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Refined Insights in the Photochromic spiro-Dihydroindolizine/ Betaine System Amendra Fernando, Aruni P. Malalasekera, Jing Yu, Tej B. Shrestha,† Emily J. McLaurin, Stefan H. Bossmann,* and Christine M. Aikens* Department of Chemistry, Kansas State University, 213 CBC Building, Manhattan, Kansas 66506, United States

Downloaded by UNIV OF BRITISH COLUMBIA on September 7, 2015 | http://pubs.acs.org Publication Date (Web): September 3, 2015 | doi: 10.1021/acs.jpca.5b05262

S Supporting Information *

ABSTRACT: We have revisited the photochromic spirodihydroindolizine/betaine system by comparing state-of-theart density functional theory calculations with experimental data. Time-dependent density functional theory calculations are employed to examine the transformations occurring after photoexcitation. This study confirms that photoexcitation of the spiro-dihydroindolizine leads to the formation of the cisbetaine. However, isomerization to the trans-betaine follows through a complicated and formerly unknown potential energy landscape, which consists of a network of transition states and intermediates. The available pathways across this potential energy landscape will determine the kinetics of the forward reaction from the cis-betaine to the trans-betaine and then, even more importantly, the back-reaction. Virtually all practical applications of this optical switch rely on these reactions and, therefore, occur within this landscape. Predicting the network of transition states and intermediates for substituted spirodihydroindolizine/betaine systems will enable the in-silico design of optical switches with enhanced performance.

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INTRODUCTION Although a wealth of photochromic spiro-dihydroindolizines (DHI) (Figure 1) has been synthesized since their discovery by H. Dürr in 1979, there is still considerable discussion about the electronic structures of spiro-dihydroindolizines and their betaine isomers, which are formed by reverse electrocyclization from photoexcited singlet states.1,2 Betaines can exist as cis- and trans-betaines. The former undergo 1,5-electrocyclization to their corresponding spiro-dihydroindolizines.2 During the last

25 years, photochromic DHI’s have re-emerged as a very promising class of optical switches in multiple applications.3−13 Special emphasis has been directed toward chemical models for the photosynthetic process,14,15 photological devices,16,17 highdensity data storage polymer films,18 molecular machines,19 and prototypes for advanced nucleic acid sequencing.20 Although early attempts toward a theoretical understanding of the DHI− betaine transitions were made in 1985 using MINDO/3 calculations, the first modern theoretical study of this system, which is based on density functional theory (DFT), was published in 2008 and examined the relative energies of the spiro-DHI and cis- and trans-betaines.21 Here, we report a more thorough analysis of the chemical nature of the photoexcitation and isomerization reactions and the resulting potential energy landscape. Extensive studies were carried out to find different conformations of DHI and both cis- and trans-betaines by examining various bond rotations. The three principal directions of bond rotations in spiro-DHI and cis- and transbetaines are shown in Figure 2. Because all three isomers feature very different geometries, the value of in-silico performance optimization, especially of substituted spirodihydroindolizines before organic synthesis, becomes immediately apparent. The methodology that is being developed here Received: June 2, 2015 Revised: August 14, 2015

Figure 1. Structure of spiro-[9,1′(8′aH)-indolizine]-2′,3′-dicarboxylic acid. © XXXX American Chemical Society

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DOI: 10.1021/acs.jpca.5b05262 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A


Figure 2. Possible bond rotations of spiro-DHI and cis- and trans-betaines leading to many local minima on the potential energy surface (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

Figure 3. Relative energies for the “pyridine” spiro-DHI/betaine system at the B3LYP/6-31G* level of theory in the gas phase (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

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COMPUTATIONAL DETAILS Theoretical calculations were performed with the General Atomic and Molecular Electronic Structure System (GAMESS)22,23 program. Density functional theory (DFT) at the generalized gradient approximation (GGA) level with the B3LYP24,25 exchange and correlation functional combined with the 6-31G* basis set is employed for all geometry optimizations. This level of theory was chosen for consistency with ref 21. In addition, the polarizable continuum model (PCM) was used to account for solvation effects; where noted, the calculations utilize acetonitrile as the solvent (solvent radius = 2.75 Å, dielectric constant = 37.5). Møller−Plesset perturbation theory (MP2) single-point energy calculations were also employed with the 6-31G* basis set. Time-dependent density functional theory (TDDFT) optimization and singlepoint energy (SPE) calculations also employ the B3LYP/631G* level of theory. Harmonic vibrational frequencies were systematically computed for the optimized geometries; these structures correspond to local minima that have only real

will be able to support synthetic efforts in the future, with special emphasis on tailoring the photophysical properties of the organic photoswitches.

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EXPERIMENTAL SECTION The synthesis of H-fluorene-[9,1′(8′aH)-indolizine]-2′,3′-dicarboxylic acid, 2′,3′-dimethyl ester (“pyridine” DHI), is described in ref 21. UV/vis spectra of DHI/betaine mixtures and thermal ring closure kinetics were recorded at 293 K in acetonitrile (ACS spectrograde) using a Varian Cary 500 UV/ vis−NIR spectrophotometer and 4.0 mL quartz cuvettes. The DHI was isomerized into its corresponding betaine by means of irradiation with a 150 W Xe arc lamp (USHIO) powered by a PTI PS-220 power supply at 293 K. A water filter was used. 13C NMR spectra were recorded at 253 and 293 K using a Varian 400 MHz NMR spectrometer. The betaine was formed by means of irradiating “pyridine” spiro-DHI 1 (2 × 10−3 M) for 600 s at 200 K in a quartz NMR tube, which was then transferred to the NMR spectrometer. B


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bond character between C1C2C3N4 atoms (Figure 4) but show rather more delocalized bonding (Table 2). C1C2

frequencies or saddle points with only one imaginary frequency. MP2 and DFT single-point energy calculations were performed with the 6-311G** basis set at the optimized cis- and transbetaines and spiro-DHI configurations for more accurate relative energy predictions.


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RESULTS AND DISCUSSION Relative Stability of spiro-DHI vs cis- and transBetaine. The structures and relative energies of the spiroDHI and cis- and trans-betaine are shown in Figure 3. The first difference with regard to the results of the previous DFT calculations reported in ref 21 is that for the “pyridine” spiroDHI/betaine system the relative stabilities of the spiro-DHI and cis- and trans-betaine have to be revised: The new stability pattern of this system in the gas phase is spiro > cis > trans. The previously reported pattern was spiro > trans > cis, but a lower energy cis isomer has been identified in our current work. B3LYP optimizations with acetonitrile solvent medium suggest that the cis isomer is even lower in energy than the spiro-isomer, which may be due to the polarity of the cis isomer because of the orientation of its ester groups (Table 1). Because

Figure 4. Pyrrolidine ring of spiro-DHI, which opens up upon photoexcitation, with the numbering of atoms described in the text (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

Table 1. Relative Energies (kcal mol−1) of spiro-DHI and cisand trans-Betaines level of theory

spiroDHI

Gas Phase B3LYP/6-31G* 0.00 B3LYP/6-311G**//B3LYP/6-31G* 0.00 MP2/6-31G*//B3LYP/6-31G* 0.00 MP2/6-311G**//B3LYP/6-31G* 0.00 Acetonitrile Phase B3LYP/6-31G* 0.16 B3LYP/6-311G**//B3LYP/6-31G* 0.12 MP2/6-31G*//B3LYP/6-31G* 0.00 MP2/6-311G**//B3LYP/6-31G* 0.00

cisbetaine

transbetaine

3.33 3.16 9.63 10.60

3.54 3.62 11.04 12.46

0.00 0.00 5.06 6.06

0.95 1.15 9.11 8.77

Table 2. Critical Bond Lengths (Å) bond

spiro

cis

trans

C1−C2 C2−C3 C3−N4 N4−C5

1.53 1.36 1.37 1.48

1.38 1.45 1.42 1.37

1.37 1.46 1.41 1.38

bond lengths in betaines are found that are closer to double bond character than single bond character compared to standard CC and CC lengths of 1.33 and 1.57 Å, respectively. On the contrary, C2C3 bond lengths in betaines are more likely to have a mixture of single and double bond character than pure double bond character, and C3−N4 bond lengths are in the range of a typical CN single bond. UV/vis Absorption Spectra of spiro-DHI and Betaines. TDDFT single-point energy calculations on B3LYP/6-31G* optimized structures were used to account for the experimental UV/vis spectral features of the DHI/betaine system. There are two characteristic peaks in the experimental transient absorption spectrum in the microsecond time scale corresponding to betaines: one sharp peak around 440 nm and a broad peak around 600 nm. Our theoretical spectra (Figure 5) for cisand trans-betaines also show two characteristic peaks around 400−450 nm and 500−600 nm. In the experimental spectra obtained on a time scale of seconds, growth of the 400 nm peak at the expense of the 440 nm peak as shown by the isosbestic points indicates the regeneration of DHI at 293 K.2,21 Figure 6 shows the experimental betaine and DHI spectrum (after cyclization) of the “pyridine” DHI. This can be accounted for by the characteristic peak observed around 400 nm in our theoretical spiro-DHI spectrum. TDDFT theoretical spectra support the fact that spiro-DHI and cis- and trans-betaines are collectively contributing to the experimental spectra observed at these time domains. The characteristic peak observed at 500−600 nm is distinctive of the presence of betaines. This demonstrates that upon photoexcitation, a fraction of DHI is converted to either cis- or trans-betaine. An extended discussion of the absorption spectra for other rotamers of cis- and trans-betaines and spiro-

the spiro structure is expected to be the lowest in energy, we have also deployed MP2 single-point energy calculations on these isomers to further test the effect of solvent on rotamer energies (Table 1). Overall, the spiro compound is preferentially stabilized in MP2 calculations relative to the cis and trans isomers. These calculations suggest that the correct stability pattern is spiro > cis > trans. Single-point calculations at the MP2/6-311G** level of theory in acetonitrile solvent show a stability pattern comparable to results of MP2/6-31G* calculations. At the B3LYP/6-31G* (gas phase) level of theory, the cis isomer is 0.21 kcal mol−1 lower in energy than the trans isomer; single-point energy calculations at the B3LYP/6-311G** level of theory in the gas phase predict that this gap is 0.46 kcal mol−1. The highest energy difference in the gas phase is obtained between the spiro and trans structures. When determining the properties and other reaction dynamics, we have used the gas phase calculation geometries unless otherwise mentioned. The other rotamers we have observed from rotations among C1−C2, C2−C3, and C3−N4 bonds are given in the Supporting Information. Optimized Geometries for spiro-DHI, cis-Betaine, and trans-Betaine. In principal agreement with the initial DFT study,21 our new geometries for the zwitterionic betaine system also do not follow the “classic” betaine single−double−single C


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Figure 5. TDDFT UV/vis absorption spectra obtained at the B3LYP/6-31G* level of theory for cis- and trans-betaines and spiro-DHI.

Figure 6. UV/vis-extinction (E) spectra of H-fluorene-[9,1′(8′aH)-indolizine]-2′,3′-dicarboxylic acid, 2′,3′-dimethyl ester (“pyridine” DHI (9.25 × 10−5 mol L−1 in acetonitrile)) and its corresponding betaine, recorded at 20 °C after irradiation for 5 min with a 150 W Xe arc lamp (USHIO).

(Figure 7); optimization of the T1 state leads to a closed-ring structure that is 37.48 kcal mol−1 higher in energy than the ground state, so the ring-opening occurs only for the S1 state. This suggests that upon excitation, spiro-DHI follows a path with the least motion of the pyridine and ester groups to form the cis-betaine rather than the trans isomer. The vibrationally excited cis-betaine may then isomerize to higher energy cis rotamers or to trans rotomers or cyclize to regenerate the DHI compound. Production of more cis than trans isomer experimentally determined to be in a 95:5 ratio21 can be explained from this observation as well as the greater stability of the cis isomer as described above. Formation of cis and trans isomers depends on the transition state barrier height between them. Knowledge about this conversion is crucial in optimizing practical applications of this photochromic system. We have extensively studied this conversion and found that the previous idea of one barrier height is not consistent with this isomerization. There are many transition states between these isomers that connect respective rotamers. The potential energy landscape between the cis and trans isomer is a rather complicated one that consists of a network of transition states and intermediates. It is also noteworthy that among these transition states we have

DHIs is provided in the Supporting Information section. Most importantly, the spectra of the other isomers display similar peaks with respect to the lowest energy geometries: The transitions responsible for the main peaks are the same for the various rotamers but show slight shifts in peak energies (Supporting Information), which is likely one reason for the broad peaks observed in the experimental absorption spectra. Inclusion of acetonitrile solvent in the TDDFT calculations again leads only to very slight shifts in peak energies (Supporting Information). With the exception that the lowest energy betaine absorption peaks are broader than anticipated at the B3LYP/6-31G* level of theory, excellent fits are observed for both DHI and betaine absorption. Note that the absence of well-defined isosbestic points indicates that more than two isomers are involved. This is in excellent agreement with the occurrence of potential landscapes in photochromism, resulting in the presence of several rotamers (see below). Potential Energy Landscapes in Photochromism. TDDFT excited state optimization calculations with the B3LYP/6-31G* level of theory on spiro-DHI revealed electrocyclic ring opening of the pyrrolidine ring upon excitation to the lowest singlet state to form the lowest energy cis-betaine D


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Figure 7. Simplified illustration of photochemical reaction scheme showing the generation of cis-betaine and trans-betaine after excitation of spiroDHI (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

Figure 8. Reactant and products connected to transition state A offset by 3 times the normal mode eigenvectors. Relative energies are given with respect to the lowest energy spiro-DHI (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

observed “cis-like” and “trans-like” transition states where “trans-like” transition states are higher in energy than “cislike” transition states. Most of the transition states have a low imaginary vibrational frequency around 13−21 cm−1, which corresponds to vibrational motions of the pyridine ring and ester groups. Overall,

these transition state vibrations connect different rotamers of cis- and trans-betaines when they are offset slightly according to their normal mode eigenvectors. To understand some features observed, we report two transition states here and some of other transition states are given in the Supporting Information. Among these transition states, a structure with the two large E


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Figure 9. Transition state for an ester group rotation between the lowest energy trans-betaine and a trans rotamer. Relative energies are given with respect to the lowest energy spiro-DHI (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

aromatic groups essentially perpendicular (Figure 8, transition state A) provides good evidence of connectivity between the cis side and the trans side of the potential energy surface. The barrier height for transition state A is only 1.74 kcal mol−1. When transition state A is offset by 3 times the normal mode eigenvectors, one offset direction leads to a structure that is a mirror image of the trans-betaine isomer shown in Figure 1, and the other offset direction optimized into a higher energy “cislike” rotamer. A larger offset of the normal mode eigenvectors (30 times) optimized into the lower energy cis rotamer and to the same trans isomer. Transition state A has a dihedral angle of 92.5° between C1−C2−C3−N4 atoms; for the lowest energy cisbetaine, the dihedral angle is 45.5° and for the trans-betaine it is 229.9°. Notably, we also calculated a transition state (transition state J, Figure S5 in the Supporting Information) that corresponds to conrotatory rotation of the cis-betaine to generate the spiro-DHI isomer. This isomer has a barrier height of 23.29 kcal mol−1 in the gas phase and 21.79 kcal mol−1 in the acetonitrile phase, so this back-reaction to reactants should be accessible under the reaction conditions. We have found several transition states demonstrating the motion of ester group rotation. Transition state B in Figure 9 represents the distinctive motion of an ester group between two trans isomers. The imaginary vibrational frequency for this ester rotation (50.36i cm−1) is higher than that for the cis−trans conversion in transition state A. Is there an easily discernible experimental consequence of the existence of a rather complicated potential energy landscape for the thermal isomerization from the trans-betaine to the cisbetaine? Let us consider the observed cyclization kinetics of trans-betaine to DHI in acetonitrile as a function of temperature (Figure 10). If a single energy barrier between trans-betaine and cis-betaine exists, the cyclization kinetics from trans-betaine to spiro-DHI should be strictly first order, because the energy barrier(s) between cis-betaine and spiro-DHI are significantly smaller, resulting in a cyclization time from cis-betaine to spiroDHI on the order of 200−300 μs.21 However, at 30 °C the observed cyclization kinetic is clearly monoexponential (k = (3.70 ± 0.11) × 10−3 s−1), significant deviations from

Figure 10. Cyclization kinetics of the corresponding betaine of Hfluorene-[9,1′(8′aH)-indolizine]-2′,3′-dicarboxylic acid, 2′,3′-dimethyl ester (1.5 × 10−5 mol L−1 in acetonitrile) recorded at 10, 20, and 30 °C at λ = 578 nm after separate irradiation for 5 min with an 150 W Xe arc lamp (USHIO) at the corresponding temperature.

monoexponential kinetics are observed at 20 °C (initial k = (2.50 ± 0.08) × 10−3 s−1), and even more so at 10 °C (initial k = (1.22 ± 0.06) × 10−3 s−1). According to the calculations reported here, the system has sufficient energy to navigate through the potential energy landscape at 30 °C, whereas fractions of the molecules “get lost” at 20 °C and especially at 10 °C and remain as one of the possible trans-betaine rotamers. This behavior is reversible. Furthermore, the residual betaine color disappears upon heating to 30 °C, demonstrating that the remaining absorption band from 500−600 nm does not result from irreversible chemical reactions. 13 C NMR Characterization of spiro-DHI and transBetaine. 13C NMR spectroscopy is a suitable tool to measure electron densities in diamagnetic molecules.26 We have utilized this technique to determine the amount of electron delocalization in spiro-DHI and trans-betaine. The resulting 13 C NMR spectra are shown in Figures 11 and 12. A comparison of experimental and calculated data is shown in the Supporting Information. NMR predictions were obtained F


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Figure 11. 13C NMR (d, ppm) spectrum of “pyridine” DHI 1 at 298 K in CDCl3.

Figure 12. 13C NMR (d, ppm) spectrum of the betaine corresponding to “pyridine” spiro-DHI at 253 K in CDCl3. The betaine was formed by means of irradiating “pyridine” spiro-DHI (2 × 10−3 M) for 600 s at 200 K in a quartz NMR tube.

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from the Web site www.nmrdb.org (NMRDB.org, Tools for NMR spectroscopists, EPFL de Lausanne27,28). The following conclusions can be drawn from the 13C NMR experiments: (1) A clean isomerization from spiro-DHI (>99.5% pure) to trans-betaine can be achieved by irradiating at 253 K. trans-Betaine is virtually stable at 253 K in CDCl3. These results are in principal agreement with the kinetic experiments reported in Figure 10. (2) The symmetry of the fluorenyl section of spiro-DHI is broken, due to the spirodihydroindolizine geometry. (3) The 13C NMR signals of the fluorenyl section and the dimethyl ester section of spiroDHI are in excellent agreement with predictions. However, the 13 C NMR signals of the spirodihydroindolizine and especially the spiro-carbon (C7) are not in good agreement with predictions. This can be regarded as an indirect proof that the “real” electron densities in spiro-DHI cannot be described/ predicted by “classic” line-angle formulas. (4) There are two resonance structures of trans-betaine (Supporting Information). The 13C NMR experiment indicates that, as could be expected, none of the resonance structures properly describes the electron densities in trans-betaine. Again, there is good agreement between calculated and experimental 13C-signals of the fluorenyl and the dimethyl ester sections. There is also better agreement (than for spiro-DHI) between calculated and experimental 13C-signals of the (now aromatic) pyridinium section, although not for the carbons in the meta-position with respect to the ring nitrogen. Again, classic descriptions fail to describe the electron densities between the fluorenyl and the pyridinium groups of trans-betaine. These findings clearly demonstrate the need for electronic structure calculations in organic chemistry.

CONCLUSIONS

The state-of-the-art density functional theory calculations reported here were able, for the first time, to elucidate the chemical nature of the photoexcitation and the thermal backreaction of this photochromic system. Overall, photoexcitation of spiro-dihydroindolizine into the singlet state easily finds the cis-betaine isomer. Isomerization to the trans structure follows through a complicated potential energy landscape, which consists of network of transition states and intermediates. The highest transition state we found related to the cis- and trans-betaines is around 6 kcal mol−1 relative to the lowest energy spiro-dihydroindolizine, which provides a small kinetic barrier to generate the trans-betaine, and, more importantly, for the thermal back-reaction of the trans-betaine to the cis-betaine. We also found a transition state for conrotatory motion of the cis-betaine to regenerate the spiro-DHI. This transition state has a barrier height of 23.29 kcal mol−1 in the gas phase and 21.79 kcal mol−1 in the acetonitrile phase, so this back-reaction to reactants should be accessible under the reaction conditions. We found experimental evidence that a fraction of the transbetaines becomes entrapped in the potential energy landscape at lower cyclization temperatures. This study clearly demonstrates that in-silico studies can significantly contribute toward the design of photochromic spiro-dihydroindolizine systems with regard to the efficiency of the photochemical spiro-DHI to trans-betaine isomerization, as well as the lifetime of the transbetaine after photoswitching. 13 C NMR spectroscopy indicated that spiro-DHI can be completely converted to trans-betaine at 253 K. The comparison of experimental and calculated 13C NMR signals demonstrated that classic line-angle drawings are unable to describe the real electron distributions in this photochromic switch. G


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(8) Bleisinger, H.; Scheidhauer, P.; Dürr, H.; Wintgens, V.; Valat, P.; Kossanyi, J. Photophysical Properties of Biphotochromic Dihydroindolizines. Ring-Opening into Extended Bis-Betaines. J. Org. Chem. 1998, 63, 990−1000. (9) Dernbecher, K.; Gauglitz, G. Solvent Effects on the Thermal Reaction of Photochromic Dihydroindolizines: Friction or Polarizability Effect within the Alkanes. J. Chem. Phys. 1992, 97, 3245−3251. (10) Dürr, H.; Thome, A.; Kranz, C.; Kilburg, H.; Bossmann, S.; Braun, B.; Janzen, K. P.; Blasius, E. Supramolecular Effects on Photochromism-Properties of Crown Ether-Modified Dihydroindolizines. J. Phys. Org. Chem. 1992, 5, 689−698. (11) Fromm, R.; Born, R.; Dürr, H.; Kannengießer, J.; Breuer, H. D.; Valat, P.; Kossanyi, J. Spirodihydroazafluorenes - A New Type of CisFixed Photochromic Molecule with Rigid Region B Showing Extremely Fast Back Reaction. J. Photochem. Photobiol., A 2000, 135, 85−89. (12) Hartmann, T.; Shrestha, T. B.; Bossmann, S. H.; Hubner, C.; Renn, A.; Durr, H. A Light-Induced Photochromic Nanoswitch Capable of Non-Destructive Readout via Fluorescence Emission: Cluster vs. Single-Molecule Excitation of Dihydroindolizines. Photochem. Photobiol. Sci. 2009, 8, 1172−1178. (13) Shrestha, T. B.; Kalita, M.; Pokhrel, M. R.; Liu, Y.; Troyer, D. L.; Turro, C.; Bossmann, S. H.; Dürr, H. Maleimide-Functionalized Photochromic Spirodihydroindolizines. J. Org. Chem. 2013, 78, 1903− 1909. (14) Kodis, G.; Terazono, Y.; Liddell, P. A.; Andréasson, J.; Garg, V.; Hambourger, M.; Moore, T. A.; Moore, A. L.; Gust, D. Energy and Photoinduced Electron Transfer in a Wheel-Shaped Artificial Photosynthetic Antenna-Reaction Center Complex. J. Am. Chem. Soc. 2006, 128, 1818−1827. (15) Straight, S. D.; Andréasson, J.; Kodis, G.; Moore, A. L.; Moore, T. A.; Gust, D. Photochromic Control of Photoinduced Electron Transfer. Molecular Double-Throw Switch. J. Am. Chem. Soc. 2005, 127, 2717−2724. (16) Straight, S. D.; Andréasson, J.; Kodis, G.; Bandyopadhyay, S.; Mitchell, R. H.; Moore, T. A.; Moore, A. L.; Gust, D. Molecular AND and INHIBIT Gates Based on Control of Porphyrin Fluorescence by Photochromes. J. Am. Chem. Soc. 2005, 127, 9403−9409. (17) Terazono, Y.; Kodis, G.; Andréasson, J.; Jeong, G.; Brune, A.; Hartmann, T.; Dürr, H.; Moore, A. L.; Moore, T. A.; Gust, D. Photonic Control of Photoinduced Electron Transfer via Switching of Redox Potentials in a Photochromic Moiety. J. Phys. Chem. B 2004, 108, 1812−1814. (18) Weitzel, T.; Wild, U.; Amlung, M.; Dürr, H.; Irie, M. New Photochromic Materials for Holographic Recording. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2000, 344, 191−198. (19) Masson, J.-F.; Hartmann, T.; Dürr, H.; Booksh, K. S. SolidPhase Synthesis and Photochromic Switching of a Polymeric Photochromic Layer on a Gold Surface. Opt. Mater. 2004, 27, 435− 439. (20) Gogritchiani, E.; Hartmann, T.; Palm, B. S.; Samsoniya, S.; Dürr, H. Photochromic Nucleic Base Units Suitable for Nucleic Acid Labelling. J. Photochem. Photobiol., B 2002, 67, 18−22. (21) Shrestha, T. B.; Melin, J.; Liu, Y.; Dolgounitcheva, O.; Zakrzewski, V. G.; Pokhrel, M. R.; Gogritchiani, E.; Ortiz, J. V.; Turro, C.; Bossmann, S. H. New Insights in the Photochromic SpiroDihydroindolizine/Betaine-System. Photochem. Photobiol. Sci. 2008, 7, 1449−1456. (22) Dykstra, C. E.; Frenking, G.; Kim, K. S.; Scuseria, G. E.; Gordon, M. S.; Schmidt, M. W. Advances in Electronic Structure Theory: GAMESS a Decade Later. Theory and Applications of Computational Chemistry: The First Forty Years; Elsevier: Amsterdam, 2005; pp 1167− 1189. (23) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347−1363.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b05262. Structures, relative energies, and absorption spectra of other spiro-DHI and cis- and trans-betaine rotamers in gas phase and in acetonitrile solvent. Transition states between cis- and trans-betaines. Carbon atom numbering for spiro-DHI and resonance structures for trans-betaine. Comparison of experimental and predicted 13C NMR peaks for spiro-DHI and trans-betaine. XYZ coordinates of the structures calculated in this work (PDF)

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AUTHOR INFORMATION


Corresponding Authors

*S. H. Bossmann. Phone: 785-532-6668. Fax: 785-532-6666. Email: [email protected]. *C. M. Aikens. Phone: 785-532-6668. Fax: 785-532-6666. Email: [email protected]. Present Address †

Department of Anatomy & Physiology, Kansas State University, Coles Hall 228, Manhattan, KS 66506, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Science Foundation (DMR #1242765, CBET #1337438) and from the Terry C. Johnson Cancer Center at Kansas State University. The authors thank Dr. Leila Maurmann for assistance with the NMR experiments.

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REFERENCES

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