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Paper Crystal Engineering, Solid State Spectroscopy and Time-Resolved Crystal engineering, solid state spectroscopy and time-resolved diffraction{ Philip Coppens,* Baoqing Ma, Oxana Gerlits, Yuegang Zhang and Pankaj Kulshrestha Paper Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260-3000, Highlight USA. E-mail: [email protected] Received 12th April 2002, Accepted 29th April 2002 Published on the Web 17th July 2002 The use of calixarene-based supramolecular solids in spectroscopy and time-resolved crystallography is discussed. A series of solids with, as guests, benzophenone, benzil, decamethylruthenocene and a binuclear Rh cationic complex have been synthesized and analyzed. The excited state lifetimes that have been measured show a pronounced dependence on the molecular environment. 1 Introduction also includes the characteristics of the guest molecules embedded in the solids. The supramolecular frameworks that One of the prime attractions of the field of crystal engineering is the possibility to tailor-make solids with desirable physical or {Based on the presentation given at CrystEngComm Discussion, 29th chemical properties. This is not limited to bulk properties but June–1st July 2002, Bristol, UK. Philip Coppens received his Ph.D. from the University interest is in time-resolved photocrystallographic temperature superconductors. He then obtained his of Amsterdam and has since been employed at the studies and supramolecular chemistry. He has over Ph.D. under the supervision of Professor Philip Weizmann Institute of Science, Brookhaven National 50 scientific publications. Coppens at the State University of New York at Laboratory and the State University of New York at Buffalo in 2001, working on engineering crystals for Buffalo, where he is currently Distinguished Professor excited state X-ray diffraction studies. He is currently of Chemistry. He is a Corresponding Member of the a postdoctoral appointee working on high energy Royal Dutch Academy of Sciences and a Doctor X-ray powder diffraction with Dr Peter L. Lee at Honoris Causa of the University of Nancy, France. Advanced Photon Source at Argonne National He is a Past President of the American Crystal- Laboratory. lographic Association and has served as President of the International Union of Crystallography from 1993 to 1996. Among his awards are the Gregori Aminoff Prize of the Royal Swedish Academy of Sciences and the Martin Buerger Award of the American Crystal- lographic Association. His research interests include photocrystallography, the study of light-induced metastable and transient species in crystals, the latter by use of time-resolved diffraction, X-ray charge density analysis, synchrotron radiation crystal- lography and the combination of experimental results Baoqing Ma with parallel quantum mechanical calculations. His most recent book, entitled X-Ray Charge Densities Oksana O. Gerlits was born in 1973 in Krasnodar, and Chemical Bonding, was published in 1997. He is a Russia. She received her M.Eng. in Chemical member of the Editorial Board of CrystEngComm. Technology of Organic Compounds in 1999 from the Moscow Academy of Fine Chemical Technology under the supervision of Dr Alla Frolkova. Since 2000, she has been a graduate student at the State University of Yuegang Zhang New York at Buffalo in the laboratory of Professor Pankaj Kulshrestha was born in 1978 in Ranchi, India. Philip Coppens, where she is involved in engineering of He received his B.S. in Pharmacy from Birla Institute Organometallic/Organic Host–Guest Crystals for of Technology, Mesra, Ranchi, India in the year 2000. Time-resolved Crystallography. Since 2001, he has been a graduate student at SUNY, Buffalo in the laboratory of Professor Philip Coppens, where he is involved in energy calculations on supramolecular structures. Philip Coppens Bao-Qing Ma was born in 1972 in Hebei province, China. He earned his Ph.D. under the supervision of Professors Guang-Xian Xu and Song Gao at National Key Laboratory of Rare Earth Chemistry and Applications from Peking University in 2000, where he worked on the molecular-based magnetic materials Oxana Gerlits and functional crystal engineering. Immediately fol- Yuegang Zhang was born in 1973 at Kunming, China. lowing his Ph.D. he joined Professor Philip Coppens’ He received his B.S. in 1996 at University of Science research group at the State University of New York at and Technology of China (Hefei, China) working with Buffalo as a post-doctoral associate. His current Professor Yitai Qian and Dr Kaibin Tang on high Pankaj Kulshrestha 302 CrystEngComm, 2002, 4(54), 302–309 DOI: 10.1039/b203602a This journal is # The Royal Society of Chemistry 2002 form the backbone of the engineered solids provide the opportunity to embed molecules in different environments and to achieve a solid-state dilution previously only attainable in amorphous glasses. Spectroscopists have routinely used dilution in amorphous matrices to isolate photoactive mole- cules from each other and to reduce exciton–exciton annihila- tion limiting lifetimes of excited species. In supramolecular solids these goals can be achieved without sacrificing three- dimensional periodicity. This means that molecular changes and the resulting distortions of the environment can be studied by diffraction methods, which have the ability to yield unambiguous evidence on molecular geometry. Though time-resolved diffraction at atomic resolution is still in its infancy and being developed,1,2 the search for suitable solids and the exploration of the spectroscopic properties of the incorporated guest molecules must proceed in parallel. The advantages of dilution of the photoactive species with retention of crystallinity are manifold: (1) the number of photons to be absorbed to achieve a Fig. 1 Time structure of the stroboscopic experiment. certain conversion percentage is reduced, which relaxes the requirements for the intensity of the laser pulses and the concomitant danger of overheating (and possibly destroying) experiment the lifetime of the excited species is 50 ms. A the sample crystal; frequency of 5100 Hz and an X-ray pulse length of 37 ms were ˚ (2) the absorption of light in the crystal, i.e. the extinction used. The observed contraction of 0.28(9) A was used to test coefficient, is reduced, and thus uniformity of illumination quantum-mechanical DFT calculations of the excited state, and was found to be reproduced by some but not all of the throughout the crystal is improved; 3 (3) for a given conversion percentage the distance between theoretical methods tested. excited molecules is increased, and the likelihood of exciton– A major factor limiting the accuracy of the experiment is the exciton annihilation reduced; small population of excited state molecules in the crystal, only (4) by varying the nature of the framework, the dependence 2% in the experiment described above. It is here that solid-state of the spectroscopic properties on the molecular environment dilution has a crucial role to play by reducing the incident- can be studied. photon to photoactive-molecule ratio. In the experiment On the other hand, much more complicated crystal struc- described above the number of photons focused on the crystal and the number of photoactive molecules in the crystal are tures must be analyzed. In this respect modern area detector 14 14 data collection techniques become indispensable as do high estimated at 2.3 6 10 and 1.1 6 10 , respectively, a 2 : 1 speed computing methods, as is the case for all of modern excess of photons. This is not adequate as not all photons crystal engineering, the development of which depends should be absorbed in the crystal to ensure uniformity of crucially on the capability for rapid structure determination illumination. The low excess of photons accounts to a large of complex solids. extent for the small conversion percentage achieved in the experiment. There are three solutions to this problem: (i) increasing the laser intensity with increased chance of destroy- ing the crystal; (ii) reduction of crystal size, which requires 2 Time-resolved diffraction at atomic resolution more intense X-ray beams; or (iii) solid-state dilution by Considerable progress has been made in time-resolved crystal- assembly of supramolecular solids. lography of macromolecules with the polychromatic Laue method, which allows very rapid collection of vast amounts of data. For very short timescale experiments the use of 3 Choice of suitable frameworks polychromatic methods may be unavoidable. However, to 3.1 General considerations attain atomic resolution, monochromatic methods offer the distinct advantage of higher resolution and better accuracy. We Though solid-state dilution has promising advantages for time- have developed a stroboscopic method in which a pulsed laser resolved diffraction studies, a number of conditions must be beam is synchronized with probing X-ray pulses, as shown satisfied. First of all, the molecules composing the framework schematically in Fig. 1. Immediately following the exciting in the solids must be transparent to the exciting radiation and laser pulse the crystal is probed by the X-ray beam for a time photochemically inert, which rules out most supramolecular span smaller that the excited state lifetime, and not exposed to solids containing metal atoms. Second, no phase transitions the X-ray beam until the excited state population is replenished leading to disorder or deterioration of crystal quality must by the next laser pulse. With a mechanical chopper, X-ray take place on cooling to the low temperatures required for the pulses of microsecond length can be easily obtained, thus experiment. Finally, the guest molecules should show no or at achieving a time-resolution that is much shorter than the read- most minor disorder, the reason we discontinued studies on out time of the CCD detector which is counted in seconds.1 To cyclodextrin inclusion complexes,4 in which guest molecules ensure heat dissipation and increase of spatial resolution, the seemed invariably disordered. sample is cooled in a liquid helium gas flow while its diffraction MacGillivray et al. designed a family of solids in which pattern and phosphorescence are being recorded.
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