Article pubs.acs.org/JPCC
Tuning the Aromaticity of s‑Triazine in the Crystal Phase by Pressure Samuele Fanetti,† Margherita Citroni,*,†,‡ and Roberto Bini†,‡ †
LENS, European Laboratory for Nonlinear Spectroscopy, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy Dipartimento di Chimica “Ugo Schiff” dell’Università degli Studi di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Firenze, Italy
‡
ABSTRACT: The effect of pressure on the electronic properties of crystalline s-triazine has been studied up to 14 GPa by using two-photon induced fluorescence. Emission and excitation spectra have been measured as a function of pressure on samples compressed in a diamond anvil cell. The different two-photon absorption cross sections to the nπ* and ππ* excited states account for the selectivity in the excitation wavelength observed in the high pressure photoinduced reactivity. The comparison between excitation and emission spectra highlights a remarkable red shift with rising pressure of the higher electronic excited states having ππ* character, which contrasts with the pressure insensitivity of the lowest nπ* states. Pressure is therefore extremely efficient at progressively destabilizing the π bonding orbitals, causing a reduction of the ring aromaticity, and driving the high pressure reactivity.
1. INTRODUCTION Pressure is an extremely powerful tool to modify the structural and electronic properties of soft materials like molecular crystals. These changes are intimately related to the chemical transformations observed in many molecular systems at high pressure.1 While the relation between crystal structure and reactivity is rather well established, at least from a qualitative point of view, through the topochemical principle2 and the lattice phonon effect,3 by far less understood is that related to the change of the electronic properties owing to the experimental and computational difficulties to access this information. Since the fundamental work of Drickamer et al.,4 where a rationalization of the pressure capability in building thermally accessible electronic excited states was attempted, important advances in this specific field were only recently performed by correlating reactive processes to pressure induced modifications of the electronic states. Two-photon (TP) induced excitation and fluorescence spectra were measured in crystalline benzene evidencing the formation of structural excimers which act as nucleation sites for the transformation of benzene into an amorphous hydrogenated carbon.5 The excimer formation was observed to be tuned by pressure and was ascribed to a progressive reduction of the energy barrier separating the monomeric and dimeric forms in the lowest electronic excited state. The comparative study of the high pressure reactivity of the simplest alcohols, methanol and ethanol, through TP induced dissociation processes6−8 provided evidence of a reduction of the dissociative character of the lowest electronic excited state along the OH coordinate. These examples demonstrate how the electronic information is crucial to identify the reactive pathways and how they can be modified or selected by tuning suitable conditions of photoexcitation, temperature, and pressure. For example, a consistent lowering of the reaction pressure has been found in several cases, remarkable examples being benzene9 and © 2014 American Chemical Society
ethylene,10 while the selective excitation of compressed fluid butadiene led to the quantitative switch from dimerization to polymerization reactions.11 Recently, the high pressure transformation of crystalline striazine (1,3,5-triazine) to an extended nitrogen rich amorphous carbon was investigated in different pressure, temperature, and photoirradiation conditions in order to disentangle their contributions to the chemical reaction.12 Pressure induced reactivity of s-triazine is an interesting issue because of the possibility to exploit the symmetric arrangement of the nitrogen atoms in the ring, as well as the favorable nearest neighbor interaction geometry of the molecules in the crystal, to synthesize appealing technological materials. Among them are worth being mentioned high energy density materials (HEDMs),13 layered graphitic (C,N) materials which are of interest as twodimensional materials in nanoelectronics,14 as metal-free extended photocatalysts for hydrogen production from water splitting,15,16 or as intermediates for the production of superhard carbon nitrides.17 Photoirradiation was shown to be extremely efficient in lowering the reaction threshold pressure of s-triazine from 8 to 4 GPa, also allowing the attainment of a material with increased mechanical properties and a darker coloration with respect to that obtained by pressure alone. These properties were speculated to be due to a clustering of planar conjugated domains. Interestingly, the photoinduced reaction is extremely sensitive to the wavelength used, being triggered by the TP absorption of 457 nm photons but not when radiation at 514.5 nm is employed. This occurrence is difficult to explain since both wavelengths should be TP absorbed because the lowest electronic transitions of the isolated molecule, all having nπ* Received: February 26, 2014 Revised: May 21, 2014 Published: June 3, 2014 13764
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character and being TP allowed, are centered at about 270 nm at ambient pressure.18 Nevertheless, for a reliable interpretation of the laser assisted high pressure reactivity of s-triazine, a characterization of the electronic properties as a function of pressure is mandatory. In this work the electronic properties of crystalline s-triazine have been investigated as a function of pressure up to and above the reaction threshold pressure (∼8 GPa) by means of TP excitation and fluorescence spectra. A selective increase in the energy of the π molecular orbitals is evidenced with rising pressure. This is likely the reason for the destabilization of the aromatic ring leading to the high pressure chemical instability of the s-triazine crystal.
2. EXPERIMENTAL SECTION Transparent crystals were obtained by sublimation of s-triazine powder from Sigma-Aldrich (97%) and loaded without any pressure medium in a membrane diamond anvil cell (MDAC) equipped with low fluorescence synthetic IIa diamonds. The samples were contained by rhenium gaskets. In order to avoid interference in the fluorescence measurements the sample was loaded with no pressure gauge and the pressure was obtained by the peak frequency of the ν3 vibrational mode whose pressure shift was previously determined by measuring the FTIR spectrum using the ruby fluorescence method for pressure calibration.12 Infrared absorption measurements were performed with a Bruker-IFS 120 HR spectrometer suitably modified for high pressure experiments with an instrumental resolution of 1 cm−1.19,20 An optical parametric generator (Ekspla PG401), pumped by the third harmonic of a picosecond Nd:YAG laser (Ekspla PL2143A), was employed to excite the fluorescence which was detected in a backscattering geometry. A careful description of the entire setup is reported elsewhere.5,21 Shortly, an achromatic doublet with a 100 mm focal length was employed to focus the beam matching the sample diameter, and the fluorescence was collected backward by a parabolic aluminum mirror, filtered by a monochromator, and finally revealed by a cooled Electron Tubes 9235QB photomultiplier. The energy on the sample was set to ∼100 nJ per pulse at a repetition rate of 10 Hz. Fluorescence spectra were measured with a resolution of ∼1 nm, and the excitation spectra were sampled in steps of 1 nm.
Figure 1. Left: one-photon absorption spectra of s-triazine measured at room conditions in an isoctane solution (red trace) and in the rhombohedral crystal (green trace); the assignment of the 0−0 transition to the 1E″ nπ* state in the crystal is from ref 25. Right: molecular structure of s-triazine (top); single molecule scheme of the HOMO−LUMO transitions and irreducible representations to which each molecular orbital belongs (bottom);red and blue arrows indicate the lowest and highest energy nπ* transitions, respectively.
induced, overlap giving rise to a broad and strong absorption band in one-photon absorption spectra. In cyclohexane solution this band is centered at about 272 nm shifting to higher energy with increasing the polarity of the solvent as expected when nonbonding electrons are involved in the transition.18 Vapor and polarized crystal absorption experiments,25 TP photoacoustic spectroscopy,26 and jet cooled laser excitation27 confirmed the calculation results,22,23 providing evidence that the lowest excited state has E″ symmetry with the origin located at 30870 cm−1 (323.9 nm) in the gas phase,25,26 shifting to 30014 cm−1 (333.3 nm) in the monoclinic crystal at 4.2 K.25 The second transition is toward the symmetry allowed 1A2″ state, but Jahn−Teller distortion of the 1E″ state and the pseudo Jahn−Teller interaction with the 1A″2 state25 prevent a reliable assignment of the higher energy vibronic features.26,27 The first set of ππ* transitions involves the couple of π and π* molecular orbitals both belonging to the E″ irreducible representation giving rise to A′1, A′2, and E′ states. Only the transition to the 1E′ state is one-photon allowed in the isolated molecule, and it is observed in the gas phase, 0−0 transition, at 61300 cm−1 (163.2 nm).28 In the gas phase, the lowest ππ* transition has a one-photon absorption intensity about 3 orders of magnitude smaller than that to the 1E′ excited state. It is observed in solution at 45350 cm−1 (220.5 nm) independently of the solvent polarity and is assigned to the dipole moment forbidden 1A′1 → 1A′2 ππ* transition.18 The UV absorption spectra of s-triazine measured on an isooctane solution and on a thin crystal at room conditions are reported in Figure 1 in perfect agreement with the previous literature. In studying the electronic properties of crystalline s-triazine it is useful to recall that the crystal structure at ambient conditions is rhombohedral R3̅c (D63d; Z = 2), indicated as phase I, transforming below 198 K and ambient pressure to a monoclinic C2/c structure (C62h; Z = 2).29,30 This transition has also been observed by compressing the phase I crystal at temperatures above 200 K.31−33 At ambient temperature the I−II phase transition occurs at 0.5 GPa and, according to group theory, all the aforementioned electronic transitions are predicted to be both one- and two-photon allowed in the high pressure monoclinic phase.
3. ELECTRONIC PROPERTIES OF S-TRIAZINE The highest energy occupied molecular orbitals (HOMO) of striazine are of nonbonding n-type character, mainly localized on the three nitrogen atoms, while the lowest unoccupied molecular orbitals (LUMO) are π*.22 As reported in the inset of Figure 1, two groups of nπ* transitions are predicted. Using the single molecule description, point group D3h, the lowest energy nπ* transition set involves the n and π* molecular orbitals of E′ and E″ symmetry (red arrows in Figure 1), respectively, giving rise to three singlet excited states of A″2 , E″, and A″1 symmetry. Calculations indicate that the lowest energy singlet nπ* state belongs, in the isolated molecule, to the E″ symmetry,22,23 but only the transition to the 1A″2 state is dipole allowed.24 The second set of nπ* transitions involves the lowest energy nonbonding orbital of A1′ symmetry and the E″ π* orbitals (blue arrows in Figure 1), giving rise to another E″ singlet state, which can interact through higher order configurational interaction with the state of the same symmetry obtained by the lowest energy nπ* transitions set. All the transitions to the nπ* states described so far, symmetry allowed or vibronically 13765
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4. RESULTS AND DISCUSSION We performed a two-photon induced fluorescence study at ambient temperature as a function of pressure in the whole stability range of the monoclinic phase II up to and above the reaction threshold pressure, evidencing how the electronic structure is modified by the density increase. Triazine is known to be fluorescent in the vapor phase emitting from the lowest electronic excited state 1E″ having nπ* character.27,34,35 The TP excitation spectrum recorded at ambient conditions is reported in Figure 2. A remarkable difference with respect to the OP
structure. Nevertheless, it is extremely evident that the emission spectrum is almost insensitive to pressure, at least on its blue edge, whereas a macroscopic red shift of the edge of the ππ* transitions follows the compression. The extremely small TP cross section of the transitions to the nπ* states, which prevents a sufficient population of the π antibonding states, nicely accounts for the reported inability to photoinduce the reaction in compressed s-triazine at 4 GPa by exciting at 514 nm,12 a wavelength that is TP resonant with the nπ* states. On the contrary, taking into account the relevant red shift of the ππ* states, excitation at 457 nm gives access to the edge of the ππ* states thus explaining the reaction efficiency. Insight into the pressure effects on the electronic properties can be gained by plotting the energy of the red edge of the transition to the lowest ππ* state as a function of pressure (Figure 4). The reported
Figure 2. TP excitation spectrum of rhombohedral s-triazine at ambient conditions revealing the fluorescence at 370 nm. For the sake of clarity, we also reported the center of the absorption bands as determined in the OP spectra shown in Figure 1 and relative to the transitions to the nπ* and ππ* states.
spectra concerns the relative intensity of the nπ* and ππ* transitions which is inverted in the two spectra. In fact, while the TP transitions to the nπ* states are barely detectable, a steep increase of the emission signal is observed when we pump the crystal above 450 nm, therefore resonant with a TP transition to the lowest ππ* state. Two-photon excitation and fluorescence spectra recorded with increasing pressure are reported in Figure 3. Owing to the small TP cross section of the nπ* transitions, the fluorescence spectra could only be measured by exciting on the ππ* states. This prevented the study of the red part of the emission because of the overlap between the excitation and detection wavelengths. The two sets of spectra appear unstructured not allowing the identification of a vibronic
Figure 4. Pressure evolution of the edge of the ππ* transition estimated from the TP excitation spectra. The onset pressure of the purely pressure induced reaction obtained from the infrared spectra12 is also reported.
energy values are the intercepts between the linear fits of the low energy band wing and of the baseline. This information is only qualitative, because the energy value obtained by this procedure can be affected by a pressure dependent band broadening besides the transition energy shift.4 Nevertheless, interesting information gained from this evolution regards the pressure at which the red shift of the ππ* transition edge stops, which corresponds to the pressure where the chemical transformation of s-triazine is induced by compressing the sample at ambient temperature.12 The comparison of excitation and emission spectra provides fundamental information about the modification of the electronic structure of crystalline s-triazine with compression. The blue edge of the fluorescence emission, corresponding to the energy separation between the ground and the lowest nπ* state, is not affected by the compression. On the other hand, from the red edge of the excitation spectra the energy separation between the ground and the ππ* excited state can be inferred. The latter is strongly modified by pressure, with its red edge decreasing by ∼800 cm−1/GPa up to 8 GPa, thus indicating that the energies of the nπ* and ππ* excited states get closer with the compression. This effect of pressure on the electronic excited states, which implies a destabilization of the π molecular orbitals, i.e., an aromaticity reduction, driven by pressure, was first reported by Mitchell et al. studying polycyclic aromatic hydrocarbons both in the isolated molecule and in the crystal phase.36,37 More recently, energy inversion of excited states at high pressure has been
Figure 3. Two-photon excitation (a) and corresponding fluorescence (b) spectra of solid s-triazine as a function of pressure. The excitation spectra are recorded revealing the fluorescence around the maximum of emission, and the fluorescence spectra are all recorded exciting at 436 nm. The peak at 355 nm in the fluorescence spectra is due to scattered third-harmonic emission from the Nd:YAG laser employed to pump the OPG. 13766
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(3) Ciabini, L.; Santoro, M.; Gorelli, F. A.; Bini, R.; Schettino, V.; Raugei, S. Triggering Dynamics of the High-Pressure Benzene Amorphization. Nat. Mater. 2007, 6, 39−43. (4) Drickamer, H. G.; Frank, C. W.; Slichter, C. P. Optical Versus Thermal Transitions in Solids at High Pressure. Proc. Natl. Acad. Sci. U.S.A. 1972, 69, 933−937. (5) Citroni, M.; Bini, R.; Foggi, P.; Schettino, V. Role of Excited Electronic States in the High-Pressure Amorphization of Benzene. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 7658−7663. (6) Ceppatelli, M.; Fanetti, S.; Citroni, M.; Bini, R. Photoinduced Reactivity of Liquid Ethanol at High Pressure. J. Phys. Chem. B 2010, 114, 15437−15444. (7) Fanetti, S.; Ceppatelli, M.; Citroni, M.; Bini, R. Changing the Dissociative Character of the Lowest Excited State of Ethanol by Pressure. J. Phys. Chem. B 2011, 115, 15236−15240. (8) Fanetti, S.; Ceppatelli, M.; Citroni, M.; Bini, R. High-Pressure Photoinduced Reactivity of CH3OH and CD3OH. J. Phys. Chem. C 2012, 116, 2108−2115. (9) Ciabini, L.; Santoro, M.; Bini, R.; Schettino, V. High Pressure Photoinduced Ring Opening of Benzene. Phys. Rev. Lett. 2002, 88, 085505. (10) Chelazzi, D.; Ceppatelli, M.; Santoro, M.; Bini, R.; Schettino, V. High Pressure Synthesis of Crystalline Polyethylene Using Optical Catalysis. Nat. Mater. 2004, 3, 470−475. (11) Citroni, M.; Ceppatelli, M.; Bini, R.; Schettino, V. Laser-Induced Selectivity for Dimerization versus Polymerization of Butadiene under Pressure. Science 2002, 295, 2058−2060. (12) Citroni, M.; Fanetti, S.; Bini, R. Pressure and Laser-Induced Reactivity in Crystalline s-Triazine. J. Phys. Chem. C 2014, 118, 10284− 10290. (13) Gao, H.; Shreeve, J. M. Azole-Based Energetic Salts. Chem. Rev. 2011, 111, 7377−7436. (14) Lu, Y. F.; Lo, S. T.; Lin, J. C.; Zhang, W.; Lu, J. Y.; Liu, F. H.; Tseng, C. M.; Lee, Y. H.; Liang, C. T.; Li, L. J. Nitrogen-Doped Graphene Sheets Grown by Chemical Vapor Deposition: Synthesis and Influence of Nitrogen Impurities on Carrier Transport. ACS Nano 2013, 7, 6522−6532. (15) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water Under Visible Light. Nat. Mater. 2009, 8, 76−80. (16) Belen Jorge, A.; Martin, D. J.; Dhanoa, M. T. S.; Rahman, A. S.; Makwana, N.; Tang, J.; Sella, A.; Corà, F.; Firth, S.; Darr, J. R.; McMillan, P. F. H2 and O2 Evolution from Water Half-Splitting Reactions by Graphitic Carbon Nitride Materials. J. Phys. Chem. C 2013, 117, 7178− 7185. (17) Teter, D. M.; Hemley, R. J. Low-Compressibility Carbon Nitrides. Science 1996, 271, 53−55. (18) Hirt, R. C.; Halverson, F.; Schmitt, R. G. s-Triazine. II. The Near Ultraviolet Absorption Spectrum. J. Chem. Phys. 1954, 22, 1148−1149. (19) Bini, R.; Ballerini, R.; Pratesi, G.; Jodl, H. J. Experimental Setup for Fourier Transform Infrared Spectroscopy Studies in Condensed Matter at High Pressure and Low Temperatures. Rev. Sci. Instrum. 1997, 68, 3154−3160. (20) Gorelli, F. A.; Ulivi, L.; Santoro, M.; Bini, R. The ϵ Phase of Solid Oxygen: Evidence of an O4 Molecule Lattice. Phys. Rev. Lett. 1999, 83, 4093−4096. (21) Fanetti, S.; Citroni, M.; Bini, R. Pressure-Induced Fluorescence of Pyridine. J. Phys. Chem. B 2011, 115, 12051−12058. (22) Brinen, J. S.; Goodman, L. Sequence and Spacing of n → π* Transitions in s-Triazine. J. Chem. Phys. 1959, 31, 482−487. (23) Goodman, L.; Harrell, R. W. Calculation of n → π* Transition Energies in N-Heterocyclic Molecules by a One-Electron Approximation. J. Chem. Phys. 1959, 30, 1131−1138. (24) Halverson, F.; Hirt, R. C. Near Ultraviolet Solution Spectra of the Diazines. J. Chem. Phys. 1951, 19, 711−718. (25) Fischer, G.; Small, G. J. Jahn-Teller Distortion of s-Triazine in its Lowest Excited Singlet State. J. Chem. Phys. 1972, 56, 5934−5944.
reported in the pyridine crystal, where the inversion between between nπ* and ππ*, due to the pressure driven enforcement of intermolecular CH···N interactions, was revealed by a huge increase of the fluorescence intensity.21 The connection between the pressure induced destabilization of the π bonding states and the reactivity observed around 8 GPa (see Figure 4) is straightforward. When the transitions to the ππ* states overlap with those to the nπ* ones, we do not observe any additional red shift with further increasing the pressure and the chemical reaction is induced. The overlap of the transitions to the ππ* and nπ* states marks the completion of the ring aromaticity destabilization driving the reactivity with neighboring molecules. It is thus evident that in s-triazine we do not observe a stabilization with pressure of the nonbonding orbitals, as would be provided by an enforcement of the hydrogen interaction like in the pyridine case.21 This observation supports the view that neighboring s-triazine molecules do not interact via preferential N···H directions,12 and accounts for the lower chemical stability of s-triazine with respect to pyridine at high pressure.
5. CONCLUSIONS The two photon induced emission from s-triazine was studied as a function of pressure in the range of stability of the monoclinic phase and above the threshold pressure of the spontaneous reaction taking place at about 8 GPa. The pressure evolution of the TP excitation spectra reveals that the high pressure photoinduced reactivity is only triggered by the population of ππ* excited states because of the negligible TP cross section of nπ* transitions. In spite of the remarkable pressure shift to lower frequencies of the ππ* transitions, there is no detectable change in the S0−S1 transition energy as deduced by the emission spectra. These occurrences are interpreted on the basis of a progressive reduction of the ring aromaticity due to the destabilization of the π bonding orbitals with increasing the pressure and to a lack of stabilization of the N nonbonding orbitals due to interactions in the crystal. Once the energy difference between π and n orbitals vanishes, the destabilization process is completed and a chemical reaction with the neighboring molecules occurs. These results represent a sharp demonstration of how the characterization of the electronic properties as a function of pressure represents a rich source of information about the mechanisms driving the instability of molecular crystals under pressure.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Supported by the Italian Ministero dell’Istruzione, dell’Università e della Ricerca MIUR (Grant FIRBFuturo in Ricerca 2010 RBFR109ZHQ), and by the Deep Carbon Observatory initiative (Physics and Chemistry of Carbon at Extreme Conditions from the Alfred P. Sloan Foundation).
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REFERENCES
(1) Bini, R.; Schettino, V. Materials Under Extreme Conditions Molecular Crystals at High Pressure; Imperial College Press: London, U. K., 2014. (2) Cohen, M. D. The Photochemistry of Organic Solids. Angew. Chem., Int. Ed. Engl. 1975, 14, 386−393. 13767
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(26) Webb, J. D.; Swift, K. M.; Bernstein, E. R. Vibronic Assignments and Vibronic Coupling in the 1E″ State of sym-Triazine by Two Photon Spectroscopy. J. Chem. Phys. 1980, 73, 4891−4903. (27) Heaven, M.; Sears, T.; Bondybey, V. E.; Miller, T. A. The Free Jet Cooled, Laser Induced Fluorescence Spectrum of sym-Triazine. J. Chem. Phys. 1981, 75, 5271−5279. (28) Bolovinos, A.; Tsekeris, P.; Philis, J.; Pantos, E.; Andritsopoulos, G. Absolute Vacuum Ultraviolet Absorption Spectra of Some Gaseous Azabenzenes. J. Mol. Struct. 1984, 103, 240−256. (29) Smith, J. H.; Rae, A. I. M. The Structural Phase Change in sTriazine. I. The Crystal Structure of the Low-Temperature Phase. J. Phys. C: Solid State Phys. 1978, 11, 1761−1770. (30) Prasad, S. M.; Rae, A. I. M.; Hewatt, A. W.; Pawley, G. S. The Crystal Structure of s-Triazine at 5K. J. Phys. C: Solid State Phys. 1981, 14, L929−L931. (31) Oron, M.; Zussman, A.; Rapoport, E. 14N PNQR Study of the Effect of Pressure on the Phase Transition and Molecular Reorientation in s-Triazine. J. Chem. Phys. 1978, 68, 794−798. (32) Eckert, J.; Fincher, C. R., Jr.; Heilmann, I. U. Neutron Scattering Study of the Phase Transition in s-Triazine at High Pressure. Solid State Commun. 1982, 41, 839−842. (33) Dove, M. T.; Ewen, P. J. S. A Raman Scattering Study of the Pressure Induced Phase Transition in s-Triazine. J. Chem. Phys. 1985, 82, 2026−2032. (34) Ohta, N.; Baba, H. An Anomalous Broad Emission of s-Triazine Vapor. Chem. Phys. Lett. 1984, 106, 382−386. (35) Ohta, N.; Sekiguchi, O.; Baba, H. Fluorescence Polarization and Intramolecular Dynamics in S1 of Pyrazine, Pyrimidine, and s-Triazine Vapors. J. Chem. Phys. 1988, 88, 68−78. (36) Mitchell, D. J.; Schuster, G. B.; Drickamer, H. G. Effect of Pressure on the Fluorescence of 9-Carbonyl Substituted Anthracenes. J. Am. Chem. Soc. 1977, 99, 1145−1148. (37) Mitchell, D. J.; Schuster, G. B.; Drickamer, H. G. High Pressure Studies of Fluorenone Emission in Plastic Media. J. Chem. Phys. 1977, 67, 4832−4834.
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