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Photo-Induced Reversible Electron Transfer Between the Benzhydryl Radical and Benzhydryl Cation in Amorphous Water-Ice Soumya Radhakrishnan, Joel Mieres Perez, Murthy S. Gudipati, and Wolfram Sander J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05466 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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The Journal of Physical Chemistry

Photo-induced Reversible Electron Transfer Between the Benzhydryl Radical and Benzhydryl Cation in Amorphous Water-Ice Soumya Radhakrishnan, Joel Mieres-Perez, Murthy S. Gudipati1,*, and Wolfram Sander* Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany 1

Science Division, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.

* Corresponding authors

1

Also a senior research scientist, IPST, University of Maryland, College Park, MD 20742, USA.

ACS Paragon Plus Environment


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ABSTRACT: The benzhydryl radical is generated in high yields by flash-vacuum-thermolysis of 1,1,2,2-tetraphenylethane with subsequent trapping of the product in argon or amorphous water at 3 – 4 K. Photoionization of the radical with various UV light and electron sources produces the benzhydryl cation which was identified by IR and UV-vis spectroscopy. In solid argon, the formation of the benzhydryl cation is irreversible, whereas in amorphous water ice the electron transfer is reversible, and irradiation into the major absorption band at 443 nm of the cation leads back to the radical by electron attachment. Applications of ionization of organic matter trapped in water-ice to icy environments in astrophysics, planetary sciences, including Earth are discussed.

Earth and in a wide variety of environments in our Solar System and beyond.17-18

Introduction Co-existence of organic matter and water is one of the prerequisites for life to have evolved on earth. Though we still do not understand how exactly origin of life occurred, other necessary conditions are understood to be energy and minerals. Interestingly, all these four conditions are already met even before our Solar System had formed – during the interstellar molecular clouds phase. Interstellar ice grains of micron-size are constantly bombarded with cosmic rays, and photons from nearby stars. These interstellar ice grains1 are composed of micron-sized silicate (mineral) dust nucleus upon which water, carbon monoxide, carbon dioxide, ammonia, methanol, sulfur-containing molecules such as OCS, and perhaps even large polycyclic aromatic hydrocarbons (PAHs) condense forming the interstellar ice grain.2 Photochemical evolution of the interstellar ice grains is an important branch of astrochemistry and tremendous progress has been made in the past few decades that shows that building blocks of life could have been already produced in the interstellar ice grains.2-3 Once the dense molecular clouds in the interstellar medium collapses forming the first phase of a star and a solar system – called protostar and protoplanetary disk,4 these ice grains transform into planets, moons, asteroids, and the reservoirs comets at the outer rim of the Solar System, known as Kuiper Belt Objects (KBOs).5-6 Complex organic matter made in the interstellar medium and preserved through KBOs and comets and asteroids could have been delivered to Earth7-10 during the early stages of our Solar System formation (about 4 billion years ago),11 which also coincides with the time during which Life has evolved on Earth through dating the bacterial fossils.12 Ionized molecules are energy-rich and highly reactive species, making further chemical reactions mostly barrier-less processes. We found that even at 5 K, ionized PAHs readily undergo hydrogenation and oxygenation (forming hydroxy-PAHs).13 Understanding radiationinduced chemical pathways of organic matter in waterdominated ice medium is also important for our understanding of how pollution effects Earth’s cryosphere (polar ice caps to snow and glaciers to cirrus cloud dominated by ice grains).14-16 Finally, organic chemistry in ice medium also helps understand cryosolvation and radiation biology. Thus, photoionization of organic molecules to generate highly reactive species is the center of many complex chemical reaction pathways both on

Our earlier work showed that ionization of organic molecules, specifically PAHs is very efficient in water-ice medium.19-20 Both the ionized PAH (cation) and electron are stabilized in water-ice. However, the fate of electron is still not completely understood. The research work presented here specifically addresses two key questions: (a) How are organic radicals different from neutral molecules towards ionization in water-ice? Is it possible to attach electrons to aromatic radicals forming negatively charged anions in water-ice? We set out with the choice of diphenylmethyl (also known as benzhydryl) radical (1a) that is ideal candidate to answer these questions. Benzhydryl radical 1a can be efficiently produced through flash-vacuum-thermolysis (FVT) of 1,1,2,2-tetraphenylethane. One of the questions we addressed is if photoionization of 1a results not only in the benzhydryl cation (2a) but in addition in the benzhydryl anion by electron attachment? A major difference between ionization of a neutral molecule such as a PAH and a radical such as the benzhydryl radical 1a is that the former goes from a closed-shell to an open-shell electronic configuration, whereas the latter starts with open-shell and leads to closed-shell electronic configuration upon ionization through removal of an electron (cations) or by the addition of an electron (anions). Hence, comparing these two cases would help to understand the chemical evolution of organic matter in water-ice environment. Photochemistry of Organic Radicals: Organic radicals and carbocations are fundamental reactive intermediates that are of paramount interest to organic chemistry. In principal, these intermediates can interconvert via single electron transfer (SET). Thus, the direct photoionization of free organic radicals using short wavelength UV light or X-ray irradiation is a convenient way to produce the corresponding carbocations. If the intensity of light is high enough and the exited state lifetime long enough, two photon ionizations of radicals with near UV or visible light can become efficient. In a laser flash photolysis study, Faria and Steenken demonstrated that 308 nm irradiation of the benzhydryl radical 1a in acetonitrile or aqueous ethanol produces the benzhydryl cation 2a via two photon ionization (Scheme 1).21 The lifetime of the excited state of 1a in acetonitrile is 100 – 330 ns, long enough to absorb a second photon that leads to


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ionization. The ionization yield increases in the presence of n-butylchloride, which is able to trap the ejected electron and thus prevents recombination of the initially formed cation – solvated electron pair. This solvation of electron and charged molecules plays a critical role in photoionization-mediated chemical pathways. To produce carbocations 2 via single photon ionization, the photon energy has to be larger than the ionization potential of the radical 1, which requires vacuum UV or Xray irradiation.

Scheme 3.

Scheme 1

Riedle et. al. described the ultrafast UV photochemistry of benzhydryl chloride, which predominantly results in the homolytic bond cleavage and formation of a radical pair between 1a and Cl atoms.22 Radical pairs with small interradical distances undergo efficient electron transfer (43% in CH3CN as solvent) to form benzhydryl cations and chloride anions with a mean electron transfer time of 22 ps (Scheme 2). Interestingly, the direct heterolysis of Ph2CHCl to form the ion pair is only a very minor reaction channel.

Since the photoionization in the solid state produces charged species, the polarizability of the matrix host as a bulk and polarity (dipole moment) of the individual molecules and microenvironments should have a large influence on the yield and stability of the cation. Both recombination with the free electron and reactions with reactive matrices reduces the yield of the cation. Thus, the phenyl cation 2b even reacts with molecular nitrogen in a barrierless, highly exothermic reaction. Lowtemperature amorphous water ice is known to stabilize the cationic species,25-26 and polar organic glasses are expected to stabilize cations, but might also lead to unwanted side reactions with the host matrix.

Scheme 2 Low density amorphous water ice (LDA ice) is a polar matrix that stabilizes cations and thus reduces the ionization potential of neutral compounds.20 Gudipati et al. studied the photoionization of polycyclic aromatic hydrocarbons (PAHs) in LDA ice and estimate that this matrix stabilizes the radical cations of these hydrocarbons by up to 2 eV.26-27

Photoionization can also be used to generate carbocations under the conditions of matrix isolation. Thus, vacuum UV irradiation, using an argon discharge light source, of the phenyl radical 1b during deposition of an argon matrix produces the phenyl cation 2b, one of the most reactive carbocations (Scheme 3).23-24 In solid argon at cryogenic temperatures cation 2b is stable and could be characterized by IR spectroscopy. In a very detailed study, Bally et al. used X-ray irradiation to ionize the allyl radical 1c and the benzyl radical 1d to obtain the corresponding allyl and benzyl cation 2c and 2d, respectively.25 In these experiments, iodides R-I were used as precursors for the radicals. The iodine atoms formed concomitantly serve as excellent scavenger for the electrons produced during the ionization.

Recently, we reported a new technique for the matrix isolation of the benzhydryl cation 2a and the fluorenyl cation 2e by protonation of the corresponding carbenes 3 and 4, respectively in LDA ice (Scheme 4).28-29 The singlet states of carbenes 3 and 4 are very strong, neutral bases that even at extremely low temperatures (3 K) are rapidly protonated by water molecules to produce OH- and the cations 2a and 2e, respectively. The hydroxide ion is efficiently stabilized in the hydrogen bond network of the water matrix, and recombination with the cations to form the corresponding alcohols is inhibited.



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Scheme 4

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carried out by slow deposition of 1a in highly pure argon (Messer Griesheim, 99.99%) through a pyrolysis cum sublimation oven heated electrically with a tantalum wire. The experiments in LDA ice matrix were performed in the same manner by replacing the argon with water. However, water was degassed several times before deposition.

Here, we describe the reversible photoionization of the benzhydryl radical 1a in LDA ice matrices. The efficiency of the photoionization of radical 1a depends on two main factors: (i) the capability of the matrix to stabilize the cation 2a, and (ii) efficient trapping of the electron ejected from 1a that hinders electron recombination with the cation 2a to form back the radical 1a. We therefore studied the photoionization in both apolar solid argon and polar LDA ice in the presence or absence of electron scavengers. For these studies, we had to develop a highly efficient radical source that avoids the generation of electron scavengers such as halogen atoms as byproducts. Thus, photolysis or thermolysis of halides R-X, which are frequently used radical sources, was not advised. Instead, we used the flash-vacuum-thermolysis (FVT) of 1,1,2,2tetraphenylethane 5 with subsequent trapping of the products in argon or LDA at cryogenic temperatures (4 – 20 K) to produce radical 1a in very high yield. In a similar way, we previously synthesized and matrix-isolated the benzyl radical 1d.30-31

Experimental Details Experiments were conducted both at the Ruhr-University of Bochum (RUB), Germany and the NASA Jet Propulsion Laboratory, California Institute of Technology (JPL). While RUB work focused on laser and light-emittingdiode (LED) photolysis of 1a in Argon (Ar) and water-ice matrices, JPL work was focused on deuterium-halogen and Xe lamp photolysis as well as electron codeposition/irradiation (5 eV – 2000 eV). At both laboratories infrared (FTIR) and ultraviolet-visible (UVVis) spectra were routinely measured. At JPL these spectra were simultaneously measured on the same sample. ESR experiments as well as density functional theory (DFT) vibrational spectral calculations were conducted at RUB. All chemicals and solvents were used as received without further purification. 1,1,2,2tetraphenylethane (5) was purchased from Sigma Aldrich (99% purity). Matrix Isolation Technique. At RUB, Matrix isolation experiments were performed by standard techniques using Sumitomo Heavy industries two-staged closed-cycle helium cryostats (cooling power 1 W at 4 K) to obtain temperatures around 3 K. For experiments with Argon as the matrix, flash-vacuum-thermolysis (FVT) of 5 was

Infrared spectra in the range between 400 and 4000 cm−1 were recorded on FTIR spectrometers with 0.5 cm−1 resolution. UV irradiation was carried out with LED lights of 365 nm wavelength and energy of 5W. Broadband irradiation with 280-400 nm light was carried out with mercury high-pressure arc lamps in housings equipped with quartz optics and dichroic mirrors. A 308 nm XeCl laser was also used with a frequency of 8 Hz. Matrix EPR spectra were recorded with a Bruker ELEXSYS 500 X-band spectrometer. Matrix UV-Vis spectra were recorded with a Varian Cary 5000. UV-Vis-NIR spectrophotometer in the range of 200 – 800 nm with a resolution of 0.1 nm. At JPL, Matrix isolation experiments were performed by standard techniques using closed-cycle helium cryostat reaching 4.2 K named Himalaya (Sumitomo SHI closedcycle-cryostat sold by Janis). The ultrahigh vacuum chamber containing the sapphire window onto which the sample with water vapour will be deposited is also equipped with a 2keV electron gun, a UV-Vis light source, UV detector, an IR light source, DTGS detector, and the pyrolysis oven. Samples were irradiated with a Kimball Physics electron gun system (ELG-2/EGPS-1022) with an electron energy range of 100–2000 eV and between 1 μA - 5μA in emission current control mode. UV–Vis–NIR absorption spectra were measured with an Ocean Optics fiber optics CCD spectrometer (HR4000CGUV-NIR). A dual-lamp (deuterium and halogen) light source to cover 200-900 nm wavelength region (Mikropack, DH-2000-BAL) was used for UV–VIS spectroscopy. A CaF2 plano-convex lens was used to focus the UV–Vis fiber optic light source onto the sample. Infrared spectra in the range between 400 and 4000 cm−1 were recorded on FTIR spectrometers with 1 cm−1 resolution. Preparation of Low Density Amorphous Ice (LDA). Ultrapure water was degassed by successive freeze-thaw and held at room temperature. The water vapours were allowed to co-deposit together with benzhydryl radical 1a (produced by thermolysis of 1,1,2,2-tetraphenylethane 5) into a high vacuum system in different substrates (oxygen free Cu rod, CsI and sapphire windows respectively for EPR, FT-IR and UV-vis spectroscopy). The deposition rate of the water vapour was controlled by a fine metering valve. For Electron Irradiation experiments, water vapour was allowed to co-deposit together with benzhydryl radical 1a (produced by thermolysis of 1,1,2,2-tetraphenylethane 5)


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and electrons of various energy (10 eV-50eV) from the electron gun into a high vacuum system containing a cold sapphire window. This was then followed by further irradiating the sample with electrons with an electron range of 100eV-2000eV and between 1 μA - 5μA in emission current control mode. Computations. Gas phase geometry optimizations and frequency calculations were performed using the B3LYP3234 density functional with D3 dispersion corrections35, employing the 6-311++g(d,p) basis sets as implemented in Gaussian 09 revision E.0136.

Results and Discussion Photoionization of 1a in solid argon. FVT of precursor 5 at 550°C with subsequent trapping of the products in a large excess of argon at 4 – 20 K produced very clean IR, UV-vis, and EPR spectra of the benzhydryl radical 1a. Radical 1a was identified by comparison with UV-vis22, 28, 37 and EPR38-39 spectra published in literature. The UV-vis spectrum of 1a in solid argon shows a sharp band with a maximum absorption at 324 nm and a progression at 304 and 296 nm (Figure S1) which is similar to the spectra reported in LDA ice and CH3CN.

using a XeCl excimer laser (308 nm, 0.3 J/pulse), although the ionization of 1a is still not efficient (Figure 2). These experiments demonstrate that matrix-isolated radical 1a can be ionized via one or two photon processes, however, the yield of 2a is very low, presumably because of the ejected electrons recombine with the cation. Such lowyields of photoionization of PAH molecules trapped in Ne and Ar matrices has been well documented in the literature and is attributed to electron-ion recombination within the matrix cage soon after ionization. A strategy to increase the yield of 2a is therefore to use electron scavengers such as molecular oxygen or CH2Cl2 or CCl4(reference).28 When the argon matrices were doped with 1% O2 or CH2Cl2, the yield of 2a increased drastically. With a conversion efficiency of 20% of 1a, the highest yields of 2a were obtained with CH2Cl2 as electron scavenger and 308 nm laser photolysis (Figure 1, Table 2). In these experiments, the IR bands of 1a and of CH2Cl2 decrease in intensity during irradiation, while that of 2a and of the CH2Cl radical are formed (Figure S7). Analysis of the spectra reveals that equal amounts of 2a and CH2Cl are formed, indicating that CH2Cl2 is efficiently cleaved into CH2Cl and Cl– anion upon electron attachment. Electron capture with O2 is less efficient and leads to lower yields of 2a. Presumably, O2– is formed which is not visible in the IR.

The IR spectrum of 1a shows strong bands at 1446.6, 777.8, 688.1, and 675.9 cm-1 and is in good agreement with DFT calculations (Figure S5, Table 1). In the EPR a very strong signal in the region between 330 and 340 mT was assigned to the radical 1a (Figure S2). The FVT of 5 is highly efficient, and yields of 1a of >95% can be achieved. The only major contamination observed in the spectra of these matrices is remaining precursor 5 (< 5%). The photoionization of radical 1a was investigated in solid argon using an argon discharge lamp (105 nm cutoff, corresponding to 11.6 eV) and a XeCl excimer laser (308 nm, corresponding to 4.0 eV). The ionization potential of 1a in the gas phase is calculated to be 6.5 eV at the B3LYPD3/6-311++G(d,p) level of theory, and therefore, the energy of the light of the argon discharge lamp is more than sufficient for the ionization of 1a, whereas two photons would be required with the XeCl laser (308 nm). During Ar-discharge-lamp photolysis of 1a in argon at 10 K, the matrix turned slightly yellow, and a weak visible absorption with a maximum at 443 nm is observed. This band is close to that reported for the benzhydryl cation 2a obtained by protonation of diphenylcarbene 3 in lowdensity-amorphous (LDA) ice.28 The IR spectrum of 2a in argon shows strong IR bands at 1580, 1522, and 1180 cm-1, in good agreement with predictions from DFT calculations (Figure 1). The yield of 2a obtained during the photolysis is low and does not increase after prolonged irradiation. Slightly higher yields of cation 2a are obtained if matrix-isolated radical 1a is irradiated

Figure 1. IR spectra of benzhydryl cation 2a in argon matrices at 3 K. a) Difference IR spectrum showing the formation of 2a upon 308 nm irradiation of 1a in solid argon. b) Difference IR spectrum produced after 308 nm irradiation of 1a in 1% O2-doped argon. c) Difference IR spectrum produced after 308 nm irradiation of 1a in 1% CH2Cl2 doped argon. d) Difference IR spectrum 2a - 1a calculated at the B3LYP-D3/6-311++G(d,p) level of theory. Peaks pointing downwards belong to 1a while peaks pointing upwards belong to benzhydryl cation 2a.



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Figure 2. IR spectra showing the formation of benzhydryl cation 2a from 1a trapped in 1% CH2Cl2 doped argon at 3 K. a) IR spectrum showing the formation of 1a on FVT (550 560°C) of 5. b) IR spectrum produced after 308 nm irradiation of the matrix for 3 hours. c) IR spectrum produced after 450 nm irradiation of the same matrix for 1 h. d) IR spectrum after 450 nm irradiation of the same matrix for 3 h.

Photoionization in amorphous water ice. FVT of 5 and trapping of the products with water at 4 K produced transparent matrices of LDA water ice doped with radical 1a (Figure S6). As described above, radical 1a in water was identified by IR, UV-vis, and EPR spectroscopy, and a very good match between the spectra in solid argon and LDA water ice was found (Figure S1 to Figure S6). The major difference is that the IR and EPR spectra in water show broader linewidths and less fine structure than in argon. This presumably results from the less homogeneous matrix environment in water compared to argon, as noted in earlier publications.26, 40

Figure 4. Photochemical interconversion between radical 1a and cation 2a in LDA water ice at 10 K. Red line: UV spectrum after 450 nm irradiation of 2a. Black line: UV spectrum after 308 nm irradiation of the same matrix. Dotted red line: after subsequent 450 nm irradiation of the same matrix 1a is recovered. Dotted black line: after subsequent 308 nm irradiation of the same matrix 2a is formed again.

The photoionization of 1a in LDA ice is highly efficient with the 308-nm laser, resulting in high yields (approximately 60%) of the benzhydryl cation 2a (Figures 3). The IR spectrum of 2a in water obtained by photoionization of 1a is in excellent agreement with that obtained by protonation of carbene 3 in the same matrix, clearly indicating the formation of 2a via two independent routes: radical ionization and carbene protonation (Figure 3 and Figure S8). If 1a is isolated in a D2O matrix, 308 nm photolysis leads to 2a without incorporation of deuterium, as expected. In contrast, if 2a is synthesized via carbene protonation, deuterium is incorporated in solid D2O (Scheme5). Figure 3. IR spectra of benzhydryl cation 2a in LDA ice matrix at 3 K. a) Difference IR spectrum showing the formation of 2a upon 308 nm irradiation of 1a. Peaks pointing downwards show the disappearance of 1a while peaks pointing upwards shows the appearance of 2a together with 5 (*). b) IR Spectrum of 2a obtained by protonation of 41 carbene 3 in LDA ice.

Energetics of photoionization in amorphous water ice. As seen in Figure 4, the strongest absorption band of 1a is centered around 330 nm. Hence, efficient ionization with 308 nm excimer laser is understandable, however energy available in a 308-nm photon is 4 eV, about 2.5 eV lower than the gas-phase ionization of 1a mentioned earlier. Lowering of the ionization energy of PAHs trapped in amorphous water ice has been observed earlier


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at similar magnitudes42 and theoretically corroborated.43 Thus, photoionization of 1a to form 2a with 308 nm laser is not very surprising, though multiphoton ionization cannot completely be ruled out under the conditions used for excimer laser photoionization. However, photoionization of 1a in water is also efficient with lowintensity light sources such as an in-house built (at RUB) LED lamps with narrow spectral bands. We used a 365 nm LED with full-width-at-half-maximum (FWHM) of ~30 nm spanning from 340 nm to 400 nm or a high-pressure mercury arc lamp with 320 nm cut-off filter. At JPL, we found that prolonged waiting subsequent to the FVTdeposition of 1a in LDA, while keeping the UV spectrometer light source constantly irradiating the ice sample results in continuous photoionization as well. As seen in Figure 5, growth of 2a was recorded while 1a was observed to reduce in intensity. The light source of the UV lamp is a combination of deuterium and halogen sources connected through fiber optics to the vacuum chamber, allowing light at >210 nm to pass through the sample. All these observations clearly indicate lowering of the ionization energy of 1a in amorphous water ice, in agreement with earlier studies.

spectrometer lamp on. During the entire experiment 1a is continuously depleted while 2a is produced due to photoionization of 1a. Electron Irradiation of 1a in LDA water-ice. With the aim of finding out whether low-energy electrons (5 – 50 V) would attach to 1a forming anion of benzhydryl and/or whether electron irradiation could also result in ionization of 1a to produce 2a, mimicking some of the radiation-processing that occurs in our solar system (through solar wind) and interstellar medium (cosmic ray induced local production of low-energy electrons in dense molecular clouds). For the formation of anion, there is a competition between water-ice and 1a in terms of their electron affinity. Vertical dissociation energies (VDE) of electrons attached to water clusters are experimentally measured to be between 0 and 3.5 eV, increasing with size of the cluster.43-44 If electron affinity of 1a is higher than the surrounding water matrix, one can expect electron attachment to 1a. Our attempts to conduct low-energy electron attachment while depositing 1a simultaneously with electrons or subsequent irradiation with electrons did not result in any new product that could be assigned to the benzhydryl anion. The only product we found was benzhydryl cation 2a, with higher yields when 30-50 eV electrons were co-deposited. When irradiated with higher energy electrons, though 1a was depleted, no significant yields of 2a was observed due to subsequent chemistry caused by excess energy in the electrons. These results clearly demonstrate that water-ice traps electron more efficiently than 1a. However, the results discussed in the next section show that 2a has stronger electron-affinity when electronically excited than the water-ice surroundings.

Figure 5. Top (A): FVT-deposition of 1a in amorphous water-ice followed by monitoring the changes in the UVVis spectra while waiting with UV-Vis spectrometer lamp irradiating the ice. Bottom (B): irradiation with 100 eV electrons for 5 min subsequent to the waiting shown in A, followed by another 40 min waiting with UV-Vis



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Scheme 5.

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corresponding to cation (450 nm) and radical (308 nm) of benzhydryl showing that the electron is readily accessible in the amorphous water-ice matrix either as a trapped electron (hydrated electron) or as a OH–. This is first of its kind observation. Note that not all 2a is converted back to 1a when irradiated at 450 nm, indicating that only some matrix-sites are accessible for such photoinduced backelectron-transfer (BET). Electron capture by radical 1a was not observed indicating that the electron affinity of 1a may be lower than water-ice matrix itself. However, electron irradiation at higher energies (100 eV – 2 keV) results in depletion of radical 1a and formation of cation 2a, with less efficiency as the energy increases due to further side reactions occurring caused by excess electron energy. Ionization-mediated chemical reaction pathways are general phenomena in water-ice environment. Ionized organics (charged species) are stabilized in ice while electrons are also efficiently trapped. In the light of this and other studies reported earlier, it is safe to conclude that organic chemistry in ice environment is strongly mediated by ionization – whether it is in Earth’s cryosphere or in our solar system or in the interstellar space.

Reversible photo-induced electron-ion recombination in amorphous (LDA) water ice. The photoionization of 1a in water ice is almost quantitatively reversible. When irradiated into the strong visible absorption of 2a with a maximum at 443 nm (450 nm LED, 3 W) results in the almost quantitative recombination of 2a with an electron leading to the back formation of the radical 1a (Figure 4). It is likely that the original electron is trapped very much in the vicinity of the cation 2a that is readily accessible for the excited 2a to react with. Near quantitative conversion of benzhydryl cation 2a to benzhydryl radical 1a by photo-induced capturing of an electron strongly suggests that the electron affinity of benzhydryl cation 2a is higher than water-ice. Earlier studies demonstrated thermally mediated electron-ion recombination, indicating the availability of electrons in amorphous water ice45 Further studies are warranted to understand the nature of electrons produced/trapped in amorphous water ice that are readily available for photochemical reaction pathways. Though solvated (hydrated) electron in water clusters are well studied,20, 45 identifying electrons in bulk ice is still a work in progress.

Conclusion Organic radicals behave similar to neutral PAHs that their photoionization is very efficient in amorphous water ice (LDA). Lowering of ionization energy from 6.5 eV to around 3.8 eV (LED shorter wavelength limit) is also in agreement with earlier studies. Photoinduced single electron transfer has been observed to be very efficient, near quantitative, when excited into the UV-Vis bands

ASSOCIATED CONTENT Supporting Information. Additional spectroscopic data (figures and tables) of the experiments in argon and water matrix, additional computational results including optimized structures of all reactive species.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected];

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft (DFG). Part of the work that was carried out at the Jet Propulsion Laboratory, California Institute of Technology, was under a contract with the National Aeronautics and Space Administration (NASA) through NASA Solar System Workings and Planetary Atmospheres grants to MSG.

REFERENCES 1. Greenberg, J. M., Interstellar grains. Annu. Rev. Astron. Astroph. 1963, 1, 267-&. 2. Öberg, K. I.; Boogert, a. C. A.; Pontoppidan, K. M.; van den Broek, S.; van Dishoeck, E. F.; Bottinelli, S.; Blake, G. a.; Evans, N. J., The spitzer ice legacy: Ice evolution from cores to protostars. Astrophys. J. 2011, 740, 109.


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Table 1. Experimental and calculated vibrational frequencies of benzhydryl radical 1a. a

Mode

Sym

11 12 16 17 18 19 20 22

B B B A B A B B

Calculated -1 d ν/cm (Iabs) 489 (20) 578 (12) 689 (59) 698 (6) 711 (32) 758 (5) 794 (56) 837 (1)

25

B

912 (7)

31 33 35 36 43 48 49 50 53 54 56 62 63

B B B A A A B B A B A A B

1001 (1) 1044 (8) 1102 (1) 1120 (7) 1327 (1) 1474 (7) 1496 (10) 1507 (23) 1588 (2) 1605 (6) 3138 (7) 3173 (45) 3186 (49)

b

c

Argon -1 e ν/cm (Irel) 486.6 (15.3) 563.5 (19.4) 675.9 (100) 683.8 (15.1) 688.1 (31.2) 745.9 (14.1) 777.8 (97.2) 822.4 (4.2) 894.3 (14.3) 896.2 942.4 (1.1) 1024.3 (12.4) 1092.4 (4.1) 1108.8 (6.2) 1320.0 (1.6) 1446.6 (39.8) 1473.3 (32.9) 1478.4 (20.7) 1567.3 (2.3) 1591.4 (0.7) 3013.1 (64.5) 3067.5 (73.4) 3077.3 (91.3)

LDA Ice -1 e ν/cm (Irel) 486.5 (21) 563.8 (18) 677.4 (66.2) 746.8 (42.3) 781.4 (100) -

Assignment

897.9 (34.15) 1021.6(4.5) 1109.3 (13.7) 1320.5 (11.9) 1447.5 (40.2) 1475.2(19.7)

Asymm. C-C-C str.

-

a

C-H str. C-H str. C-H str. b

c

d

Calculated at the B3LYP-D3/6-311++G(d,p) level of theory. In argon matrix at 3 K. In LDA ice at 3 K. Absolute intensities in e km/mol. Relative intensities based on the strongest observed absorption band

Table 2. Experimental and calculated vibrational frequencies of benzhydryl cation 2a. Mode

Sym

11 12 16 19 24 27 39 41

a

b

B B B B B B B B

Calculated -1 e ν/cm (Iabs) 493 (70) 578 (44) 674 (71) 783 (88) 957 (26) 1007 (46) 1214 (158) 1252 (44)

LDA Ice -1 f ν/cm (Irel.) 482.0 (4.7) 561.0 (2.2) 995.3 (2.3) 1180.0 (31.8) 1223.7 (6.7)

44

B

1369 (106)

1338.8 (14.5)

46 47 48 51 52 54

B B A B B B

1384 (180) 1458 (60) 1480 (50) 1560 (598) 1587 (44) 1623 (574)

1362.0 (6.9) 1426.0 (8.7) 1451.0 (8.7) 1521.6 (100) 1580.1 (82.9)

a

c

Argon , 1% CH2Cl2 -1 f ν/cm (Irel.)

d

Assignment

-3.5 +0.2 -0.2 +2.3

564.5 (9.4) 660.8 (0.5) 772.1 (6.3) 938.1 (0.8) 995.6 (4.7) 1180.2 (21.3) 1221.4 (3.6) 1332 (20.2) 1344.4 1361.3 (11.6) 1429.4 (21.6) 1452.4 (8.3) 1523.7 (100) 1561 (0.8) 1580.3 (87.2) b

Shift

+0.7 -3.4 -1.4 -2.1 -0.2

C-C-C asym. str. C=C str. Ring

c

d

Calculated at the B3LYP-D3/6-311++G(d,p). In LDA ice at 3 K. In 1% CH2Cl2 doped Argon matrix at 3 K. Frequency shift e f relative to benzhydryl cation 2a in LDA ice matrix. Absolute intensities in km/mol. Relative intensities based on the strongest observed absorption band


12

Photoinduced Reversible Electron Transfer ... - ACS Publications

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