Structural Dynamics of Bismuth Triiodide in Solution Triggered byPhotoinduced Ligand-to-Metal Charge TransferEun Hyuk Choi,†,‡,⊥ Doo-Sik Ahn,†,‡,⊥ Sungjun Park,†,‡ Changwon Kim,†,‡ Chi Woo Ahn,†,‡

Siin Kim,†,‡ Minseo Choi,†,‡ Cheolhee Yang,‡ Tae Wu Kim,‡ Hosung Ki,‡ Jungkweon Choi,†,‡

Martin Nors Pedersen,§ Michael Wulff,§ Jeongho Kim,# and Hyotcherl Ihee*,†,‡

†Department of Chemistry and KI for the BioCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon34141, Republic of Korea‡Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 34141, Republic of Korea#Department of Chemistry, Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Republic of Korea§European Synchrotron Radiation Facility (ESRF), 38000 Grenoble Cedex 9, France

*S Supporting Information

ABSTRACT: Bismuth triiodide, BiI3, is one of the simplest bismuth halides,which have recently attracted considerable attention because of theirpromising properties. Here, we investigate the structural dynamics of aphotoinduced reaction of BiI3 in solution phase using time-resolved X-rayliquidography (TRXL) and density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. The photoreaction was initiated byexcitation at 400 nm, which corresponds to the ligand-to-metal charge-transfer transition. The detailed structures and kinetic profiles of all relevantintermediate species from the TRXL data show that the trigonal planarstructure of BiI3, which is predicted to be the most stable structure of thelowest excited state by TDDFT calculation, was not observed, and thephotoreaction proceeds via two parallel pathways within the time resolutionof 100 ps: (i) isomer formation to produce iso-BiI2−I, which relaxes back tothe ground-state structure, and (ii) dissociation into BiI2· and I· radicals,which nongeminately recombine to generate ground-state BiI3 and I2.

Metal halides, a class of polyhalogenated compounds,have attracted much interest because of their

remarkable photophysical and photochemical properties andtheir controllability with different ligands.1−3 In particular,light-driven processes of metal halides in the liquid solutionphase have been investigated intensely because of fundamentalinterest and their wide applicability for various optoelectronicmaterials.4−9 However, because of the presence of heavyatoms, photoexcited metal halides undergo complex relaxationpathways, both radiative and nonradiative, making itchallenging to elucidate the detailed mechanisms of photo-induced processes. Among metal halides, bismuth-basedcompounds have come into the spotlight in many fields ofchemistry and physics for their low toxicity and low costcompared with other metal halide compounds.10 For example,bismuth halides have recently been proposed as one of themost promising lead-free metal halide materials for solid-stateperovskite photovoltaics.11−14 In addition, they are goodcatalysts for various organic reactions, including Diels−Alderreactions and acylation reactions.15,16 Bismuth triiodide, BiI3, isthe simplest bismuth halide and can serve as an excellentmodel system to investigate the characteristics of bismuthhalides. Initially, Molnar et al. pioneered the detailed molecular

structure of BiI3 using gas-phase electron diffraction combinedwith vibrational spectroscopy.17 While most of previous studiesof BiI3 have focused on the characterization of the structureand photophysical properties of solid-state BiI3,

18 Horvath etal. have demonstrated, in their studies on BiIn

(n−3)−, ultraviolet(UV) photoexcitation of BiI3 in solution with the solvent ofacetonitrile, subsequently leading to photoredox decomposi-tion generating molecular iodine as a final product.19,20 To thebest of our knowledge, however, the mechanism of the UV-initiated photoreaction of BiI3 in the liquid solution phase hasnever been investigated. In particular, the reaction inter-mediates involved in the photoreaction of BiI3 and theirdetailed kinetics have not been determined yet. Althoughcrystalline BiI3 in the solid phase has been a hot topic becauseof their technological applications, it is necessary to elucidatethe photochemical behavior of a constituent unit cell, that is,solvated molecular BiI3 in solution, for a better understandingof the optoelectronic properties of the material.

Received: February 7, 2019Accepted: March 5, 2019Published: March 5, 2019

Letter

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Previously, the major intermediates generated by photo-excitation of polyhalogenated compounds were identified bytime-resolved spectroscopic and X-ray scattering experi-ments.21−23 In particular, isomeric species of polyhalogenatedcompounds have been identified as key intermediates involvedin the relaxation of dissociative excited states.24,25 For example,Tarnovsky and co-workers have studied ultrafast photo-isomerization of various polyhalide compounds in solutionby using ultrafast time-resolved spectroscopy and quantumcalculations.26 Recently, time-resolved X-ray solution scattering(TRXSS), also known as time-resolved X-ray liquidography(TRXL), has been widely used to directly capture thestructures of transient intermediates.27−31 Using TRXL, Ahnet al. recently reported a global scheme for the reactionpathways of UV-excited CHI3 in cyclohexane and, especially,identified an isomeric reaction intermediate, iso-CHI2−I.32 Inaddition, using TRXL, Marcellini and co-workers observed thesolvent-dependent formation of two types of isomeric speciesof CH2IBr.

33

In this work, we investigate the photoinduced structuraldynamics of BiI3 in acetonitrile by TRXL. Upon UV excitation,BiI3 is expected to undergo photochemical reactions includingdissociation, bond formation, and isomer formation. Threepossible candidate intermediates based on the literature andour theoretical predictions by quantum calculation arepresented in Figure 1. In the present study, we employ theexcitation at 400 nm, which excites the lowest transition of BiI3in acetonitrile solution. This transition has been assigned to theLMCT transition, which generally induces intramolecularphotoredox reactions with the metal center reduced andligands oxidized.Experimental scattering curves measured at various time

delays from a TRXL experiment are shown in Figure 2a,together with theoretical scattering curves that were fit to theexperimental scattering curves using the best kinetic model,which will be discussed below.It can be seen that the experimental and theoretical curves

are in good agreement with each other, indicating therelevance of the kinetic model. In Figure 2b, difference radialintensities, r2ΔR(r), obtained by sine-Fourier transform of thedifference scattering curves in Figure 2a are shown togetherwith theoretical r2ΔR(r) curves that well reproduce theexperimental ones. The scattering signal of a solution samplecan be decomposed into (i) the solute-only term, (ii) the cageterm, and (iii) the solvent-only term, as shown in Figures S1−S3 in the Supporting Information. Note that, as shown inFigure S1, the total scattering intensity of the solution sampleis dominated by the solute-only term because of large atomicform factors of the heavy atoms in the solute molecule and thesubstantial structural changes of the solute molecule.To find the best kinetic model that can describe the

mechanism of photoreaction of BiI3, we examined fourcandidate reaction pathways: (i) formation of trigonal planarBiI3 with D3h symmetry, which is predicted as the most stablestructure of the lowest singlet excited state S1 according to ourtime-dependent density functional theory (TDDFT) calcu-lations (which will be discussed below); (ii) formation ofdissociation fragments (BiI2· + I·); (iii) formation of an isomer,iso-BiI2−I, as observed in photoreactions of various poly-halogenated compounds; and (iv) formation of a mixture ofdissociation fragments (BiI2· + I·) and iso-BiI2−I. As anexample of examining these candidate pathways, we show inFigure 3 the results of the fitting analysis for the experimental

scattering curve at 100 ps. The candidate pathway (i), that is,the formation of trigonal planar BiI3 with D3h symmetry,exhibits a marked discrepancy between the experimental curveand the theoretical curve, giving the highest χ2 value. The fitcurve of the candidate pathway (ii) agrees well with theexperimental curve up to q = 6 Å−1 but shows deviation at highq values of 6−9 Å−1. By contrast, the fit curve of (iii) givesslightly better agreement at high q values but deviatessignificantly in the low q region around 1−2 Å−1. Theexperimental curve is most satisfactorily reproduced with thecandidate pathway (iv) that combines pathways (ii) and (iii),indicating that the LMCT transition induced by 400 nmexcitation leads to both isomer formation and dissociation ofBiI3. Accordingly, the theoretical scattering curves shown inFigure 2 were obtained with the kinetic model involving thecandidate pathway (iv).This finding can be more intuitively represented when the

scattering intensities in q-space are sine-Fourier transformedinto radial intensities in real space. As X-ray crystallographycan determine the structure of a crystal in real space, the realspace representation of the solution scattering signal directlyvisualizes the average structure of the solute molecules insolution, even though the solution scattering data are one-dimensional and therefore contain much less structuralinformation than those from X-ray crystallography.34,35 To

Figure 1. Candidate intermediates of BiI3 in acetonitrile upon 400 nmexcitation of LMCT transition and the steady-state absorptionspectrum of BiI3 in acetonitrile.

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visualize the structural change of BiI3 with higher clarity, wefocus on the solute-only difference radial intensities,r2ΔRsolute(r), at 100 ps. Details of how to extract the solute-only contribution are described in the Supporting Information.In Figure 4a−d, the theoretical r2ΔRsolute(r) curves for the fourcandidate pathways are compared with the experimentalr2ΔRsolute(r) curve. In Figure 4e, various interatomic distancesin the reactant and the candidate photoproducts are indicatedby colored arrows, and these interatomic distances areindicated by vertical bars of corresponding colors at thebottom of Figure 4a−d.For example, ground-state BiI3 has two types of atomic pair

distances: the three equivalent Bi−I bond lengths of 2.82 Åand the three equivalent I···I distances of 4.26 Å. When someof these atomic pair distances are changed in the reaction,negative peaks will be observed at the original values of theinteratomic distances in the ground-state BiI3, as indicated bydownward bars in Figure 4a−d. Indeed, the experimentalr2ΔRsolute(r) has (i) negative peaks at r values corresponding tothe atomic pair distances in the ground-state BiI3 and (ii)broad positive features around 5−6 Å, which implies that oneor more Bi−I distances in the ground-state BiI3 are changedand new atomic pairs with longer distances are formed in thereaction. For candidate pathway (i) shown in Figure 4a, thereare considerable discrepancies between the theoretical andexperimental r2ΔRsolute(r) as in the reciprocal space repre-sentation shown in Figure 3. In particular, theoreticalr2ΔRsolute(r) for D3h trigonal planar BiI3 shows twoderivative-like features at ∼3 and ∼5 Å. These features arisefrom the Bi−I and I−I distances of D3h trigonal planar BiI3,which are longer than those of ground-state BiI3, as

represented by green upward bars in Figure 4a. For candidatepathway (ii) shown in Figure 4b, the theoretical curve decentlyreproduces the two negative peaks originating from thedepletion of one Bi−I and two I···I distances of ground-stateBiI3, but it does not describe at all the broad positive feature atdistances longer than ∼5 Å in the experimental r2ΔRsolute(r)because two Bi−I and one I···I distances of BiI2· are slightlyshorter than those of ground-state BiI3. For candidate pathway(iii) shown in Figure 4c, the positive and negative features ofthe theoretical r2ΔRsolute(r) are located at the same positions asthose of the experimental r2ΔRsolute(r) because of longer I···Iand Bi···I distances of newly formed iso-BiI2−I than those ofdepleted ground-state BiI3, but the intensities of those featuresin the theoretical and experimental r2ΔRsolute(r) do not matcheach other. In contrast, as shown in Figure 4d, candidatepathway (iv), which combines pathways (ii) and (iii), givesexcellent agreement between the experimental and theoreticalr2ΔRsolute(r), indicating the reaction mechanism involves thetwo parallel reaction pathways, that is, dissociation and isomerformation.The reaction mechanism of photoinduced reaction of BiI3 in

acetonitrile and the time-dependent concentrations of all thereactant and photoproduct species determined from the TRXLdata are shown in panels (a) and (b) of Figure 5, respectively.According to the TRXL results, 52.6 ± 0.6% of ground-stateBiIIII3 (where the oxidation number of Bi is indicated) isinitially excited to the LMCT state, I·BiIII2, resulting in thereduction of the central Bi3+ atom to Bi2+ and the oxidation of

Figure 2. TRXL signals of BiI3 in acetonitrile measured withexcitation at 400 nm. (a) Experimental (black) and theoretical (red)difference scattering curves, qΔS(q). (b) Difference radial intensities,r2ΔR(r), obtained by sine-Fourier transformation of qΔS(q) in panela.

Figure 3. Fits of the difference scattering curve at 100 ps using fourcandidate pathways: (i) formation of trigonal planar BiI3 with D3hsymmetry, (ii) dissociation fragments (BiI2· + I·), (iii) iso-BiI2−I, and(iv) a mixture of iso-BiI2−I and dissociation fragments (BiI2· + I·).Experimental (black) and theoretical (red) difference scatteringcurves, qΔS(q), are shown with the residuals (blue) obtained bysubtracting the theoretical qΔS(q) from the experimental qΔS(q).The χ2 value and the molecular structures of photoproducts are alsoshown for each candidate pathway. Candidate (iv) gives the best fit.

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the I− ligand to the I· radical. Subsequently, 68.3 ± 1.5% of theexcited molecules rapidly relax back to the ground state whilereleasing a part of the photoexcitation energy as heat to

surrounding solvent molecules. Among the remaining 31.7 ±1.5% of excited molecules, 12.0 ± 0.6% isomerize to form iso-BiIII2−I and 19.7 ± 1.0% dissociate to yield BiIII2· and I·radicals. Because both of these reaction pathways occur fasterthan our temporal resolution (100 ps), the kinetic componentscorresponding to them are not identified from the TRXL data.The iso-BiIII2−I isomer reisomerizes back to the ground statewith a time constant of 3.90 ± 0.41 ns. The dissociationfragments, BiIII2· and I·, nongeminately recombine to form theground state BiI3 and molecular iodine with bimolecular ratesof 3.34 (± 0.43) × 1010 M−1 s−1 and 1.90 (± 0.48) × 1010 M−1

s−1, respectively. These mixed processes of the fastunimolecular relaxation and the two slow bimolecularrecombination reactions can also be identified from rightsingular vectors (RSVs) obtained from singular valuedecomposition (SVD) of the TRXL data in a high-q region,where the solute term is dominant. As can be seen in FigureS5, the RSVs for both the experimental and theoreticalscattering curves are well described by a sum of twoexponential functions, of which one is attributed to the fastunimolecular relaxation and the other describes the slowbimolecular reactions.To confirm the validity of the reaction mechanism involving

the two parallel pathways, we performed the geometryoptimization of the ground electronic states of BiI3 usingdensity functional theory (DFT), and its results are illustratedin Tables S1 and S2. The ground-state BiI3 has a pyramidalstructure with a Bi−I bond of 2.826 Å length and two Bi−Ibonds of 2.824 Å length. Because the two types of Bi−I bondlengths are different by only 0.002 Å, the ground-state BiI3 canbe regarded as belonging to the C3v symmetry group, and welabel it as S0-C3v. We also found the optimized structure of theisomer species, iso-BiI2−I, in S0 state and labeled it as S0-iso. S0-iso is located at a shallow minimum with the energy of 1.73 eVrelative to S0-C3v. From the calculated energies of (i) thetransition state, S0-TS, between S0-iso and S0-C3v and (ii) the

Figure 4. Experimental (black) and theoretical (red) solute-only difference radial intensities at 100 ps time delay for the candidate pathways of (a),trigonal planar BiI3 with D3h symmetry (b) dissociation fragments (BiI2· + I·), (c) iso-BiI2−I, and (d) a mixture of iso-BiI2−I and dissociationfragments (BiI2· + I·). In each panel, the positions of the interatomic distances are shown with vertical bars. The upward and downward barsindicate the interatomic distances of the reaction products and the depleted ground state, respectively. The bars are colored according to the atom−atom pair distances in the chemical species relevant for the four candidate reaction pathways (that is, the ground-state BiI3 with C3v symmetry,trigonal planar BiI3 with D3h symmetry, BiI2·, and iso-BiI2−I), as shown in panel (e).

Figure 5. (a) Reaction mechanism of BiI3 in acetonitrile upon 400 nmexcitation. (b) Time-dependent concentration profiles for the reactantand photoproduct species determined by global fit analysis of TRXLdata. Dots with error bars were obtained by fitting the individualexperimental difference scattering curve at each time delay by a linearcombination of necessary components, such as solute, cage, andsolvent terms, and solid lines were obtained from the global fitanalysis.

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dissociation fragments (BiI2· + I·), the barrier from S0-iso to S0-C3v is estimated to be only 0.06 eV and the energy required fordissociation into radicals should be at least 0.39 eV. Thus, afterthe formation of S0-iso, the relaxation back to the ground state(S0-C3v) should be favored over the dissociation into theradical fragments, BiI2· + I·. This result of DFT calculation onthe energetics of S0 is consistent with the optimal kineticmodel, where iso-BiI2−I relaxes back to the ground stateinstead of being dissociated into radicals and the pathway ofisomer formation is parallel with the dissociation pathway.The optimized structural parameters of the reactant and

photoproduct species determined from the TRXL experimentand DFT calculations are presented in Figure 6. Overall, the

bond lengths obtained from the DFT calculations areoverestimated by 0.3−1.6% compared with the experimentallydetermined values. Also, the dihedral angle of iso-BiI2−I waspredicted to be −142.0° from the DFT calculation, in contrastto −64.9° determined from the TRXL experiment, meaningthat the DFT-predicted geometry of iso-BiI2−I is more planarthan the one determined by the TRXL experiment. We notethat the structure of iso-BiI2−I strongly depends on the choiceof DFT functional and the solvation field, which was also thecase for iso-CHI2−I, while the structure of BiI3 is less sensitiveto those factors, as listed in Tables S1 and S2. The dependenceof optimized structure on DFT functional and the comparisonof DFT-optimized and TRXL- determined structures arediscussed in detail in the Supporting Information.From the TRXL measurement, we found that photoexcited

BiI3 in acetonitrile undergoes two parallel reactions: (i)formation of iso-BiI2−I and (ii) dissociation into BiI2· and I·radicals. The branching ratio between the isomer formationand the dissociation is 38:62 with an uncertainty of 3. Inaddition, the detailed structures of the photoproducts, iso-BiI2−I and BiI2·, were determined by virtue of structuralsensitivity of TRXL. In particular, it should be noted that Bi−Ibonds are shortened in both BiI2· and iso-BiI2−I compared

with that in the ground-state BiI3. Such contraction of the Bi−Ibond induced by the photoreaction is consistent with recentstudies of other trihalides, AX3, where A = CH, B, or P and X =I or Br. In addition, we note that the I−I bond (3.444 Å) of iso-BiI2−I is considerably long, even longer than Bi−I bonds(2.68−2.82 Å), which are in principle expected to be longerthan I−I bonds, of the reactant and photoproducts of BiI3,including BiI2·, and iso-BiI2−I. The long X−X bond is acommon structural feature of isomeric species of othertrihalides. Especially, the I−I bond of iso-BiI2−I is significantlylonger than that of other trihalide isomers. The halogen−halogen bond in isomeric species of various halocarbons hasthe character of polar covalent bond and therefore can beaffected by the polar environment.36 According to a study onCHBr3, it was theoretically shown that the structure of iso-CHBr2−Br varies depending on the solvent polarity, especiallywith the Br−Br bond length being proportional to the solventpolarity.37 Accordingly, the I−I bond of iso-BiI2−I (3.444 Å) islong in acetonitrile, which is a polar solvent, and especiallymuch longer than the I−I bond of an isomer, iso-CHI2−I,(2.922 Å) formed from the photoreation of CHI3 in nonpolarcyclohexane, which was recently investigated using TRXL. Inaddition, according to the study on CHBr3, the lifetime of theisomeric species changes depending on the solvent polarity,from a few nanoseconds in polar solvents to microseconds innonpolar solvents. Such solvent dependence of lifetime of theisomeric species can be attributed to the polar nature of thetransition state between iso-BiI2−I and the ground state (S0-C3v) (see the Supporting Information and Table S2). Becausethe transition state can be stabilized with solvation by polarsolvent molecules, the barrier for back isomerization to theground state becomes smaller. In fact, iso-BiI2−I in (polar)acetonitrile (3.90 ns) exhibits much shorter lifetime than iso-CHI2−I in (nonpolar) cyclohexane (330 ns).The TRXL data show no evidence for the formation of

trigonal planar BiI3 with D3h symmetry, which is predicted tobe the most stable structure in the S1 state of BiI3. We proposethat, at the earliest stage of the photoreaction, the trigonalplanar BiI3 with D3h symmetry may serve as a primary reactionintermediate that branches into the two reaction pathways, thatis, the formation of iso-BiI2−I and the dissociation fragments.In this work, we investigated the spatiotemporal kinetics of

photoinduced reaction of BiI3 in solution. By analyzing thetime-dependent X-ray scattering signals, we identified thekinetics and structures of major intermediate species. We alsoperformed DFT/TDDFT quantum calculations for BiI3, whichsupport our experimental findings. Interestingly, trigonal planarBiI3 with D3h symmetry is predicted as the optimum structureof S1 according to our calculation, but it was not observed fromthe analysis of our TRXL data. This stable form of BiI3 may bethe primary intermediate formed at the earliest stage of thereaction. It should be feasible to directly observe the evolutiondynamics of this primary intermediate with ultrafast opticalspectroscopy or femtosecond X-ray scattering using an X-rayfree electron laser (XFEL). The present contribution mayinitiate further studies of BiI3 and related trihalide/metal halidesystems, which will enable us to extend our understanding ofthe underlying reaction mechanism of those systems.

Figure 6. Comparison of structures of ground-state BiI3, iso-BiI2−I,and BiI2· determined from the experimental TRXL data and theirDFT-optimized structures. The values of the bond lengths and bondangles determined from the global fit analysis of the TRXL data areshown in black font, and those from DFT calculations are shown inblue font in parentheses. For iso-BiI2−I, the dihedral angle is alsoindicated by a dashed circular arrow in the lower right panel.

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■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jp-clett.9b00365.

Details of TRXL experiment, data processing, dataanalysis, density functional theory calculation, andmolecular dynamics simulation; decomposition ofsolution scattering signal, qΔS(q, t) and r2ΔR(r, t);experimental solute-only r2ΔR(r, t); SVD analysis ofexperimental, theoretical, and mock data; polychromaticcorrection; DFT-optimized versus experimentally deter-mined structures of BiI3, BiI2·, and iso-BiI2−I (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: hyotcherl.ihee@kaist.ac.kr.ORCIDEun Hyuk Choi: 0000-0001-9051-9946Doo-Sik Ahn: 0000-0001-9154-1029Tae Wu Kim: 0000-0002-6370-0907Hosung Ki: 0000-0001-8273-6596Jungkweon Choi: 0000-0002-9979-305XMartin Nors Pedersen: 0000-0001-6975-5071Jeongho Kim: 0000-0003-4085-293XHyotcherl Ihee: 0000-0003-0397-5965Author Contributions⊥E.H.C. and D.-S.A. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by Institute for Basic Science (IBS-R004).

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