Schotte, F.

Techert, S.

Wulff, M.

English

MPG.PuRe

VOLUME 86, NUMBER 10 P H Y S I C A L R E V I E W L E T T E R S 5 MARCH 20012030Picosecond X-Ray Diffraction Probed Transient Structural Changes in Organic SolidsS. Techert,1,2,* F. Schotte,1 and M. Wulff11European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble Cedex, France2Max-Planck-Institute for Biophysical Chemistry, Department 010, 37070 Göttingen, Germany(Received 16 May 2000)In this Letter, we report on the experimental characterization of the geometry of short-lived electroni-cally excited states in organic solids by time-resolved x-ray diffraction. Here, the structure factor ofthe organic crystal is measured as a function of time. Since this technique gives complete structuralinformation, it is a very useful tool for learning more about atom motions on the excited-state energysurface—“beyond” the broad band typical of conventional spectroscopy. Although we used molecularcrystals rather than free molecules, the compounds show detectable transient structural changes on theps to ns time scale in our study.DOI: 10.1103/PhysRevLett.86.2030 PACS numbers: 61.10.Eq, 78.47.+pIntroduction.—Since the discovery of fluorescence incharge-separated systems [1], considerable effort has beenmade to gain more insight into the interaction betweengeometrical changes and repulsive electronic effects onthe excited state potential energy surface (PES). How-ever, a disadvantage of conventional time-resolved (TR)spectroscopy is the lack of structural information. Sincein these experiments the energetics of binding electronsare probed, it is impossible to deduce the atom posi-tions during structural transitions without knowledge ofthe PES. Normally, the observed transient spectroscopicbands in the ultraviolet/visible range are very broad andeven TR vibrational (IR) spectroscopy cannot cover thewhole frequency range necessary for a complete structuredetermination [2]. Experimental results in ultrafast spec-troscopy also raise the question whether ultrafast motionson the excited state PES can be described within the Born-Oppenheimer approximation [3] or whether one has to gobeyond [4].In this Letter, we describe an experimental approachusing pulsed diffraction techniques for determining pico-second structures of photosensitive organic solids, wherethe structure factors of a system are measured as a functionof time. The first nanosecond TR diffraction experimentswere carried out at the European Synchrotron RadiationFacility (ESRF) with optical light as pump and polychro-matic x-ray pulses from a synchrotron as probe on photoac-tive proteins such as the CO complex of myoglobin [5].Recently, the time resolution has been improved by use ofa mode-locked Ti:sapphire femtosecond laser which allowsTR x-ray diffraction experiments down to 60 ps [6]. Theprinciple of the experiment is summarized in Fig. 1, top:After photoinitiation of the reaction by a 150 fs opticallaser pulse, the excited state is probed with a 100 ps x-raypulse. By varying the delay between pump and probe, aseries of snapshots of the moving structure can be takenwhere each shot probes the average structure (nonexcitedand excited, including the dispersion of the latter) at agiven time ti . Since the typical lifetime of the systemsreported here is in the nanosecond range, this is equiva-0031-90070186(10)2030(4)$15.00lent to following the structural relaxation pathways duringthe lifetime of the excited state—on the excited state PES.Apart from the improvement of the time resolution, thisnew setup can also run at a high repetition frequency of897 Hz. Thus, it is now possible to accumulate monochro-matic data on an integrating detector.In this Letter, we study TR powder diffraction ofN ,N-dimethylaminobenzonitrile (DMABN, C9H10N2,Fig. 1), a compound widely discussed in the literaturewith emphasis on relaxation mechanisms after photo-excitation [2,7–10]. It is also a special case of molecularcharge transfer salt [11,12]. Our aim was to measurethe structural relaxation following photoexcitation andto test the feasibility of using this method to determinedisplacements at atomic resolution on a picosecondtime scale. One of the main questions was as follows:Does structural relaxation take place in a crystal and, ifso, what are the amplitude and time scale (see Fig. 1,bottom)? With TR x-ray diffraction, it should be possibleto distinguish between the inversion motion of the CH3groups around the N atom and the torsional motions ofthe CH3 groups as relaxation processes on the excitedstate PES. Depending on the kind of motions, electroniccoupling effects between the p electrons (torsion) orvibronic coupling as in NH3 (inversion) characterize thephotophysical behavior of DMABN.Spectroscopic characterization.—From spectroscopicobservations, some structural changes were expected evenin light-activated DMABN crystals. Purified, recrystal-lized, and crushed samples were pressed in a gasketsample diameter  250 mm until the 30 mm thicksample became optically transparent. The excitation andfluorescence spectra of the samples were recorded inthe excitation/fluorescence mode of a commercial spec-trometer (SPEX Fluorolog III, Jobin-Yvon). The spectrawere found to be identical with published ones [13].The biexponential fluorescence lifetimes of t1  1.71 nsand t2  0.53 ns at lemi  345 nm were also observed[14]. In the present work, the lifetime of the excited stateof DMABN crystals was determined to tf  1.9 ns (at© 2001 The American Physical SocietyVOLUME 86, NUMBER 10 P H Y S I C A L R E V I E W L E T T E R S 5 MARCH 2001FIG. 1. Top: principle of TR x-ray diffraction on the excited-state PES of DMABN crystals. Bottom: the intramoleculardegrees of freedom, which contribute to the relaxation process(inversion qinv and torsion ftors). C atoms of the phenyl moietyare given as open, N atoms as black, and the amino C as greycircles. Note that the H atoms are not shown, since they do notcontribute to the x-ray diffraction signal.lemi  375 nm) by using a photon-counting instrument(Edinburgh Instruments, N2 source with lexc  337 nm,500 ps resolution).Data processing and structural refinement.—The datawere collected on the ID09-TR beam line at the ESRF.A mode-locked Ti:sapphire laser [pulse width (laser) 150 fs] runs synchronously with single pulses of x rays.A synchronized chopper was used to select the singlex-ray pulses from the 16 bunches filling the storage ring[pulse width x ray  100 ps, focus  200 3 200 mm2,flux on the sample  8 3 107 photonss. The poly-chromatic beam was produced by a 162 pole undulatorwith a 20 mm magnetic period K  0.273. The experi-ment used a monochromatic x-ray probe beam at 16.5 keVlx ray  0.753 Å selected by a Si111 monochromator.High-resolution data sets were recorded using an im-age plate [size  354 3 432 mm2, pixel size  100 3100 mm2, distance (sample detector)  400 mm, expo-sure time  1500 s]. Further data sets were collectedwith an image-intensified Princeton CCD camera [size 124 3 115 mm2, pixel size  120 3 120 mm2, dis-tance sample detector  80 150 mm, exposure time 1200 s]. On the goniometer, the x-ray intensity of the di-rect beam was measured with a PIN diode before and afterthe measurement. During the measurement, the decayof the storage ring current was monitored on-line. Later,the collected spectra were intensity corrected. The pumpwavelength was lexc  267 nm (generated by frequencytripling, average power  25 30 mJ, focus  250 mm).The laser power of the direct beam was determined beforeand after every measurement and on-line monitored ona backscattered signal of a laser mirror. With 267 nm,DMABN was excited in the area of the highly vibra-tionally excited S1 and the S2 state. t  0 was roughlyset by using a fast Si avalange diode and double checkedby monitoring the temporal change in the diffractionamplitudes of the sample. The accuracy of the time zerocalibration was 30 ps, determined by the response func-tion of the detector and its oscilloscope. The repetitionfrequency of the experiment was 897 Hz. The absorbedoptical photons were partially emitted (with a fluorescencequantum yield of Ff  0.3) but also phonon-assistedemission at lemi  702 nm was observed. The overallheating of the sample could be determined from the 2Qshift of the Bragg peak maxima to DT  2 K. Thelaser and x-ray beams hit the sample in a quasiparallelconfiguration which gives the biggest overlap betweenpump and probe volume (see also [6]). The experimentswere carried out at T  298 K.The collected powder patterns were integrated using theprogram FIT2D [15]. Subtracting the radial diffraction in-tensity between the (partially) excited and nonexcited state,one obtains — after suitable normalization — the intensityof the photoexcited state. Note that nonexcited unit cellsdo not contribute to the difference signal. Because of thereversible nature of the photoreaction, the nonexcited datacould be taken at ti , 0 where the laser arrived just beforethe x rays. Figure 2, top, shows the difference pattern IQ, t  80 ps 2 IQ, t  2240 ps. The t  2240 pspattern contains the ground state p1. After laser excita-tion t  80 ps, the diffraction signal contains a mixtureof ground state p1 and excited state p2. Figure 2, bottom,shows the change of the integrated Bragg peak intensitiesas a function of time. The Bragg reflections are marked bytheir Miller indices. The intensity change ranged between1 60.5 10 60.5% of the total signal.The structures were refined applying the Rietveldmethod (program package GSAS [16]). The powder datawere analyzed in a biphasic model p1p2 keeping thephenylic moieties as rigid entities (rigid body approach[17]). The parameters of p1 were determined by the2031VOLUME 86, NUMBER 10 P H Y S I C A L R E V I E W L E T T E R S 5 MARCH 20015.0 7.5 10.0 12.5 15.0-120-100-80-60-40-20020406080[121][104][120][-122][113][021][020][-112][102][110][-111][002]Θ2 / deg∆int/ph.u.-200 0 200 400 600 800 1000 1200 1400-100-80-60-40-2002040[102]  [-112][-111] [020][110]  [121][104]  [-122]int / ph.u.t / psFIG. 2. Top: intensity difference map of the powder pattern att  80 ps and t  2240 ps. Bottom: change of the integratedBragg intensities as a function of time. The intensity changeranges between 1%–10% of the total signal. Selected Braggreflections are marked by their Miller indices.refinement of the ti , 0 structures. p1 is essentially theheated ground state structure [13,18]. The ti . 0 struc-tures were refined in p2 assuming a statistical distributionof the photoexcited molecules in the lattice. p1 acted asreference and was kept constant during analysis. In p2, thetranslational and rotational degrees of freedom (DOF) ofthe rigid body and the two intramolecular DOF, inversionand torsion (Fig. 1, bottom), were calculated and refineduntil a minimum in the least-square fit was reached. Thequality of the least-square fits and the validity of the refinedstructure was expressed by the R factor which is definedby R2F  SjIo 2 IcjSIo (Io  observed intensity atgiven Bragg angle, Ic  calculated intensity). The re-maining intramolecular DOF (especially the benzonitrile-and N ,N-dimethylamino moieties) were kept fixed duringthe refinements (for this widely used approach, see [19]).Based on the knowledge obtained, in particular, fromTR-IR measurements [2], this assumption is reasonablesince the distortions of the phenyl moiety in the singletexcited state as well as the distortions of the CN moietywere found to be negligibly small and not the drivingforces of the photophysics of DMABN [2,20]. Sincesmall distortions occur normally on extremely fast timescales, nuclear displacements of the benzonitrile moietyare to be expected on a femtosecond time scale and can2032therefore not be resolved in the experiment presented inthis Letter. 450–500 observations were used and refinedto the 12 structural DOF (four for the monoclinic unit cell,six for the rigid body molecule, and two for the moleculardistortion).The occupancies of p1 and p2 were also refined indi-cating the yield of photoexcited species compared to theground-state species. For small t, the refined occupan-cies of p2, the photoexcited state, range from 28% to32% and agree with the estimated excitation yield fromspectroscopic data (25%–30%). However, the structuraldifferences between p1 and p2 diminish for time pointsti . 800 ps leading to refined occupancies which mightbe meaningless.DMABN crystallizes in a monoclinic space groupP 21c Z  4 [13,18]. The unit cell parameter wasallowed to vary to compensate for small variations inthe sample-to-detector distance [a  6.304 65 Å,b  7.941 63 Å, c  17.233 65 Å, and b  91.5561.5±]. A preferred orientation of the powder samplewas excluded by comparing the intensity distribution ofthe various Bragg peaks and powder samples in rotatedcapillaries (where the microcrystals are assumed to berandomly orientated). No radiation damage—such asthe increase of amorphous background or the gradualdecrease of diffraction amplitudes —was observed.Discussion.—The results of the experiments and therefinements are summarized in Table I. For ti , 0 ps,the averaged inversion angle was found to be qinv 13 61±. After the absorption of an optical photon, thesystem is excited into the repulsive part of the PES of theexcited state. Here, the structure reorganizes quickly fromqinv  13 61± to qinv  3 61± bringing thesystem into a quasiplanar configuration (Table I, Fig. 3).Moreover, the CH3 groups are rotated out of the ben-zylic plane: The averaged torsional angle increases fromftors  0 61± to ftors  10 61±. If one comparesqinv and ftors of the excited state in Table I with theTABLE I. Geometries of different short-lived transientspecies of DMABN compared to high- and low-temperaturestructures. Five time points out of a series of 25 measure-ments are listed. T  temperature of the sample; t  time(error: 630 ps); qinv  averaged inversion angle (error:61%); ftors  averaged torsional angle (amino moiety, error:61±); occ.  occupancy [%] (phase 2, error: 64%), value inparentheses  fitting parameter (without physical meaning, seetext); R2F , see text.T t qinv ftors Occ. R2F301 K static 10± 0± 100% 7.90% [13]293 K static 12± 0± 100% 4.31% [18]173 K static 7± 8± 100% 4.37% [13]298 K 2240 ps 13± 0± 100% 6.40%80 ps 3± 9.5± 26% 4.89%240 ps 6± 8± 30% 5.25%470 ps 9± 6± 22% 4.80%1500 ps 14± 0± (6%) 6.90%VOLUME 86, NUMBER 10 P H Y S I C A L R E V I E W L E T T E R S 5 MARCH 2001-250 0 250 500 750 1000 1250 1500 1750024681012t / psφ tors/ degFIG. 3. Time evolution of the averaged torsional angle. Arelaxation lifetime trelax  520 ps is observed (the electroniclifetime of the excited state of a molecule in the molecular crystalis tf  1.7 1.9 ns, this Letter and [14]).known high- and low-temperature structures [13,18], thephotoexcited structures resemble the low-temperaturestructure. This means that, during photoexcitation,DMABN “cools down” in terms of the structure, whereasthe “structural cooling” is transient and nondissipative andconcerns only the photoexcited molecules in the crystal(in contrast to [21]). However, the results can also beinterpreted vice versa: It might be that the observedrelaxation process is an effect of a complex couplingbetween the electronic and vibrational states, where therelaxation pathway of the electronically excited DMABNoccurs via phonon scattering, which is shown in thedistortion of the molecule. However, taking the size of thegrains into account (about 5 mm3), this effect is expectedin 10–20 ns (further experiments and discussions willbe given in [22]). Figure 3 shows the time dependenceof ftors. The relaxation time trelax  520 ps is shorterthan the electronic lifetime of the excited state of DMABN(tf  1.9 ns, this Letter and [14]). A more complexdata analysis concerns the variation of the intermolecularparameters and will be presented in [22].Summary.— In this Letter, we have reported on thestructural characterization of ultrashort-lived organic crys-tals and their excited state geometry. Since these experi-ments require a high x-ray flux, the experiments werecarried out using femtosecond laser pump and picosecondx-ray-probe pulses generated by a synchrotron of the thirdgeneration (ID09-TR at the ESRF in Grenoble). Thoughthe work was done in crystals, on the picosecond tonanosecond time scale, the compounds showed transientstructural changes which could be characterized by anensemble of geometrical rearrangements during the relax-ation processes. In DMABN, the excited state geometryis similar to the low-temperature structure. Interactionsbetween the electronic and (intermolecular) vibrationalstates may also play a role as driving forces.The authors thank M. Hanfland, H. Staerk, J. Troe, andK. A. Zachariasse for their support and the high interest inthis work, which manifested itself in many useful discus-sions and comments.*Author to whom correspondence should be addressed.Electronic address: stecher@gwdg.dePresent address: Max-Planck-Institute for BiophysicalChemistry, Department 010, 37070 Göttingen, Germany.[1] R. S. Mulliken, J. Chem. Phys. 7, 121 (1939); R. A. Marcus,J. Chem. Phys. 65, 679 (1965); A. Weller, Pure Appl.Chem. Phys. 16, 115 (1968).[2] C. Chudoba, A. Kummrow, J. Dreyer, J. Stenger, E. T. J.Nibbering, T. Elsaesser, and K. A. Zachariasse, Chem.Phys. Lett. 309, 357 (1999).[3] M. Born and R. Oppenheimer, Ann. Phys. (Leipzig) 84,457 (1928).[4] H. Koeppel, W. Domcke, and L. S. Cederbaum, Adv. Chem.Phys. 57, 59 (1984).[5] D. Bourgeois, T. Ursby, M. Wulff, C. Pradevand, V. Srajer,A. LeGrand, W. Schildkamp, S. Laboure, C. Rubin, T.-Y.Teng, M. Roth, and K. Moffat, J. Synchrotron. Radiat. 3,65 (1996); V. Srajer, T.-Y. Teng, T. Ursby, C. Pradevard, Z.Ren, S. Adachi, W. 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Picosecond x-ray diffraction probed transient structural changes in organic solids.

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