Isotopic Mixed Crystals Research Articles

Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal

(1978). Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal. Molecular Physics: Vol. 35, No. 2, pp. 595-599.

Intermolecular vibrations of a crystalline molecular complex

The nature of intermolecular vibrations in a crystalline molecular complex is examined using a simple model of a crystal with one molecule of a 1:1 complex per unit cell. It is shown that two limiting descriptions can result depending on the coupling strength between molecular components in the complex: (i) The giant molecule limit describes the case of a strong coupling where the intermolecular motions are the motions of the complex as one entity. (ii) The sublattice limit is applicable in the weak coupling case. Here, the motion can be characterized as an intermolecular motion belonging to the sublattice of the individual components. The phonon bands of naphthalene:octafluoronaphthalene crystals are examined within the framework of the simple model. The results predict phonons to be close to the sublattice limit. Our experimental study of Raman phonon spectra using isotopic mixed crystals of the complex is in agreement with the theoretical predictions and show that the low frequency phonons belong predominantly to the C10F8 sublattice, while the high frequency phonons characterize the C10H8 sublattice motions.

Exciton percolation. II: Naphthalene 1B 2u supertransfer

The exciton percolation theory has been tested for the migration of the lowest singlet exciton in our model organic alloy system at low temperature (2 K): binary isotopic mixed crystals of naphthalene (C 10H 8—C 10D 8), with an added exciton sensor (supertrap) of betamethylnaphthalene (about 10 −3 mole fraction). At these relatively high sensor concentrations the system's migration dynamics are described quantitatively by the simple limiting case of supertransfer, i.e. by a dynamic percolation formulation of Hoshen and Kopelman that depends only on the concentrations and interaction topology of the ternary crystal. Experimentally, the exciton's migration dynamics is monitored by fluorescence spectra, taken under controlled conditions (crystal quality, purity, concentration, excitation and temperature). The effects of exciton tunneling (superexchange), exciton—phonon coupling, coherence, exciton delocalization and non-equilibrium chemical solubility are considered. We show that a complete concentration study of this simple energy transport model system reveals four kinds of exciton transfer regions, but only one (above the critical guest site percolation concentration) with long ranged, multistep, direct guest exciton transport. The same study, on the low concentration side, yields relative trapping efficiencies and thus eliminates the need for adjustable parameters. The substitutional randomness of the isotopic mixed crystal is confirmed, as well as the short range nature of the 1B 2u exciton interactions and the dominance of the interchange equivalent, nearest neighbor, pairwise (molecular) excitation ( 1B 2u) exchange integrals in the pure, perfect, low-temperature, naphthalene crystal. Our present exciton percolation study is also relevant to the primary energy transport process in heterogeneous photosynthetic units.

Phonon assisted trap–trap triplet energy migration in the 0 °K limit in crystalline benzene

The transfer of triplet excitation energy between isotopic traps in three component (host, trap, and supertrap) benzene isotopic mixed crystals has been studied. The trap–host energy difference was kept constant for the systems investigated while the trap–supertrap difference was varied. The transfer of triplet excitation energy was determined by measuring the trap-to-supertrap phosphorescence intensity ratio as a function of concentration. The results demonstrate the expected dependence of off-resonance trap–supertrap excitation energy transfer on the lattice (phonon) Franck–Condon factor. The efficiency is found to go through a maximum at a trap–supertrap energy separation of ∼50 cm−1, decreasing rapidly at higher and lower values. The observed inefficiency of trap–trap transfer (as compared to trap–supertrap transfer) is attributed to small differences in trap energies caused by crystal inhomogeneities.

Quantitative tests of mixed crystal exciton theory. II. Energy denominator study of naphthalene 1B2u resonance pairs

A spectroscopic absorption study is presented of 1-D1C10H7, 2-D1C10H7, and 1,4-D2C10H7 resonance pairs in C10D8, over a concentration range of 0.009% to 5% mole, at 2 °K, with a resolution of 0.5 cm−1. The resulting structure is interpreted in terms of the pairwise exciton exchange interactions, M1/2(a+b), Ma, Mb, Mc, Ma+c, and M1/2(a+b)+c. The energy denominator dependence is interpreted in terms of both exciton quasiresonance and exciton superexchange. The three parameter sets derived by Hong and Kopelman to fit earlier data on neat and isotopic mixed crystals are corroborated by this study, as they were in our previous study on monomers (I). However, it was not possible to narrow down completely the choice of parameters. A discussion is given on the feasibility of deriving a unique dispersion relation (and Green’s function) for the first naphthalene singlet exciton and on its likely precision.

Long range exciton percolation and superexchange: Energy denominator study on 3B 1u naphthalene

The long-range exciton percolation model is found to describe the lowest triplet exciton superexchange (“tunneling”) migration at low temperature (2 K), in our model alloy system: Binary isotopic mixed naphthalene crystals with dispersed exciton sensors (supertraps) consisting of small concentration of betamethylnaphthalene (−10 −3 mole fraction) or isotopic substituted naphthalene molecules (with lower excitation energies than the partially deuterated naphthalene guest species). While the “host” is C 10D 8 throughout, the “guest” species in our five experimental systems are: C 10H 8, 2-DC 10H 7. 1-DC 10H 7, 1,2-D 2C 10H 6 and 1,4,5,8-D 4C 10H 4. The variation in guest—host (and supertrap—guest) energy denominator in the above systems enables a quantitative test of our physical exciton superechange (tunnelling) migration model. In conjunction with a mathematical long-range percolation model (J. Hoshen, E.M. Monberg and R. Kopelman, unpublished). The experimental monitoring of the exciton migration dynamics consists of refined phosphorescence measurements to our systems, under highly controlled conditions (crystal quality, purity, concentration, temperature and excitation). Using only the known nearest neighbor (interchange-equivalent) exciton exchange interaction, quantitative agreement with the experimental dynamic percolation concentration is achieved, without adjustable parameters, for four of the five investigated systems. The fifth one is known to involve a cooperative percolation—thermalization exciton migration, and is effective in qualitative agreement with the predicted upper limit for the exciton percolation concentration. The nearest-neighbor 3B 1u excitation exchange interactions, and their square lattice topology, play the dominant role in determining the guest triplet exciton energy transfer and migration. This energy conduction involves an extremely narrow “impurity band”, on the order of 10 to 10 3 Hz, formed by the superexchange (tunneling) exciton interactions resulting from the above mentioned exciton exchange interactions (integrals). The latter are thus confirmed as the major contributors to the 3B 1u exciton transfer, migration and energy bond (3 × 10 11 Hz) in the ordinary naphthalene crystal. Just below the percolation concentration the “impurity conduction band” further shrinks by one or two orders of magnitude, resulting in a bandwidth of about one hertz or less, and thus practically resulting in the “switching off” of the exciton transport. The tunneling radius is about 30 Å or larger, depending on the system, but essentially in the ab plane.

Study on the molecular geometry of phosphorescent pyrazine by optical detection of zero-field magnetic resonance and T ← S excitation spectroscopy

The phosphorescent state of pyrazine has been studied with the T → S emission and T ← S excitation methods in combination with microwave techniques in the systems of pyrazine-h 4 in pyrazine-d 4, pyrazine-h 4 and -d 4 in p-dioxane, p-xylene, and benzene, and pyrazine-h 4 x-trap. The three out-of-plane hydrogen bending vibrations of v 10a , v 5 and v 17a are found to be active in the Tx sublevel emission of the isotopic mixed crystal. By modifying the force constants used by Scully, the frequencies of the out-of-plane vibrations were reexamined and the result was compared with the observed frequencies. The microwave-induced changes in the phosphorescence intensity were used as a measure of the relative populations in the triplet sublevels, when the system was excited with light having the wavelength corresponding to a particular vibronic band of the T ← S absorption. In this way, the vibrational structure of the absorption spectra was successfully analysed. The overtone bands of v 10a and v 5 were observed...

On the triplet energy transfer mechanism in isotopic mixed molecular crystals

The transfer of triplet excitation energy between isotopic traps in benzene isotopic mixed crystals has been investigated using both steady state and pulsed excitation sources. It has been demonstrated for the first time that the excitation–tunneling transfer mechanism [H. Sternlicht, G. C. Nieman, and G. W. Robinson, J. Chem. Phys. 38, 1326 (1963)] predicts the correct dependence of the transfer rate on trap depth. Furthermore, it is shown that the Perrin approximation, which grossly simplifies the treatment of the experimental data, is a reasonable albeit not perfect approximation for this system. A large fraction of the energy transfer is completed during times much shorter than the benzene triplet state lifetime. This is consistent with the strong distance dependence of the transfer rate constant predicted by a tunneling mechanism. Using the Perrin approximation, it has been shown that the use of a linear chain tunneling mechanism predicts the excitation exchange integral to be of the correct magnitude, a conclusion that is also consistent with a strong distance dependence of the transfer rate constant. (Energy can be transferred through 2–3 intervening host molecules.) In this system there is no evidence for the existence of the intermolecular triplet energy relaxation mechanism recently shown to be operative in chemically mixed crystals [H. C. Brenner and C. A. Hutchison, Jr., Chem. Phys. 58, 1328 (1973)].

Triplet energy transfer mechanism in isotopic mixed molecular crystals

The transfer of triplet excitation energy between isotopic traps in benzene isotopic mixed crystals has been investigated using both steady-state and pulsed excitation sources. The prediction of the correct dependence of the transfer rate on trap depth by the excitation-tunneling transfer mechanism was demonstrated. It was shown that the Perrin approximation is a reasonable albeit not perfect approximation for this system. Using this approximation, it has been shown that the use of a linear-chain tunneling mechanism predicts the excitation exchange integral to be the correct magnitude. This conclusion is consistent with a strong distance dependence of the transfer rate constant. (BLM)

Matrix perturbations of the benzene1B2ustate

The fluorescence spectrum of C6H6 in C6D6 is reported. The vibronic analysis shows the crystal-field effect to be much smaller than the Herzberg-Teller vibronic mixing present in the free molecule. Data on the benzene S 1↔S 0 gas-to-crystal shift in this and other host crystals is presented. An approximate calculation of this gas-to-crystal shift in pure crystalline benzene indicates that London dispersion interactions make a significant contribution to the gas-to-crystal shift. The calculated value indicates that the dispersion contributions to the shift are > 187 cm-1 compared to the experimental value of 248 cm-1 (both to the red). The importance of crystal packing in determining the gas-to-crystal shift is emphasized by an attempt to correlate shift data in a number of crystalline matrices. In addition, the deuterium isotope effect on the gas-to-crystal shift, reported previously for isotopic mixed crystals, is found to be generally observed for chemically mixed crystals. Crystal packing effects are also used to explain this isotope effect.

Low frequency raman spectra of isotopic mixed benzophenone crystals

In this paper, we present an experimental study of vibrational lattice modes in isotopic mixed crystals of benzophenone- h 10 and - d 10. Our results are discussed using theoretical and experimental data concerning other molecular isotopic mixed crystals. The spectra show two regions; in the first (ν < 100 cm −1) we did not observe an interaction between the various vibrational lattice bands; in the second region (ν > 100 cm −1), the interaction appears clearly. The 111.5 cm −1 and 73 cm −1 torsional modes of benzophenone- h 10 have the same behaviour as the external modes throughout the whole concentration range (0–100% of benzophenone- d 10).

Energy transfer via guest excitons in isotopic mixed naphthalene crystals

The temperature dependence of the fluorescence spectra of aggregates in naphthalene-perdeuteronaphthalene mixed crystals has been investigated between 1.4 and 70 K and for concentrations up to 50% naphthalene. It is shown that the most abundant traps — the monomer guest molecules — transfer energy like a guest exciton band 48 cm −1 below the host exciton band. With increasing temperature, the excitation energy is redistributed between the different aggregate traps by thermal activation into the monomer states. The energy transfer constant within the monomer exciton band is measured as a function of concentration. It is suggested that dipole-dipole interaction between the monomer guests is responsible for the energy transfer via guest excitons.

Structure and dynamics of the lowest triplet state in p-benzoquinone: I. An isotope effect study on the optical absorption, emission and ODMR spectra

The results of detailed spectroscopic experiments on the lowest nπ* triplet state of p-benzoquinone- h 4, - dh 3, 2,6- d 2 h 2, - d 4 and -CH 3 in mixed and isotopic mixed crystals are presented and analyzed. The origin of the lowest B 1g (nπ*) singlet-triplet transition in p-benzoquinone- h 4 (PBQ- h 4) is shown to be induced by asymmetric isotopic substitution and the oscillator strength of this origin is seen to be accounted for by a corresponding decrease in intensity of a level 16.9 cm −1 higher in energy in the pure PBQ- h 4 crystal. The combined oscillator strength of these close lying levels is measured and found to be almost independent of deuteration. These results are discussed in reference to the previously proposed double minimum potential model for the lowest nπ* triplet state in PBQ- h 4 and the applicability for this model is critically examined. Optical absorption experiments on heavily doped isotopic mixed crystals of PBQ- h 4 in PBQ- d 4 show hydrogen (deuterium) bounding effects between translational inequivalent molecules to be primarily responsible for the observed cluster states. These hydrogen bounding effects also induce the electronic origin of the B 1g (nπ*) triplet state in case of a translational inequivalent dimer. A detailed vibrational analysis of the phosphorescence spectrum of PBQ- h 4 in a PBQ- d 4 host crystal at 1.8 K is presented and it is shown that the unobserved origin of the B 1g (nπ*) triplet state of PBQ- h 4 is located at 18609 ± 1 cm −1 and that the inversion splitting in this lowest excited state amounts to 21 ± 1 cm −1 in this mixed crystal system. An isotope effect is study on the vibronic structure in the emission spectrum further indicates that the excited state structure of PBQ is isotope dependent. The observed large isotope effect on the ZFS parameters of the lowest triplet state of PBQ- h 4 is demonstrated to be an intramolecular phenomenon and explained as an isotope dependent spin-orbit contribution to the ZI-S parameters, induced by localization of the nπ* excitation on oxygen. Finally the dynamics of energy migration in the dilute PBQ- h 4 in PBQ- d 4 isotopic mixed crystal is probed by concentration and temperature dependent phosphorescence intensity measurements and it is suggested that trap-exciton band communication effects are of importance in this system.

ChemInform Abstract: ON THE NATURE OF EXCITON‐PHONON COUPLING IN CRYSTALLINE BENZENE

Chemischer InformationsdienstVolume 6, Issue 17 Physical Organic Chemistry ChemInform Abstract: ON THE NATURE OF EXCITON-PHONON COUPLING IN CRYSTALLINE BENZENE STEVEN D. COLSON, STEVEN D. COLSONSearch for more papers by this authorTHOMAS L. NETZEL, THOMAS L. NETZELSearch for more papers by this authorJEAN M. VAN PRUYSSEN, JEAN M. VAN PRUYSSENSearch for more papers by this author STEVEN D. COLSON, STEVEN D. COLSONSearch for more papers by this authorTHOMAS L. NETZEL, THOMAS L. NETZELSearch for more papers by this authorJEAN M. VAN PRUYSSEN, JEAN M. VAN PRUYSSENSearch for more papers by this author First published: April 29, 1975 https://doi.org/10.1002/chin.197517060AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume6, Issue17April 29, 1975 RelatedInformation

On the nature of exciton–phonon coupling in crystalline benzene

A new technique for determining the nature of the exciton−phonon coupling in molecular crystals is applied to the S1 (0,0) exciton band. The phonons associated with the origin line of the neat crystal fluorescence are found to derive their intensity by a vibronic mechanism. In addition, the recently proposed ’’localized exciton sideband method’’ is applied to the fluorescence and phosphorescence spectra of pure and isotopic mixed benzene crystals to further test its applicability to molecular crystals. The difficulties expected for the general application of the localized exciton sideband method are discussed in some detail. It is shown that the method can be applied to pure as well as mixed crystals, removing uncertainties about possible impurity induced perturbations of the phonon modes. The correspondence between observed and calculated features in the phonon spectrum is not as satisfactory as in the naphthalene crystal. The strength of the exciton−phonon coupling is found to be similar, but measurably different for the T1 and S1 states of benzene. Furthermore, it is shown that the exciton−phonon coupling for the S1 state is less in the pure than in the isotopic mixed crystal. Thus, even though the pure crystal excitation exchange integrals are as small as ∼2 cm−1, the delocalization of the excitation causes a ∼10% reduction in the phonon coupling.

Crystal field effects on the ground and lowest excited triplet states of benzene isotopes in a borazine host crystal

The phosphorescence spectra of the C 6H 6C 6H 5D 1 p-C 6H 4D 2 symp-C 6H 3D 3, C 6D 6 and 13CC 5D in a borazine host crystal are analyzed at high resolution. The spectral lines are sharp (∼2 cm −1 wide) indicating that the impurity molecules occupy a unique site in the borazine lattice which is probably substitutional. The phonon sidebands are weak,giving clean, well-resolved spectra much like those of isotopic mixed crystals. In contrast, however, the crystal field effects on the ground state vibrational levels are much smaller than those found for isotopic mixed crystals. The gas-to-crystal shifts are very small, the vibrational degeneracies are not removed and orientational splittings are only observable for a few select vibrational levels. For most vibrational levels and for the derivation of selection rules one can asume the effective crystal site symmetry to be D 3d. The data provide the first conclusive evidence that the splitting observed in the benzene phosphorescence spectrum results from a distortion of the molecule when excited to the zeroth vibrational level of the T 1 state. Furthermore, the data suggest that the distortion is intrinsic in nature (i.e.,is not caused by the crystal field).

An energy denominator study of the phonon side band in isotopic mixed naphthalene crystals

The fluorescence phonon sidebands of C10H8 and 1,4,5,8-C10D4H4 guests in C10H8 are studied and compared at 2 K. The mild differences are attributed to the increased delocalization of the C10D4H41B2u electronic state. The relative importance of the diagonal (Franck-Condon) versus off-diagonal (Herzberg-Teller) electron-phonon coupling terms is discussed. We also demonstrate that the dispersion force coupling (and not the resonance force coupling) dominates the 1B2u (and 3B1u) C10H8 in C10D8 phonon sideband structure, and is responsible for its intensity distribution (and the difference therein between fluorescence and phosphorescence).

Quantitative tests of mixed crystal excition theory. I. Naphthalene monomer 1B 2u and 3B 1u spectra

A quantitative, computer processed spectroscopic study, using photon counting, on the first excited triplet and singlet states of dilute isotopic mixed crystals of naphthalene at 2 K is presented for C 10H 8; 1-DC 10H 7; 2-DC 10H 7; 1,4-D 2C 10H 6; 1,4,5-D 3C 10H 5; 1,4,5,8-D 4C 10H 4; 1,2,4,5,8-D 5C 10H 3; a β-D 4C 10H 4 and a β 2-D 6C 10H 2 as guests in C 10D 8 host crystals (and, for comparison, also for the same guests in a durene host crystal). The guest—host relative polarization Rashba formula has been verified quantitatively, and, as an added bonus, the elusive polarization ratio of the pure naphthalene crystal singlet Davydov components has been found to be 80 ± 20 ( b/a), which is in poor agreement with the transition octupole—transition octupole model. The experimental guest energies and their concomitant quasiresonance shifts for bound singlet states (as well as the occurrences of unbound states) are in excellent quantitative agreement (about 1 cm −1) with those calculated using a Green's function formalism based on the ideal mixed crystal approximation and on a restricted Frenkel type dispersion relation derived from resonance pairs. The same Green's function also accounts quantitatively (within 10%) for the guest singlet state exciton localizations (guest excitation amplitudes). The triplet exciton state reveals an orientational site splitting (about 0.7 cm −1) for the 0—0 transition of the I-DC 10H 7 guest in C 10D 8 host. The order of the α and β substituted deuteronaphthalenes in the triplet state is reversed from that of the singlet state. The last two observations are related to the different nature of the lowest Π-Π * singlet and triplet states of naphthalene.

ChemInform Abstract: PHONON SIDEBANDS OF LOCALIZED EXCITONS IN MOLECULAR CRYSTALS WITH METHYL TORSIONS, HEXAMETHYLBENZENE

Fluorescence and phosphorescence phonon sidebands of isotopic mixed hexamethylbenzene crystals at 2°K are presented. The external phonons can be observed separately from the semi‐internal (methyl torsion) ones. The nature of the electronic or vibronic state has observable but not drastic effects on the exciton‐phonon coupling function. Likewise, the exciton delocalization is of minor importance to the exciton‐phonon function in hexamethylbenzene. The coupling between external and internal vibrations is also weak in this system. The exciton‐phonon coupling appears comparable for the optical and acoustic phonons in hexamethylbenzene. The phonon sidebands give some of the phonon singularities of the low‐temperature crystal.

Phonon sidebands of localized excitons in molecular crystals with methyl torsions: Hexamethylbenzene

Fluorescence and phosphorescence phonon sidebands of isotopic mixed hexamethylbenzene crystals at 2°K are presented. The external phonons can be observed separately from the semi-internal (methyl torsion) ones. The nature of the electronic or vibronic state has observable but not drastic effects on the exciton-phonon coupling function. Likewise, the exciton delocalization is of minor importance to the exciton-phonon function in hexamethylbenzene. The coupling between external and internal vibrations is also weak in this system. The exciton-phonon coupling appears comparable for the optical and acoustic phonons in hexamethylbenzene. The phonon sidebands give some of the phonon singularities of the low-temperature crystal.