Insights from Theory and Experiment on the Photochromic spiro

Article pubs.acs.org/JPCA

Insights from Theory and Experiment on the Photochromic spiroDihydropyrrolo−Pyridazine/Betaine System Amendra Fernando,† Tej B. Shrestha,†,‡ Yao Liu,§ Aruni P. Malalasekera,† Jing Yu,† Emily J. McLaurin,† Claudia Turro,§ Stefan H. Bossmann,*,† and Christine M. Aikens*,† †

Department of Chemistry, Kansas State University, 213 CB Building, 1212 Mid-Campus Drive North, Manhattan, Kansas 66506, United States ‡ Department of Anatomy & Physiology, Kansas State University, Coles Hall 228, 1600 Denison Avenue, Manhattan, Kansas 66506, United States § Department of Chemistry and Biochemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: We elucidated the photochromic spiro-4a,5dihydropyrrolo[1,2-b]pyridazine/betaine (DPP/betaine) system by comparing state-of-the-art density functional theory calculations with nanosecond/millisecond UV−vis absorption spectroscopy, as well as steady-state absorption and cyclization kinetics. Time-dependent density functional theory calculations are employed to examine the transformations occurring after photoexcitation. This study shows that the photochromic spiro-4a,5-dihydropyrrolo[1,2-b]pyridazine and spiro-1,8a-dihydroindolizine (DHI) systems react according to similar pathways. However, notable differences exist. Although photoexcitation of the spiro-DPP system also leads to cisbetaines, which then isomerize to trans-betaines, we found two distinct classes of cis isomers (cis-betaine rotamer-1 and cis-betaine rotamer-2), which do not exist in spiro-1,8a-dihydroindolizine. Similar to our previous study on the spiro-DHI/betaine system, a complicated potential-energy landscape between cis and trans isomers exists in the spiro-DPP system, consisting of a network of transition states and intermediates. Because the spiro-DPP/ betaine is even more complicated than the spiro-DHI/betaine system, (substituted) photochromic systems featuring a 4a,5dihydropyrrolo[1,2-b]pyridazine functional unit will require thorough in silico design to function properly as logical gates or in devices for information storage.

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INTRODUCTION

(DFT) analysis of the spiro-DHI/betaine system was published in 2008.17 It was followed by a recent DFT study, which revealed the existence of a formerly undiscovered potentialenergy landscape between cis- and trans-betaines.11 The same methodology was used here in analyzing the spiro-DPP/betaine system. This study reveals both similarities and differences between the spiro-DHI/betaine and the spiro-DPP/betaine systems. Although both feature rich potential-energy landscapes, the spiro-DPP/betaine system features two distinct classes of cis isomers that differ in the orientation of the pyridazine ring. This dualism complicates the time-resolved and steady-state UV−vis-absorption spectra of spiro-DPP/betaine systems, as well as the observed cyclization kinetics.

The first spiro-4a,5-dihydropyrrolo[1,2-b]pyridazine/betaine (DPP/betaine) system was reported by H. Dürr in 1979.1 Since that time, numerous papers have been published in which the substitution of one CH group by N in the DPP/betaine system was generally treated as a minor variation of the spirodihydroindolizine (DHI; Figure 1).2−10 Here, we present strong evidence that the photochemical reactivity of the spiroDPP/betaine system differs significantly from that of the spiroDHI/betaine system.11 This is of great importance for the prospective use of spiro-DPP/betaine derivatives in photological devices12,13 and high-density data storage polymer films.14,15 Replacing spiro-DHI species with spiro-DPP species is highly recommended because the latter are significantly more photostable.2,7 Betaines, which exist as cis and trans isomers, are formed from spiro-DPP systems by reverse electrocyclization from photoexcited singlet states.1,7,16 The former undergo 1,5electrocyclization to their corresponding spiro-dihydropyrrolo[1,2-b]pyridazines.1,16 The first density functional theory © XXXX American Chemical Society

Received: October 13, 2015 Revised: January 25, 2016

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DOI: 10.1021/acs.jpca.5b10020 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

processed by a Tektronics 400 MHz oscilloscope (TDS 380).18,19 Computational Details. The General Atomic and Molecular Electronic Structure System (GAMESS)20,21 program was used to perform all calculations. DFT with the B3LYP22,23 exchange and correlation functional in conjunction with the 6-31G* basis set is utilized for all geometry optimizations. Time-dependent density functional theory (TDDFT) single-point energy and optimization calculations also employ the B3LYP/6-31G* level of theory. This level of theory was chosen for consistency with refs 11 and 17. Harmonic vibrational frequencies were calculated for optimized geometries, and these structures correspond to local minima that have only real frequencies or transition states with only one imaginary frequency. The polarizable continuum model was used to incorporate solvation effects; all the DFT and TDDFT calculations employ acetonitrile as the solvent (solvent radius = 2.75 Å, dielectric constant = 37.5). DFT single-point energy calculations were also performed with the 6-311G** basis set for the optimized cis- and trans-betaines and spiro-DPP configurations for more accurate relative-energy predictions.

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RESULTS AND DISCUSSION Relative Stability of spiro-DPP Versus cis- and transBetaine. Our previous interesting findings with the spirodihydroindolizine/betaine system led us to investigate related systems. We use the same techniques that were employed before to observe the spiro-DPP/betaine system. On the basis of the available literature2−10 we initially expected a behavior similar to the spiro-DHI/betaine system, but to our surprise the inclusion of the additional nitrogen introduced many more variations to the system. Our first step was to detect the existence of possible isomers. Compared to the spiro-DHI, the spiro-DPP system also consists of many rotamers originating from several possible bond rotations making it rather a complex system to study (see Figure 2). Hereafter, the terms spiro-, cis-, and trans-betaine isomers will correspond to the spiro-DPP/ betaine system unless otherwise mentioned in comparison.

Figure 1. (a) Structures of spiro-[9H-fluorene-9,5′(4′aH)-pyrrolo[1,2b]pyridazine]-6′,7′-dicarboxylic acid, 6′,7′-dimethyl ester (1; spiroDPP) and spiro-[9,1′(8′aH)-indolizine]-2′,3′-dicarboxylic acid (2; spiro-DHI). (b) Resonance structures of cis- and trans-betaines (reaction of the cis isomer from rotamer-1 to rotamer-2 is discussed in the text).1,16

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METHODS Experimental Methods. The synthesis and characterization of spiro-[9H-fluorene-9,5′(4′aH)-pyrrolo[1,2-b]pyridazine]-6′,7′-dicarboxylic acid, 6′,7′-dimethyl ester (1) (spiro-DPP) is described in the Supporting Information section. UV−vis spectra of spiro-DPP/betaine mixtures and thermal ring closure kinetics were recorded at 303, 293, and 283 K in acetonitrile (ACS spectrograde) using a Varian Cary 500 UV− vis−NIR spectrophotometer and 4.0 mL quartz cuvettes. SpiroDPP 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 “pyridazine” spiro-DPP (2 × 10−3 M) for 600 s at 200 K in a quartz NMR tube, which was then transferred to the NMR spectrometer. Laser Flash Photolysis. Transient absorption spectra and lifetimes were measured on a home-built instrument pumped by a frequency tripled (355 nm) Spectra-Physics GCR-150 Nd:YAG laser (full width at half-maximum (fwhm) 8 ns, 5 mJ per pulse unless indicated otherwise). The output from a 150 W Xe arc lamp (USHIO) powered by a PTI PS-220 power supply was focused onto the sample at 90° with respect to the laser beam. The white light transmitted by the sample was collimated and focused onto the entrance slit of a Spex HR-20 single monochromator (1200 gr/mm) and was detected utilizing a Hamamatsu R928 photomultiplier tube and

Figure 2. Possible bond rotations for the cis-betaine rotamer-1 isomer (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

At the B3LYP/6-31G* level of theory in acetonitrile, spiroDPP is the lowest-energy structure, followed by the transbetaine with an energy difference of 3.06 kcal mol−1 (Figure 3). This energy difference is predicted to be 3.11 kcal mol−1 at the B3LYP/6-311G** level of theory. We found two distinct classes of cis-isomers (exemplified by cis-betaine rotamer-1 and cis-betaine rotamer-2). Their importance will be discussed below. The lowest-energy cis-betaine rotamer-2 is 2.13 kcal B


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Figure 3. Isomers and relative energies (B3LYP/6-31G* in acetonitrile) for the spiro-DPP/betaine system (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

mol−1 higher in energy (2.03 kcal mol−1 at the B3LYP/6311G** level of theory) than the lowest trans-betaine isomer. The highest energy gap was obtained between spiro-DPP and cis-betaine rotamer-1 structures, which is 6.93 kcal mol−1 with the 6-31G* basis set. For the lowest-energy cis-betaine rotamer2, which has the pyridazine ring rotated compared to rotamer-1, this gap is 5.19 kcal mol−1. Single-point energies calculated with the 6-311G** basis set at the optimized 6-31G* geometries yielded 6.71 and 5.14 kcal mol−1 energy gaps, respectively. In contrast to the spiro-DHI/betaine system in which the cis isomer was lower in energy than the trans isomer,11 we observed in the current system that the trans-betaine isomer is more stable than the cis isomer; thus, the relative stability pattern for the pyridazine spiro-DPP/betaine system is spiro > trans > cis (Figure 3). The other low-energy rotamers calculated in this work and their relative energies are provided in the Supporting Information. Frequency calculations for all structures in Figure 3 were performed, and these structures represent minima on the potential-energy surface. In agreement with our findings in the spiro-DHI/betaine system, the calculated geometries for the zwitterionic DPP/ betaine systems do not follow the classic betaine single− double−single bond character between C1−C2−C3−N4 atoms (atom numbers are labeled in Figure 4). C1−C2 bond lengths in betaines are closer to double-bond character, and C2−C3 bond lengths are more likely a hybrid of single and double bonds (Table 1). C3−N4 and N4−C5 are in the range of a typical CN single bond. Calculated Absorption Spectra. TDDFT single-point energy calculations with the B3LYP/6-31G* level of theory with continuum acetonitrile solvent were used to calculate the theoretical UV−vis spectra. We noticed two characteristic peaks for the lowest-energy betaines (Figure 5): one sharp peak at ∼412−430 nm and another at ∼532−554 nm. The cis-betaine rotamer-1 also shows two peaks: one broad peak at ∼402−452 nm and one sharp peak at ∼638 nm. In contrast, spiro-DPP exhibits a peak at 406 nm (Figure 5). UV−vis spectra of higherenergy rotamers are given in the Supporting Information. It is important to note that the longer wavelength (lower energy)

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

Table 1. Critical Bond Lengths (Å) of spiro-DPP and Betaines bonds

spiro-DPP

cis rotamer-1

cis rotamer-2

trans

C1−C2 C2−C3 C3−N4 N4−C5

1.54 1.36 1.36 1.48

1.38 1.44 1.40 1.37

1.37 1.48 1.39 1.36

1.37 1.47 1.39 1.36

peaks of the betaines for each isomer have a greater variability in their positions than the peaks near 400 nm. For example, the higher-energy peak (∼412 nm) of cis-betaine rotamer-2 varies at ∼16 nm depending on the isomer, and the lower-energy peak (∼554 nm) differs by up to 78 nm. The cis-betaine rotamer-1 also shows similar variations, where the higherenergy peak has a range of ∼11 nm, and the lower-energy peak varies up to 42 nm. With trans-betaines there is a difference of 35 nm in the higher-energy peak and a 65 nm variation in the lower-energy peak. This suggests that a broad low-energy absorption peak, such as the one observed experimentally, may arise in part due to the existence of several rotamers. C


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Figure 5. TDDFT UV−vis absorption spectra calculated at the B3LYP/6-31G* level of theory (acetonitrile phase) for spiro-DPP and betaines. The intensity given on the y-axis is proportional to the oscillator strength.

Figure 6. Illustration of photochemical reaction scheme showing the generation of cis- and trans-betaine isomers after photoexcitation of spiro-DPP (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen). The scheme is not drawn in proportion to the energies but instead represents the relevant structures on the potential-energy surface that are discussed in the text. The potential-energy surface drawn between the cisbetaine rotamer-2 and the trans-betaine demonstrates the complicated potential-energy landscape consisting of networks of intermediate states and transition states.

previously observed for the spiro-DHI system.11 Therefore, our theoretical understanding of this reaction is based on the paradigm that upon photoexcitation, the transient species follows a path to generate the cis-betaine rotamer-1. This

Potential Energy Surfaces. TDDFT optimization calculations of the S1 state with the B3LYP/6-31G* level of theory on the spiro-DPP system (Figure 6) suggest that the electrocyclic ring opening occurs in a similar manner to that D


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Figure 7. Transition states for the pyridazine ring rotation of the cis-betaine isomers. Relative energies (B3LYP/6-31G* in acetonitrile) are given with respect to the lowest-energy spiro-DPP and imaginary frequencies are shown below the transition states. (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

Figure 8. Reactants and products connected to transition state C. Transition state C is offset by three times the normal mode eigenvectors. Relative energies (B3LYP/6-31G* in acetonitrile) are given with respect to the lowest-energy spiro-DPP, and imaginary frequencies are provided below the transition states. (color code: black = carbon, red = oxygen, blue = nitrogen, gray = hydrogen).

mol−1, and the higher-energy transition state B has a barrier of 13.12 kcal mol−1. Because of the flexibility of the functional groups, the potential-energy surface is quite complicated, both in the cis region of the surface and in the trans region of the surface. As discussed above, many different rotamers may be present, and the coordinates and energies of these structures are given in the Supporting Information. In addition, many other transition states can be located between different rotamers. For example, transition state C (Figure 8) is one of the many transition states

isomer will then proceed through one of two transition states (transition state A or B, Figure 7) to form the lowest-energy cisbetaine rotamer-2 with the pyridazine ring rotated. The two low-energy transition states between the two cis isomers (rotamer-1 and rotamer-2) that are generated from clockwise and counterclockwise rotations of the pyridazine ring are shown in Figure 7. The Hessian calculations of these transition states show only one imaginary frequency. The lowest-energy transition state A has a barrier of 11.22 kcal E


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unchanged, but the signal at ∼650 nm decays monoexponentially with τ = 291 ns with the concomitant growth of a transient feature with maximum at ∼580 nm. The shape of the spectrum collected at 200 ns does not change significantly to 300 μs, but it decays with τ = 254 μs. According to our TDDFT single-point energy calculations (Figure 5), maxima at 460 and 650 nm can be attributed to the cis-betaine rotamer-1; this is the only species with a peak between 600 and 700 nm. This isomer clearly dominates the transient absorption spectrum of spiro-DPP at early times and decays within 291 ns to generate the cis-betaine rotamer-2, with a broad absorption in the region of 580 nm, which is a hallmark of the rotated pyridazine ring found in this isomer and is also present in the spectrum collected at 300 μs. Finally, there is also an absorption at 540 nm assigned to trans-betaine, which is also present in both spectra. In the case of spiro-DHI, however, only a single cis isomer exists, and the spectral features in Figure 9a are assigned to this isomer. The ring-opening process is quick and faster than our instrument resolution (99.5% pure) to trans-betaine is achieved by irradiating at 253 K for 600 s. As for the “pyridine system”,11 the trans-betaine of the “pyridazine system” is virtually stable at 253 K in CDCl3. These findings are in agreement with the results of the thermal ring-closure experiments from trans-betaine to spiro-DPP, as shown in Figure 10. (2) In contrast to the spiro-DHI system,11 the symmetry of the fluorenyl-section of spiro-DPP appears to be intact. This is a first indication that the electron distributions in spiro-DHI and spiro-DPP are quite different. This finding is also an important result of the calculations reported here and in ref 11. (3) The 13C NMR signals of the double-bond region, the ester region (with the exception of both methyl groups), and the dihydropyridazine region of spiro-DPP are in very good agreement with 13NMR predictions. However, the 13C NMR signals of the fluorenyl group and the spiro-region are not in good agreement with predictions. This finding demonstrates that the “real” electron densities in spiro-DPP cannot be described/predicted by “classic” line-angle formulas. (4) There are two resonance structures of trans-betaine possible (see Supporting Information). The 13C NMR experiment indicates that there is good agreement between 13C NMR predictions and experimental findings for the pyridazinium and the fluorenyl regions. However, there is virtually no agreement between 13C NMR prediction and observed chemical shifts for every atom between the fluorenyl (anion) and the pyridazinium section of trans-betaine. As already demonstrated for the photochromic spiro-DHI/betaine system,11 quantum spin dynamic simulations based on classic molecule descriptions are not capable of correctly predicting the observed 13C NMR shifts in spiro-DPP/betaine. For example, the trans-betaine can not be represented by a single Lewis structure; two resonance structures are important for the betaine structures (Figure 1b). DFT can inherently treat this, but the 13C NMR calculations assume a single resonance structure is sufficient. These findings clearly demonstrate that there is a critical need for electronic structure calculations in organic chemistry.

Figure 11. Cyclization kinetics of the spiro-4a,5-dihydropyrrolo[1,2b]pyridazine/betaine (1.5 × 10−5 mol L−1 in acetonitrile) recorded at 283, 293, and 303 K at λ = 518 nm after separate irradiation for 5 min with an 150 W Xe arc lamp (USHIO) at the corresponding temperature. Note that, after 30 min at 293 K and 15 min at 303 K, no residual vis absorption was found in the vis spectra, which are shown in the Supporting Information.

potential-energy landscape between cis and trans isomers proves (again) true. The experimental behavior of spiro-DHI/ betaine3 and spiro-DPP/betaine (Figure 11) is indeed very similar at lower temperatures. 13 C Nuclear Magnetic Resonance Characterization of spiro-DPP and trans-Betaine. As discussed in ref 11, 13C NMR spectroscopy is a standard physical-organic chemistry method to probe electron densities in diamagnetic molecules.24 Here, we are following the previously established procedure of 13 C NMR structure elucidation for spiro-DPP and trans-betaine. An overlay of the resulting 13C NMR spectra is shown in Figure 12. A comparison of experimental and calculated data is shown in the Supporting Information. NMR predictions were obtained

Figure 12. 13C NMR (δ, ppm) spectra of spiro-DPP at 298 K and the betaine corresponding to spiro-DPP at 253 K in CDCl3. The betaine was formed by means of irradiating “pyridazine” spiro-DPP (2 × 10−3 M) for 600 s at 200 K in a quartz NMR tube. The solvent peak is omitted. G


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CONCLUSIONS We have employed DFT calculations to interpret the results obtained in steady-state and transient UV−vis absorption spectroscopy experiments studying the photochromic spiro4a,5-dihydropyrrolo[1,2-b]pyridazine/betaine (DPP/betaine) system. On the basis of this approach, initial photoexcitation of spiro-DPP leads to cis-betaine rotamer-1, which can undergo electrocyclic back reaction to regenerate the starting material. Alternatively, it may proceed through a transition state with a barrier height of ∼11 kcal mol−1 to reach a lower-energy (pyridazine ring rotated) cis-betaine rotamer-2, a process observed to take place with a time constant of 291 ns. The latter then decays to form the trans-betaine isomer with τ = 254 μs. Unlike the DHI/betaine system, the trans-betaine is lower in energy than the cis-betaine for the DPP/betaine system. Experimental evidence in the spiro-DPP/betaine system suggests that similar to the spiro-DHI/betaine system, a fraction of the trans-betaines also becomes entrapped in the potentialenergy landscape at lower cyclization temperatures. 13C NMR spectroscopy indicated that spiro-DPP is quantitatively converted to trans-betaine at 253 K. This photochromic transition is thermally reversible above 273 K. The comparison of experimental and calculated 13C NMR signals demonstrated that both spiro-DPP and the corresponding trans-betaine are nonclassical systems, which can be far better described by using DFT calculations. Overall, the spiro-DPP/betaine system is even more complicated than the spiro-DHI/betaine system. Since spiroDPP/betaine is photochemically more stable than spiro-DHI/ betaine, it is the more attractive system for applications as a photochemical switch in bionano- and logical devices. However, in silico studies of substituted spiro-DPP/betaine are mandatory to exactly predict their reactivity so that their potential as optical switches can be achieved.

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and from the Terry C. Johnson Cancer Center at Kansas State Univ. The computing for this project was performed on the Beocat Research Cluster at Kansas State Univ, which is funded in part by NSF Grant Nos. CNS-1006860, EPS-1006860, and EPS-0919443.

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(1) Hauck, G.; Dürr, H. 1,8a-Dihydroindolizines as Components of Novel Photochromic Systems. Angew. Chem., Int. Ed. Engl. 1979, 18, 945−946. (2) Ahmed, S. A.; Hozien, Z. A.; Abdel-Wahab, A.-M. A.; Al-Raqa, S. Y.; Al-Simaree, A. A.; Moussa, Z.; Al-Amri, S. N.; Messali, M.; Soliman, A. S.; Dürr, H. Photochromism of Dihydroindolizines. Part 16: Tuning of the Photophysical Behavior of Photochromic Dihydroindolizines in Solution and in Polymeric Thin Film. Tetrahedron 2011, 67, 7173− 7184. (3) 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. (4) Deniel, M. H.; Tixier, J.; Lavabre, D.; Micheau, J. C.; Dürr, H. Kinetic Modelling of the Photochromism of Dihydroindolizines. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1997, 298, 129−135. (5) 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. (6) Dürr, H.; Schommer, C.; Münzmay, T. Dihydropyrazolopyridine and Bis(dihydroindolizine)Novel Mono- and Bifunctional Photochromic Systems. Angew. Chem., Int. Ed. Engl. 1986, 25, 572−574. (7) García, N. A.; Rossbroich, G.; Braslavsky, S. E.; Dürr, H. The Measurement of Energy by Short-Lived Species with Conventional Photoacoustic Spectroscopy. NATO ASI Series 1985, 85, 163−165. (8) García, N. A.; Rossbroich, G.; Braslavsky, S. E.; Dürr, H.; Dorweiler, C. Photoacoustic Measurements and mindo/3 Calculations of Energy Storage by Short-Lived Species: the Spiro[1,8-a]dihydroindolizine-Betaine System. J. Photochem. 1985, 31, 297−305. (9) Gauglitz, G.; Scheerer, E. Method for the Determination of Absorption Coefficients, Reaction rate Constants and Thermodynamic Data in the System. J. Photochem. Photobiol., A 1993, 71, 205−212. (10) Gross, H.; Dü rr, H.; Rettig, W. Emission Spectra of Photochromic Spiro[1,8-a]dihydroindolizines and Mechanism of the Electrocyclic Ring Opening Reaction. J. Photochem. 1984, 26, 165− 178. (11) Fernando, A.; Malalasekera, A. P.; Shrestha, T. B.; McLaurin, E.; Bossmann, S.; Aikens, C. M.; Yu, J. Refined Insights in the Photochromic Spiro-Dihydroindolizine/Betaine System. J. Phys. Chem. A 2015, 119, 9621−9629. (12) 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. (13) 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. (14) 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. (15) Weitzel, T.; Wild, U.; Amlung, M.; Dürrb, H.; Irie, M. New Photochromic Materials for Holographic Recording. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2000, 344, 191−198. (16) Dürr, H. Perspectives in Photochromism: A Novel System Based on the 1,5-Electrocyclization of Heteroanalogous Pentadienyl Anions. Angew. Chem., Int. Ed. Engl. 1989, 28, 413−431.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b10020. Syntheses of studied compounds. Structures, relative energies, and absorption spectra of other spiro-DPP and cis- and trans-betaine rotamers in acetonitrile solvent. Transition states between cis- and trans-betaines. Carbon atom numbering for spiro-DPP and resonance structures for trans-betaine. Comparison of experimental and predicted 13C NMR peaks for spiro-DPP and transbetaine. XYZ coordinates of the structures calculated in this work. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: 785-532-6668. Fax: 785-532-6666. E-mail: [email protected]. (S.H.B.) *Phone: 785-532-6668. Fax: 785-532-6666. E-mail: cmaikens@ ksu.edu. (C.M.A.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Science Foundation (DMR No. 1242765, CBET No. 1337438) H


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Insights from Theory and Experiment on the Photochromic spiro

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