ± 0.02 0.10 ± 0.02 0.51 ± 0.02 0.45 ± 0.02 0.39 ± 0.02 0.56 ± 0.01 0.47 ± 0.02 0.51 ± 0.02 0.37 ± 0.02 0.59 ± 0.01 0.44 ± 0.02 0.52 ± 0.02 0.65 ± 0.02 0.31 ± 0.02 1-HN on forsteriteb,d 0.81 ± 0.04 0.16 ± 0.02 1.7 ± 0.2 0.10 ± 0.02 0.62 ± 0.06 0.32 ± 0.03 0.79 ± 0.04 0.17 ± 0.03 0.81 ± 0.05 0.16 ± 0.03 0.82 ± 0.04 0.14 ± 0.03 0.70 ± 0.07 0.09 ± 0.06 0.63 ± 0.03 0.36 ± 0.02 0.46 ± 0.02 0.51 ± 0.02 0.73 ± 0.06 0.23 ± 0.03


(5 ± 1)⋅10–21 (2.3 ± 0.6)⋅10–20 (8 ± 2)⋅10–21

29 ± 7 6±2 18 ± 4

(2.5 ± 0.4)⋅10–20 (2.3 ± 0.4)⋅10–20 (2.3 ± 0.4)⋅10–20 (1.6 ± 0.6)⋅10–20 (1.4 ± 0.4)⋅10–20 (2.5 ± 0.6)⋅10–21

6±1 6±1 6±1 9±3 10 ± 3 60 ± 10

(2.5 ± 0.5)⋅10–21 (1.2 ± 0.1)⋅10–21 (2.5 ± 0.7)⋅10–21 (2.5 ± 0.7)⋅10–21 (2.5 ± 0.1)⋅10–21 (1.6 ± 0.5)⋅10–21 (6 ± 1)⋅10–21 (2.11 ± 0.09)⋅10–21 (1.36 ± 0.09)⋅10–21

60 ± 10 130 ± 10 60 ± 20 60 ± 20 62 ± 3 100 ± 30 26 ± 5 75 ± 3 116 ± 8

(1.6 ± 0.2)⋅10–21 (1.3 ± 0.1)⋅10–21 (9 ± 1)⋅10–22 (1.1 ± 0.2)⋅10–21 (1.1 ± 0.1)⋅10–21 (1.8 ± 0.4)⋅10–21 (8 ± 1)⋅10–22 (1.0 ± 0.1)⋅10–21 (1.1 ± 0.2)⋅10–21 (1.09 ± 0.06)⋅10–21

90 ± 10 110 ± 9 150 ± 20 130 ± 20 130 ± 10 80 ± 20 170 ± 30 150 ± 20 130 ± 20 131 ± 7

(5.4 ± 0.7)⋅10–20 (3.9 ± 0.7)⋅10–20 (5.4 ± 0.5)⋅10–20 (3.9 ± 0.5)⋅10–20 (5.0 ± 0.7)⋅10–20 (2.9 ± 0.2)⋅10–20 (3.9 ± 0.5)⋅10–20 (5.4 ± 0.7)⋅10–20 (3.9 ± 0.2)⋅10–20

2.9 ± 0.4 4.1 ± 0.7 2.9 ± 0.2 4.1 ± 0.5 3.2 ± 0.4 5.3 ± 0.4 4.1 ± 0.5 2.9 ± 0.4 4.1 ± 0.2

(4.4 ± 0.7)⋅10–19 (8.6 ± 0.9)⋅10–19 (9 ± 2)⋅10–19 (1.6 ± 0.2)⋅10–19 (1.8 ± 0.2)⋅10–19 (2.9 ± 0.5)⋅10–19 (1.6 ± 0.5)⋅10–19 (2.0 ± 0.2)⋅10–19 (1.6 ± 0.2)⋅10–19 (6 ± 1)⋅10–19

0.35 ± 0.05 0.18 ± 0.02 0.17 ± 0.04 1.0 ± 0.1 0.9 ± 0.1 0.53 ± 0.08 1.0 ± 0.3 0.77 ± 0.09 1.0 ± 0.1 0.26 ± 0.06

25


Page 26 of 56

a

The cross sections were calculated with A0 = 0.0707 cm2 (irradiated spot radius = 1.5 mm) and Φtot = 5.72×1015 s–1; the cross sections were calculated with A0 = 0.0707 cm2 (irradiated spot radius = 1.5 mm) and Φtot = 5.20×1015 s–1; c total irradiation power = 110 mW; dtotal irradiation power = 100 mW; ecouple of overlapping peaks analyzed as a whole. b

Data in Table 1 show that forsterite is responsible for higher rates of photo-induced processes, with the exception of 1,6-DHN, whose bands decay faster in the case of anatase. Apparent differences in the degradation rates of the various bands, e.g. the 1524 cm–1 band of 1,8-DHN and bands at 1464, 1412, 781 cm–1 in 1,6-DHN, can be ascribed to the intervention of complex pathways in addition to photodestruction (the latter notably featuring general band loss with considerably similar degradation rates), along with the complex nature of the species produced by UV irradiation and their different photoreactivity. In the case of 1-HN, photodesorption may play an important role. Band loss rates depend considerably on the substitution pattern in the case of DHN, with 1,8-DHN displaying faster decaying rates. In the case of forsterite, whose DRIFTS spectrum is compatible with the presence of hydroxyl defects on its surface,83–86 HNs are supposed to interact mainly via their OH functionalities. Therefore, DHNs should be bound more strongly to forsterite with respect to 1HN. 1,8-DHN, in particular, would be adsorbed on forsterite with higher affinity with respect to 1,6-DHN.

H

H O

O

H O

H O

H

H O H


O O

Forsterite

26

Page 27 of 56 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


Figure 15. Proposed adsorption arrangements of HNs on forsterite: 1-HN (left), 1,8-DHN (mid) and 1,6-DHN (right). This graph aims only at suggesting possible interactions between OH groups on the molecule and the mineral surface. A schematic illustration supporting the above arguments is given in Figure 15. It is shown that 1-HN features a single anchor point, although interactions between aromatic cloud and mineral surface might be also involved (needing tilted or face-to-face configuration, not shown). 1,8-DHN features two OH groups on the same side, hence it can interact strongly with the mineral even in a perpendicular side-to-face arrangement, minimizing the interactions between its non-polar moiety and mineral surface. 1,6-DHN on the other hand features two anchor points only in a tilted or face-to-face configuration, causing the possible involvement of aromatic cloud. These differences in adsorption arrangement may impact significantly on the available photo-induced processes, justifying the different trends seen during the UV-irradiation experiments. The arrangements proposed in Figure 15 might also be relevant to the interactions between anatase and adsorbed HNs, although the slower rates of photodegradation with anatase may reflect different photo-induced processes. Clearly, the above rationalization is speculative and further studies, current underway, are required to better clarify the underlying phenomena. It is worth noting, moreover, that UV and IR have very different penetration depths: UV radiation interacts just with a few molecular monolayers, decaying very fast as it penetrates, contrarily to IR radiation that penetrates deeper (µm) into the sample. Therefore, information inferable from DRIFTS spectra concerns even sample portions not affected by UV radiation, justifying the nonnull values of the C term in Table 1.


27


In a subsequent experiment, the photochemical behavior of 1,8-DHN on mineral surface was studied under lower pressure and temperature conditions (~10–5 mbar and 80 K). Data in Figure 16 show a marked alteration of the OH stretching band during UV irradiation at 80 K and the appearance after 10 minutes of irradiation of a characteristic peak at 2340 cm–1 attributed to CO2 asymmetric stretching. This signal could not be detected at high temperatures since the vacuum pump rapidly scavenges any gaseous CO2, but could be observed at 80 K since produced CO2 can accumulate on the powder surface as dry ice, becoming detectable with DRIFTS techniques.

1,8-DHN on forsterite (80 K) 75

70

Before irradiation 10 min 1.75 h 4.75 h

65

60

55

50

45

40 2360 4000

3800

3600

3400

3200

3000

2800

2600

2340 2400

2320

35 2200

Wavenumber (cm−1)

Figure 16. DRIFTS analysis of the effects of UV irradiation on 1,8-DHN adsorbed on forsterite (80 K). Inset: magnification of the peak at 2340 cm–1. From this set of experiments, it could be concluded that: a) forsterite has a significant catalytic effect on UV-induced degradation, suggesting a specific role in favoring evolution of


28

Reflectance (%)

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

Page 28 of 56

Page 29 of 56 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


excited states possibly via interaction with reactive OH groups through its basic sites, in agreement with previous studies;87 b) with all HNs examined, extensive loss of OH stretching bands in the 3600-3000 cm–1 region is observed, denoting significant modification of the photolabile phenolic groups;88,89 c) irradiation of 1,8-DHN at room temperature induces specific photodegradation pathways absent in the case of 1-HN and 1,6-DHN, suggesting on one side formation of polymeric species (spectral changes above 4000 cm–1) and the other side the generation of species with distinct well-defined spectral features (see bands at 1666, 1517, 1456, 1363 cm–1). Formation of CO2 detected at 80K suggests moreover profound photodestruction of 1,8-DHN via intermediate carboxyl formation followed by decarboxylation. Whether heating phenomena enhanced by the forsterite are implicated in the loss of CO2 could not be assessed. However, irradiation of pure HNs with high photon fluxes resulted in sublimation/deposition phenomena around the UV-light cone, suggesting local heating of the sample (see SI), in agreement with the behavior described in previous works.90 Similar effects were observed even after irradiation of HN-spiked minerals (see SI). Proper decrease of the total power of the incident radiation (1/3 of the power leading to sublimation/deposition phenomena) led to a complete abatement of such phenomena, thus ruling out the effect of sublimation on degradation rates. It is therefore concluded that the intense peak loss observed in the case of 1-HN may be due only in part to photodesorption phenomena in addition to photodegradation. As mentioned previously, the strength of the interaction between HNs and mineral surface may be responsible for the evident difference observed during UV irradiation, with photodesorption quantum yields being much lower with DHNs. Autoxidation experiments. Although the UV irradiation experiments were run under a flux of nitrogen to decrease availability of oxygen, some oxidation might have occurred at the


29


Page 30 of 56

expenses of residual oxygen or by intermolecular redox exchange following excitation/ionization of stacked species in the solid state, giving rise to polymerization products. In accord with this view, both the loss of OH stretching bands and the virtually featureless absorption increase above 4000 cm–1 (in the case of 1,8-DHN) suggested possible conversion of phenolic groups into carbonyl-containing oligomeric/polymeric species.76 Accordingly, to confirm the proposed generation of polymeric species during UV irradiation, in separate experiments the HN samples adsorbed on forsterite were subjected to autoxidation and the DRIFTS spectra of the resulting polymers were compared with those of the UV irradiation experiments (Figures 17-22). To induce autoxidation of the HNs adsorbed on forsterite we referred to a protocol recently developed to overcome solubility issues in the preparation of melanin thin films on various substrates, i.e. the Ammonia-Induced Solid-State Polymerization (AISSP) protocol. The procedure was not intended to reproduce astrophysically relevant environments but only to obtain polymeric species in the solid state in the absence of chemical oxidants. It exploits the high susceptibility to oxidation of phenolic compounds when exposed to an alkaline environment, giving rise to melanin-type materials polymeric in nature even in the solid state.77 Typically, the solid sample is exposed to gaseous ammonia in equilibrium with a saturated aqueous solution in a closed chamber. Activation of phenolic groups to autoxidation is supposed to be due to proton abstraction from the hydroxyl group, favoring electron transfer to oxygen from the more oxidizable phenolate species. The resulting free radicals can then undergo further reactions, e.g. by coupling with other radical species giving rise to oligomers and then to extended quinones which would be prone to attack by the monomers thus being involved in chain growth. No significant incorporation of ammonia usually occurs with the AISSP process, as verified in previous experiments on melanin thin films.77


30

Page 31 of 56

1-HN on anatase 120

AISSP Before irradiation

100

Irradiated (1.5 h)

60 60 50 40

Reflectance (%)

80

70

40

30 20

20

10

2000

1800

1600

1400

0 1000

1200

0 7400

6400

5400

4400

3400

2400

1400

400

Wavenumber (cm−1)

Figure 17. DRIFTS analysis (inset: expansion of the 1000–2000 cm–1 region) of the effects of AISSP vs UV irradiation on 1-HN adsorbed on anatase. 1-HN on forsterite 85 75 65 55 55 45 45 35 35

Reflectance (%)

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


25

AISSP Before irradiation Irradiated (1 h)

15 5 1900 7400

1700

1500 6400

1300

1100 5400

900 4400

25 15

700

5 3400

2400

1400

400

Wavenumber (cm−1)

Figure 18. DRIFTS analysis (inset: expansion of the 700–2000 cm–1 region) of the effects of AISSP vs UV irradiation on 1-HN adsorbed on forsterite.


31



AISSP Before irradiation Irradiated (5 h)

70 60

40 35 30

50

25

40

20

30

15 10

Reflectance (%)

50 45

80

20

5 10

0 2000

1800 7400

1600

1400

1200

6400

1000

5400

800

0

4400

3400

2400

1400

400

Wavenumber (cm−1)

Figure 19. DRIFTS analysis (inset: expansion of the 800–2000 cm–1 region) of the effects of AISSP vs UV irradiation on 1,6-DHN adsorbed on anatase. 1,6-DHN on forsterite 85

75


65

Irradiated (6.25 h) 55

55 45

45

35

35

Reflectance (%)

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

Page 32 of 56

25 25 15 15 5 1900

1700

1500

1300

1100

900

700 5

7400

6400

5400

4400

3400

2400

1400

400

Wavenumber (cm−1)

Figure 20. DRIFTS analysis (inset: expansion of the 700–2000 cm–1 region) of the effects of AISSP vs UV irradiation on 1,6-DHN adsorbed on forsterite.


32

Page 33 of 56

1,8-DHN on anatase 90 80

60 50 40 30

AISSP Before irradiation Irradiated (5.5 h)

Reflectance (%)

70

20 10 0

7400

6400

5400

4400

3400

2400

1400

400

Wavenumber (cm−1)


AISSP Before irradiation Irradiated (5.5 h)

40

30

20

Reflectance (%)

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


10

0 2000

1800

1600

1400

1200

1000

800

Wavenumber (cm−1)

Figure 21. DRIFTS analysis (below, expansion of the 800–2000 cm–1 region) of the effects of AISSP vs UV irradiation on 1,8-DHN adsorbed on anatase.


33


1,8-DHN on forsterite 85 75

55 45


35

Irradiated (5 h)

25

Reflectance (%)

65

15 5 7400

6400

5400

4400

3400

2400

1400

400

Wavenumber (cm−1) 1,8-DHN on forsterite 55


45

Irradiated (5 h) 35

25

Reflectance (%)

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

Page 34 of 56

15

5 1900

1700

1500

1300

1100

900

700

Wavenumber (cm-1)

Figure 22. DRIFTS analysis (below, expansion of the 700–2000 cm–1 region) of the effects of AISSP vs UV irradiation on 1,8-DHN adsorbed on forsterite. DRIFTS analysis performed on HN samples on forsterite after AISSP revealed an alteration of the OH stretching band at ν෤ > 3000 cm–1, consistent with conversion of OH groups to free radical or ketone/quinone species (Scheme 1).76


34

Page 35 of 56 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


X

OH

NH4+ X

NH3

O-

O2

O

X

X

O

X

. O

O2.-

.

. x2

x2 2,4'-dimer

4,4'-dimer

2,2'-dimer [O]

Polymer

Scheme 1. First stages of the oxidative polymerization triggered by AISSP. 1-HN (X = H); 1,6DHN (X = 6-OH); 1,8-DHN (X = 8-OH). Oxidative processes inducing polymerization in the HN samples are expected to proceed via phenoxyl radicals which would then couple to give carbonyl-containing species as in Scheme 1. This conclusion was supported by the loss and broadening of the OH stretching band due to conversion to complex species lacking well defined phenolic moieties. In the case of DHNs, ammonia-induced pathways, supposedly leading to polymeric products, led to intense broadening and merging of the C–C stretching peaks to form a broad spectral feature centered at 1600 cm–1. This behavior is indicative of a great variability in the C– C population causing broadening and merging of the relevant IR bands. From comparison of these data it can be concluded that: a) autoxidation pathways of HNs are substantially different from UV-induced pathways, b) a generalized increase in the NIR region may be diagnostic of oxidative polymerization processes (melanization); and c) detectable oxidative polymerization may occur by photolysis of 1,8-DHN on forsterite. 1,8-DHN was the


35


Page 36 of 56

only HN showing intense band broadening and merging in the C–C stretching region, rather than a general band loss like that observed in the case of 1-HN and 1,6-DHN. To orient further investigations into the nature of HN photodegradation products, the IR absorption features of potential photoproducts of 1,8-DHN, the most reactive of the compounds tested, were briefly explored by DFT calculations in vacuum (Figure 23) and the computational results were compared with experimental data. Quantum chemistry calculations were performed with the Gaussian package of programs.78 Structures were geometry-optimized in vacuo at the DFT level, with a hybrid functional (PBE0)79 and a reasonably large basis set [6-31+G(d,p)]. For the simulation of infrared spectra, harmonic frequencies were scaled by a factor of 0.0954780 and a Gaussian line shape with 20 cm–1 half-width at half-maximum was imposed. Although the study was preliminary and the simulation conditions (in vacuo) were far from reproducing the actual situation of 1,8-DHN photodegradation products adsorbed on forsterite, it was interesting that the simulated spectra of the hypothesized oxidation products displayed bands that were missing in the starting monomer and that matched fairly well with some of the bands generated by irradiation of 1,8-DHN. In particular, bands around 1180 and 1440 cm–1 were predicted for a representative free radical species and around 1650 cm–1 for a carbonyl-containing/extended quinone species. These bands in the simulated spectra matched fairly well with the developing bands in the UV-irradiated 1,8-DHN sample on forsterite at 1160, 1450 and 1620–1660 cm–1. As emphasized above, it is not possible to draw any conclusion about the impact of forsterite surface on the UV photochemistry of the 1,8-DHN based on preliminary DFT calculations.


36

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Figure 23. IR spectra computed in vacuum for 2,2’ carbon-carbon coupling dimer from 1,8DHN, phenoxyl radical form (A) and for carbon-carbon coupling dimers from 1,8-DHN, quinone forms (B). Intensity is reported in arbitrary units.


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A more accurate simulation of the experimental conditions reported in this study, including the role of forsterite, will be the focus of a separate computational study. However, with due caution, it seems plausible to conclude that the photochemical processes induced on mineral surface result in part in the conversion to free radical and carbonyl-containing groups. To put the results of this study in a proper astrochemical perspective, a brief comment on the expected properties of oxyPAHs versus PAHs seems in order. Compared to PAHs, oxyPAHs display polar oxygenated functions that may significantly affect the physicochemical properties and photoreactivity of the aromatic system. In particular, hydroxylated PAHs are anticipated to be more oxidizable than PAHs because of the electrondonor properties of phenolic OH groups. To verify this prediction, vertical ionization energies of HNs versus naphthalene were determined at the PBE0 / 6-31+G(d,p) level. Consistently with the observed reactivity trend, data indicated ionization potentials decreasing in the order: naphthalene (D2h) 7.96 (exptl 8.144) > 1HN (Cs) 7.59 (exptl 7.76) > 1,6-DHN (Cs) 7.47 > 1,8-DHN (Cs) 7.32 eV.91 These results suggest that oxyPAHs, although possibly present in low levels in astrochemical contexts, may be more susceptible to photodegradation than PAHs, thus impacting strongly in the overall fate of PAH-related species. Available data from DRIFTS spectra, as corroborated by DFT predictions on putative photooxidation products, provide a sufficient body of evidence to delineate the basic events in DHN photolysis, as illustrated in Scheme 2. Carbonyl α-cleavage pathways would require an oxidizing agent, e.g. hydrogen peroxide, which may be produced starting from mineral residual water and/or from dioxygen traces in the irradiation chamber, as a result of radiation-induced processes,92–95 maybe with the participation of the extended quinone itself.96,97


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It is plausible that the ortho-OH group supports the decarboxylation step.98–101 The apparent difference in the photochemical behavior of 1,8-DHN vs 1,6-DHN, which both display a meta-like substitution pattern, may be due to the intramolecular interactions between the hydroxyl groups operating in the case of 1,8-DHN but not of 1,6-DHN. In this perspective, comparative experiments at low temperature would be highly clarifying.

OH O

OH OH

.

[O] 1,8-DHN radical

1,8-DHN

OH O

H

OH OH H

O

[O]

OH OH OH 2,2'-dimer

OH OH [O] O.

OH O

OH O

2,2'-dimer radical

extended quinone carbonyl α-cleavage

OH

-CO2

O OH OH

OH

[O]

OH COOH O OH OH

Scheme 2. Proposed conversion pathways of 1,8-DHN under UV irradiation conditions on forsterite. In the preliminary low-temperature test performed with 1,8-DHN, evident changes occurred in the OH stretching band, while the CO2 peak slowly disappeared as a consequence of sublimation from the surface of mineral powder. These changes suggest that UV irradiation


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promoted the formation of species which could not further react at 80 K, probably due to a low thermal energy, whereas at higher temperatures their thermal energy would become high enough to allow for further conversion. These species are most likely free radicals generated by photoinduced homolytic dissociation of OH bonds (Scheme 3), in agreement with the ionization potential trend predicted by DFT calculations. H O

H

. O

H O

O

O

O

. .

.O

H

H O

O

H O

O

O

. . Scheme 3. Resonance structure of 1,8-DHN radical highlighting H-atom exchange between the two oxygen atoms. The intramolecular H-atom transfer proposed for 1,8-DHN radical would account for high reactivity rates even at low temperatures, since the lack of energetic molecular motions would be compensated by extensive free radical delocalization. As far as decarboxylation is concerned, it is worth noting that a similar loss of one-carbon fragments (carbon dioxide and formaldehyde) has been reported by photolysis of various PAHs, including anthracene, pyrene and benzo[ghi]perylene,102 as well as coronene103 in water ice at 15 K. In these studies, carbon dioxide was proposed to arise from carbon monoxide via subsequent oxygenation. No evidence is available in the present study that favors this mechanism over the direct decarboxylation pathway, and more work is necessary to definitively clarify this issue.


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Although the previous observations refer to conditions of closer relevance to the ISM rather than to planetary environments, as in the present study, these studies together point to a general tendency of PAHs to undergo photodecomposition with ring cleavage, and that hydroxylated derivatives are involved in the process. CONCLUSIONS The present investigation has delineated the photochemical behavior on mineral surface of HNs as representative members of the family of oxygenated PAH derivatives (oxyPAHs), to model and inquire into astrochemical processes of possible relevance to Martian surface and planetary conditions. To aid and support product identification, autoxidation experiments have also been performed in the solid state under the same conditions, and the IR spectra of putative free radical intermediates and oligomer products from 1,8-DHN have been simulated by DFT calculations. Overall, the results can be summarized as detailed in the following. A first important observation is that minerals, especially forsterite, markedly enhance susceptibility of HNs to photodegradation in the solid state. The factors and mechanisms underlying this effect are currently under investigation and are likely to open new vistas into the role of minerals as catalysts of processes of planetary relevance. The second key observation is that UV-induced decomposition of HNs, as enhanced by interactions with the mineral surface, proceeds via complex sequential and/or possibly competing pathways which involve oxidation and conversion to carboxyl groups, ring cleavage and decarboxylation, along with some polymerization. In these pathways, the presence of hydroxyl groups on the aromatic scaffold would account for both polymerization and photodestruction via hydroxylation/oxygenation steps. Formation of polymeric products was measured by the


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absorption enhancement above 4000 cm–1, a characteristic spectral change accompanying autoxidation and overall did not seem to provide the primary decomposition pathway under UV irradiation conditions. Mechanistically, a basic gap remains as to the mechanism of decarboxylation and the role of water and mineral surface. While ample evidence for loss of CO2 in water ices is available, the occurrence of the process in the solid state on the mineral surface requires detailed separate studies and is currently under assessment in our laboratory. Altogether, these preliminary results open interesting scenarios from the viewpoint of prebiotic processes. The main ingredients of the reported chemistry, oxyPAHs and minerals, are all likely to be active participants in prebiotic scenarios under planetary conditions. OxyPAHs have also been specifically implicated as container elements, favoring energy transduction and acting as templates in the synthesis of putative precursors for genetic components, such as RNA, for early life forms (the so-called PAH world hypothesis).5,90 Moreover, they have been found to exert cholesterol-like stabilizing effects in a simulated prebiotic membrane, lowering the permeability of fatty acid bilayers to small solutes up to 4-fold relative to PAHs.104 In this paper, we propose that oxyPAHs and their photodecomposition products may contribute to mineral coating and may play both protective and catalytic roles in prebiotic processes. The definitive identification and characterization of PAHs and related compounds on Mars may corroborate this view. The investigation of TRAPPIST-1 planetary system is in this respect a major focus in prebiotic chemistry, since its rocky planets are suitable environments for chemical formation and transformation of PAHs under mild conditions.105

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org at DOI: UV-visible spectrum of the Newport Hg-Xe 300W lamp; DRIFTS spectra of pure HNs under UV irradiation, of pure minerals; sublimation/deposition phenomena images; IR spectra computed in vacuum for 1,8-DHN and some of its representative oxidation products. AUTHOR INFORMATION Corresponding Author *Prof. Marco d’Ischia Phone: +39081674132 Fax: +39081674393 e-mail: [email protected] Author Contributions S.P., P.M. and A.N. performed HNs autoxidation. T.F. optimized mineral spiking and irradiation techniques. S.P. and G.P. performed UV-irradiation experiments. S.P. and J.R.B. performed DRIFTS analysis. O.C. and V.B. performed theoretical calculations. All authors discussed the results, contributed to data analysis and wrote the main manuscript text. M.d.I. contributed to refining the manuscript. ACKNOWLEDGMENTS This work was supported by INAF-Astrophysical Observatory of Arcetri and Italian Space Agency (ASI) grant agreement n. 2015-002-R.0; M.d.I. thanks the inter-university center “STAR” for support. We thank the referees for their helpful suggestions.


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ABBREVIATIONS PAH, polycyclic aromatic hydrocarbon; oxyPAH, oxygenated derivative of polycyclic aromatic hydrocarbon; HN, hydroxynaphthalene; DHN, dihydroxynaphthalene; DFT, density functional theory; ISM, interstellar medium; IDP, interplanetary dust particle; UCAMM, ultracarbonaceous Antarctic micrometeorites; CC, carbonaceous chondrite; IOM, insoluble organic matter; SAM, Sample Analysis at Mars; FT-IR, Fourier-transform infrared spectroscopy; DRIFTS, diffuse reflectance infrared Fourier-transform spectroscopy; AISSP, ammonia induced solid state polymerization; KM, Kubelka-Munk; NIR, near infrared; RNA, ribonucleic acid. REFERENCES (1)

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Solid State Photochemistry of Hydroxylated Naphthalenes on Minerals

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