Reversible Photochemical Isomerization of N,N′-Di(t-butoxycarbonyl

Article pubs.acs.org/JPCA

Reversible Photochemical Isomerization of N,N′‑Di(t‑butoxycarbonyl)indigos Dominik Farka, Markus Scharber, Eric Daniel Głowacki,* and Niyazi Serdar Sariciftci Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Altenbergerstrasse 69, A-4040 Linz, Austria ABSTRACT: We report on the photophysics of N,N′-di(t-butoxycarbonyl)indigos (tBOC indigos), finding that reversible photochemical trans−cis and cis−trans isomerization reactions proceed with high quantum yields (0.10−0.46). Absorption of wavelengths in the 500−600 nm region induces trans−cis isomerism, while blue light leads to the reverse cis−trans process. Like their parent indigos, trans-BOC indigos have low fluorescence yields (∼1 × 10−3), while the cis isomers have no measurable emission. These compounds are the first examples of photoisomerizable indigoid dyes in which photochemical isomerism effectively outcompetes radiative decay processes. Though indigo dyes typically have poor solubility in organic solvents, tBOC indigos can be dissolved at concentrations up to 8 w % in common organic solvents like acetone. Furthermore, unlike other photoisomerizable indigoids, tBOC indigos are not sensitive to quenching by proton and electron donors. These features, combined with high quantum yields of reversible photoisomerism induced by relatively low-energy photons (∼2 eV), make tBOC indigo derivatives potentially interesting for photochromic applications, such as photomechanically actuated materials.

■

around the central double bond, has fluorescence yields of around 0.9.13,3 Trans−cis photoisomerizable compounds convert light energy into mechanical work and thus have found application in photoalignment coatings, especially for liquid-crystal optoelectronics, optical data storage,14 photoactuating polymeric materials, photoswitchable optical elements, and permeation membranes.15 In these applications, azobenzene and its derivatives dominate, with spiropyrans and spirooxazines,16 fulgides,17 and various diarylethenes18 being other notable photochromic material classes. Indigoids, especially thioindigos, had been explored relatively early in some of these applications and were thought attractive because of lowerenergy green or red light providing photoswitching, as opposed to the UV light normally necessary for azobenzenes and other photochromic moieties. Thioindigo, selenoindigo, and several N,N′-substituted indigos with dimethyl or diacyl substituents all undergo reversible photochemical trans−cis and cis−trans photoisomerization. A major disadvantage of the photoisomerizable indigos, however, was low photochemical isomerization yields, combined with the dominance of other undesired photophysical processes, e.g. thioindigo was reported to have a ϕf of 0.47.11 N,N′-Dimethylindigo19 affords a ϕtrans−cis of ∼0.01 with a very rapid thermal reverse isomerization. N,N′Diacetylindigo has been reported to have a ϕtrans−cis of ∼0.1 as well, with higher yields of fluorescence.11 The low solubility of indigos in organic solvents and polymers also hampered applications. Photochromic thioindigo was explored as a potential dye for optical information recording,12 and N,N′-

INTRODUCTION Indigoids are molecules with a rich cultural and scientific history. The use of indigo and derivatives, such as 6,6′dibromoindigo (Tyrian purple), has been a part of civilization from ancient Egypt and the Mediterranean Greco-Roman world, through southeast Asia, and the Americas for thousands of years. In 19th century Europe, the commercial significance of indigo became one of the driving forces behind the development of modern organic synthetic chemistry and chemical industry.1,2 Indigos, in their hydrogen-bonded form (Figure 1a) are remarkable for their photostability and low fluorescence yield (ϕf ∼ 1 × 10−3). This property has been attributed to rapid excited-state proton transfer, leading to a keto−enol tautomerization process that efficiently dissipates the excitedstate energy.3−5 Photochromic effects were reported in various indigoids already in the 1930s and 1940s,6 with the generation of a cis isomer proposed as the possible origin of this effect. Wyman and Zenhausern7 showed in 1965 that the optical spectra of N,N′-oxalylindigo, an indigoid that is covalently locked in the cis configuration, closely match the photoinduced spectral changes observed in the case of photochromic indigoids. This led to the conclusion that indeed two photoisomers exist. Subsequent experimental and theoretical work on indigoids proved this to be the case. Substituting the NH protons with methyl,8 acyl,9,10 aryl, chalcogenides like −S− leads to indigoids (Figure 1b) where photochemical isomerization occurs, competing with fluorescence and nonradiative decay. Trans−cis quantum yields range from around 0.1 in the case of N,N′-diacetyl indigo11 and thioindigo to a maximum of around 0.3 in the case of some thioindigo derivatives.12 By way of comparison, the fused-ring cibalackrot indigo (Figure 1c), which lacks NH protons and also does not allow rotation © 2015 American Chemical Society

Received: December 11, 2014 Revised: March 11, 2015 Published: March 26, 2015 3563

DOI: 10.1021/jp512346z J. Phys. Chem. A 2015, 119, 3563−3568

Article

The Journal of Physical Chemistry A

acetate. Spectroscopic-grade anhydrous acetonitrile, acetone, toluene, chlorobenzene, and bromobenzene were used for all optical experiments. UV−vis spectra were obtained using a PerkinElmer Lambda 1050. Photoluminescence spectra were recorded using a PTI fluorometer equipped with a photomultiplier tube. Fluorescence quantum yield was calculated using Rhodamine 6G in ethanol as a standard, according to the IUPAC standardized method.25 Photochemical switching was achieved using either a 532 nm diode laser or light from a xenon lamp coupled through a grating monochromator, as indicated. To correctly calculate both the trans−cis and cis− trans photochemical isomerization yields, the extinction coefficients for each species must be known. The extinction coefficients of the pure trans and cis isomers were calculated analytically using the method introduced by Blanc and Ross.11 This method can be applied provided that at least one of the two interconvertible isomers is luminescent at a wavelength where the emission is not reabsorbed and at which there is no luminescence from the other isomer. The method also assumes that the Beer−Lambert law holds. Both conditions are satisfied in the case of molecules 1−3 evaluated here, and as discussed in the results and discussion section, only the trans isomers are luminescent, emitting at wavelengths >600 nm, where no reabsorption occurs. The method for calculating the extinction coefficient for the cis isomer is described as follows (assuming optical path length 1 cm): While the fluorescence intensity is measured, with a constant excitation intensity, the concentration [trans]1 changes to [trans]2, and the optical density of the sample changes from D1 to D2. We define the ratio, R, of the trans isomer concentrations as

Figure 1. (a) Photophysics of indigo. The dominant process upon excitation is radiationless internal conversion, believed to occur via excited-state proton transfer followed by a keto−enol−keto tautomerization process. Indigoids without NH protons (b), such as N,N′dimethylindigo and thioindigo, have competing luminescence and trans−cis photoisomerization. Removing protons and blocking possible isomerism results in the highly luminescent cibalackrot compound (c). (d) The three indigos used in this work (1−3) contain bulky tBOC groups and undergo reversible photoisomerism.

⎛ I ⎞⎛ D ⎞⎛ 1 − 10−D2 ⎞ [trans]1 ≡ R ⎜ F1 ⎟⎜ 1 ⎟⎜ ⎟ [trans]2 ⎝ IF2 ⎠⎝ D2 ⎠⎝ 1 − 10−D1 ⎠

(1)

The optical density at a given wavelength, Dλ, will constitute the sum of the absorption contributions of the trans and cis isomers, according to eq 2, where ε is the molar extinction coefficient: λ D λ = (εtrans − εcisλ )[trans] + εcisλ [total]

disubstituted indigos were proposed for storage of solar energy in the form of the photochemically generated higher-energy cis isomer.20 However, because of the low ϕtrans−cis and limited solubility, these indigoids were overlooked in favor of some of the material classes mentioned above. Recent work published by de Melo and co-workers21 showed that indigo in its reduced form, leuco-indigo, can have a ϕtrans−cis of 0.9, with the photochemically generated cis isomer being fluorescent. These results, combined with recent demonstrations of indigoids as promising semiconducting materials,22 motivate their further exploration as photoswitching chromophoric systems. The tBOC indigos reported here, 1−3, combine excellent solubility, relatively high trans−cis and cis−trans yields, negligible fluorescence, and resilience to quenching and might thereby revitalize exploration of the indigo family of molecules for photoswitching applications.

(2)

The value of R, the knowledge of the total concentration, and the absorption spectra in states 1 and 2 are sufficient to construct the absorption spectrum of the cis isomer by combining eqs 1 and 2 to give eq 3. εcisλ =

R(D2λ) − (D1λ) [total](R − 1)

(3)

To establish the quantum yield of photoisomerization, the setup shown in Figure 2 was used. A monochromator is used to tune the wavelength of light from a xenon lamp. The light was focused onto the sample: a 0.1 mM solution in a 1 cm quartz cuvette. This served as the pump to induce the desired photoisomerization process. Changes in transmission of the sample over time were measured using chopped light from a tunable supercontinuum white light laser source (NKT SuperK Extreme) detected using a Si photodiode (Hamamatsu), which was synchronized to the chopper frequency using a lock-in amplifier. Intensity of the probe is maintained much less than that of the pump to not contribute to the photochemical process. Another calibrated photodiode was placed in such a way that it could be slid directly into the pump beamline in the position where the sample was placed to measure the photon

■

MATERIALS AND METHODS Indigo was obtained from BASF and purified by temperature gradient sublimation. Both 5,5′- and 6,6′-dibromoindigo were obtained from the corresponding 2-nitrobenzaldehydes according to the Baeyer−Drewsen synthesis.23 All were converted to their N,N′-diBOC forms 1−3 according to published procedures24 and purified by recrystallization from ethyl 3564

DOI: 10.1021/jp512346z J. Phys. Chem. A 2015, 119, 3563−3568

Article

The Journal of Physical Chemistry A

We observed photochromism in 1−3. The absorption spectrum of a 0.2 mM solution of 1 in acetonitrile is shown in Figure 3 before and after irradiation with a 532 nm laser (2

Figure 2. Setup for measuring photoisomerization quantum yield. Changes in transmission over time of the sample during excitation by the pump are measured using chopped laser light and a Si-photodiode with lock-in detection.

Figure 3. Reversible photoisomerization of a 0.2 mM solution of 1 in acetonitrile. Black arrows show the thermal recovery of the original absorption.

mW), followed by the thermal relaxation in the dark of the cisrich solution back to trans over 120 min. Isosbestic points are observable at 398 and 498 nm. This process is reversible for at least 5 cycles under these conditions. As mentioned above, indigoids lacking intramolecular H-bonding are photochromic compounds, undergoing reversible trans−cis isomerization. Single-crystal X-ray diffraction revealed that N,N′-diBOC indigos are highly strained with respect to the central ethylenic carbons connecting the two indole rings, which helps to explain why the barrier for isomerization is easily overcome by energy from photons ∼2 eV. The absorption spectra for 1−3 are shown as solid red traces in Figure 4. Irradiation with the 532 nm laser leads within 120−150 s to a photostationary state shown in solid blue lines. Photoluminescence spectra were recorded before and after photoirradiation. A weak emission without visible vibrational structure was recorded in all cases. Using Rhodamine 6G in ethanol according to IUPAC standards,25 a ϕf of ∼1 × 10−3 was calculated for all three compounds. The excitation spectra, shown as dotted colored lines in Figure 4, were found to follow the absorption spectrum of the trans-rich solutions, suggesting that luminescence originates from only the trans isomer. No detectable luminescence attributable to the cis-isomer was found, including when irradiating cis-rich solutions in the 420−450 nm region, where cis absorption is at its maximum. A method for calculating the absorption spectra of two isomers reversibly interchangeable by a photochemical isomerization where only one isomer is luminescent was introduced by Ross and Blanc.11 We followed this methodology to calculate the extinction coefficients of the pure trans and cis species (Figure 5). The cis absorption profile is similar to that of oxalylindigo, an indigoid that is known to have cis configuration.7 The absorption maxima of both trans and cis isomers follow the principles of H-chromophore absorption first outlined by Lü ttke and co-workers, i.e., that the fundamental indigo chromophore is the cross-conjugated system of electron-rich NH groups and electron-poor carbonyl groups, and the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) transition features a charge-transfer from NH to CO.29,30 Thus, πelectron donors at 5,5′ and 6,6′ positions yield bathochromic

flux. Photon flux was calculated on the basis of the manufacturer responsivity calibration data for the photodiode, corrected for reflection losses in the setup, and kept around 1 × 10−5 einsteins s−1. The well-known thioindigo was used as a “standard” to cross-check our methodology. With the knowledge of extinction coefficients for each isomer, photoisomerization quantum yields can be calculated according to the method of initial rates.26 The photochemical isomerization is a first-order process, and the quantum yield ϕi→f is thus given by eq 4, where Iabs is the moles of photons absorbed by the sample per second and V is the total volume of the solution.

■

⎛ ⎞ ⎜ ∂c ⎟ ϕi → f = −⎜ ∂t i ⎟ × V ⎝ Iabs ⎠t = 0

(4)

RESULTS AND DISCUSSION In recent publications, the synthesis of N,N′-diBOC indigos was described.24,27 The point of that work was to use the tBOC function as a protecting group that would render indigo derivatives transiently soluble in organic solvents and thus amenable to further chemical manipulation. Indigos are normally remarkably planar molecules because of O···HN− hydrogen-bonding. Introduction of tBOC groups eliminates these H-bonding interactions and introduces substantial steric bulk as well. For various applications of photoisomerizable dyes, solubility is important. It is the low solubility of photoswitching indigoids, in fact, that was the major factor in them being overshadowed by other molecular systems.20,28 Compound 1 was found to produce stable solutions up to 75− 85 mg/mL in acetone, dichloromethane, and chloroform, with slightly lower solubility (∼65−70 mg/mL) in aromatic solvents like toluene and chlorobenzene. Brominated 2 and 3 have lower solubility, 25−35 mg/mL in the same solvents, with 3 being the least soluble of the series. In contrast, thioindigo can be dissolved with maximum concentration of ∼1 mg/mL in organic solvents. 3565

DOI: 10.1021/jp512346z J. Phys. Chem. A 2015, 119, 3563−3568

Article

The Journal of Physical Chemistry A

Figure 5. Molar extinction coefficients of the trans and cis isomers of 1−3 calculated according to the analytical method reported in ref 11.

Figure 4. Original trans-rich solutions (solid red traces) for 1−3 in acetonitrile solution. Photostationary states obtained with 532 nm laser irradiation are shown in solid blue traces. Solid black lines show the photoluminescence of the original trans-rich solutions, while the dotted black traces show the photoluminescence of the photostationary cis-rich solutions. The dotted red and blue lines show the excitation spectra for luminescence at 620 nm of the trans-rich and cisrich solutions, respectively.

measured using a Si photodiode with phase-sensitive detection. The calculated photoisomerization yields, shown in Table 1, are found to be significantly higher in the case of 1 than for brominated 2 and 3. Photoisomerization in indigos has been proposed to proceed via a singlet intermediate excited state. The finding that both brominated derivatives 2 and 3 have lower photoisomerism yields is consistent with the singlet model: Bromine atoms are expected to increase the population of triplet excited states because of spin−orbit coupling, and bromination in this case is found experimentally to decrease and not increase the photoisomerism yield. Increasing triplet population decreases the singlet excited state that can lead to isomerization. It is important to note here that the comparison of 5,5′ and 6,6′ dibrominated derivatives helps to exclude other electronic effects on the photoisomerization process, as substitution at 5,5′ versus 6,6′ is known to have opposite influences on the Hchromophore of indigos.31,32 According to the “heavy-atom solvent” method introduced by Kasha and co-workers,33 employing solvents with heavier atoms are expected to increase triplet population. We measured yields of compounds 1−3 in the solvent series toluene, chlorobenzene, and bromobenzene. The values of ϕtrans−cis decreased in the presence of heavieratom solvents (Table 2), supporting the hypothesis that photoisomerism proceeds via a singlet excited state. Because the combined quantum yields of both luminescence and photoisomerism do not exceed 0.1−0.46, a natural question is how the rest of the energy is dissipated. Based on the differences in photoisomerization yields of 1 and brominated 2

and hypsochromic shifts, respectively.31,32 For example, the Br atoms at the 5,5′ positions of 2 inductively push electron density onto the NH groups, giving a bathochromic shift. Compounds 1−3 follow this H-chromophore principle in both trans and cis isomeric forms. The inductive effects on the photoisomerism yields, however, remain unclear and would have to be resolved with the help of time-resolved optical measurements on the intermediate transition states. The knowledge of the extinction coefficients of both isomers is necessary both to calculate the photoisomerism yield and to choose photoswitching wavelengths that selectively excite one or the other isomer as desired. The rate of trans−cis photoisomerization is linear with respect to excitation intensity, indicating a first-order photochemical reaction. Thus, based on the method of initial rates outlined by Lippert and Luder,26 the quantum yields of trans−cis and cis−trans photochemical isomerization could be calculated. For trans−cis reaction of 1, for example, we used monochromatic light from a xenon lamp in series with a grating monochromator at a frequency of 562 nm; for cis−trans, 433 nm was used. Molar absorptivity based on calculated extinction coefficients (Figure 5) was assumed. The setup (Figure 2) for these measurements featured a cw pump illumination and a chopped probe, with transmission 3566

DOI: 10.1021/jp512346z J. Phys. Chem. A 2015, 119, 3563−3568

Article

The Journal of Physical Chemistry A

Table 1. Extinction Coefficients and Photophysical Data for 1−3 in Acetonitrile Solution, with Measurement of Thioindigo Shown for Comparison compound

trans λmax (nm), ε (mol−1 cm−1 L)

1 2 3 thioindigo

546, 557, 543, 542,

cis λmax (nm), ε (mol−1 cm−1 L)

7 626 8 604 6 840 17 900

438, 450, 426, 483,

5 885 7 577 4 246 13 800

Table 2. Φtrans−cis for 1−3 in Solvents with Potential Increasing Heavy Atom Effect 1

2

3

toluene chlorobenzene bromobenzene

0.13 0.09 0.08

0.09 0.04 0.04

0.05 0.04 0.04

ϕf

ϕtrans−cis

ϕcis−trans

606 621 600 592

0.001 0.001 0.002 0.55

0.13 0.01 0.06 0.10

0.46 0.02 0.20 0.38

photobleaching experiment was found to slow bleaching in the case of 1 but had no effect on 2 or 3. The survival of these samples under relatively intense illumination for such periods of time is encouraging for many applications. Under normal room lighting, samples were found to be stable for at least 6 months. For photoswitching applications, understanding of the isomerization process in solid films is likewise of interest. The photochemical behavior of compounds 1−3 was measured in spin-cast 100 nm-thick polymer films of poly(methyl methacrylate) and poly(styrene). When these dyes are dissolved in the polymer matrix at a ratio of 1:5 dye:polymer by weight, their absorption and emission spectra resembled closely those obtained from dyes in solution. Photochromism was not observed for the brominated derivatives. In the case of 1, however, photoswitching between the trans−cis and cis−trans was observed, albeit with a much lower quantum yield ranging around 1 × 10−8. Conversely, as can be expected from the suppression of isomerism, photoluminescence yield increases to ∼0.15. Photoluminescence yields of these samples were estimated based on the method reported by Langhals et al., using N,N′-bis(1-hexylheptyl)-3,4,9,10-perylene bis(dicarboximide) as a standard.42 These results were the same in both polymer matrices. On the basis of the very low photoisomerization yields and the moderate photoluminescence efficiency, ∼85% of the excitation energy must be dissipated by vibrational relaxation processes. However, these experiments do not exclude the possibility that in the polymeric matrix the photochemical forward and back conversion are too rapid to observe photochromism.

ϕtrans−cis solvent

fluorescence λmax (nm)

and 3, the contribution from population of the triplet states is not a significant channel. The remaining option is efficient vibrational relaxation. Yang et al. have recently reported34 on the basis of theoretical calculations, that because of the torsional degrees of freedom offered by the tBOC groups on 1, configurational relaxation processes are expected to be a major pathway of nonradiative energy dissipation. In reports on thioindigo and similar indigoids, both proton donors35,36 and electron donors37 were found to quench trans− cis isomerism. Proton quenching is seen as evidence that in indigo compounds photoinduced proton transfer is a prevalent process that results in nonradiative deactivation of the excited state mediated by keto−enol tautomerism. This mechanism has been postulated for the negligible photoluminescence combined with photostability of indigo itself.38 Indeed, 4,4′dihydroxythioindigo was recently found to be a mimetic of indigo in that efficient photoinduced intramolecular proton transfer from hydroxyl groups on the phenyl rings quenches luminescence and isomerization.5 Theoretical studies also support the model of excited-state proton transfer reactions outcompeting other photophysical processes.4,39 In the case of 1−3, however, quenching was observed only with quencher concentrations greater than ∼5000× that of 1−3. Phenol was used as the proton donor as in the case of earlier reports. Thus, the tBOC indigos are remarkably unaffected by protonquenching. It is likely that the steric bulk of the tBOC groups hinders the phenol−indigo interaction that would cause quenching. Stronger acids (e.g., mineral acids) were not used because these would lead to hydrolysis of the tBOC functional groups. Photoreduction has been reported when thioindigo and other indigiods are irradiated in the presence of electron donors.37,40,41 For compounds 1−3, we found that these materials show the same insensitivity to electron donors as they do to proton donors. A ∼104× excess of triethylamine was needed to observe any quenching. Overall, the idea that N,N′substitution of bulky groups can help to preserve photoisomerism in environments with potential quenching species may be useful for applications. Considering the stability of tBOC indigos with respect to quenching, we evaluated also the stability to photobleaching. It was found that solutions of compounds 1 and 2, when placed on a solar simulator with an intensity of 100 mW/cm2, began to show irreversible photobleaching after ≈17−18 h. Compound 3 was less stable, beginning to degrade already after ≈4 h of illumination. Saturating the acetonitrile solutions with Ar or N2 prior to the

■

CONCLUSIONS It has been shown that tBOC indigos have promising photoisomerization properties, where relatively high photoisomerism quantum yields outcompete radiative processes. The tBOC indigos are resilient to quenching by proton and electron donors, which normally quench photoisomerizable indigos. Combined with their high solubility in organic solvents and ability to dissolve and even isomerize in glassy polymer matrices, they are potentially advantageous for photoswitching and photomechanical applications.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful for support by the Austrian Science Foundation, FWF, within the Wittgenstein Prize of N. S. Sariciftci Solare Energie Umwandlung Z222-N19 and the Translational Research Project TRP 294-N19 “Indigo: From ancient dye to modern high-performance organic electronics 3567

DOI: 10.1021/jp512346z J. Phys. Chem. A 2015, 119, 3563−3568

Article

The Journal of Physical Chemistry A

(23) Baeyer, A.; Drewsen, V. Darstellung von Indigoblau Aus Orthonitrobenzaldehyd. Chem. Ber. 1882, 15, 2856−2864. (24) Głowacki, E. D.; Voss, G.; Demirak, K.; Havlicek, M.; Sunger, N.; Okur, A. C.; Monkowius, U.; Gąsiorowski, J.; Leonat, L.; Sariciftci, N. S. A Facile Protection−Deprotection Route for Obtaining Indigo Pigments as Thin Films and Their Applications in Organic Bulk Heterojunctions. Chem. Commun. (Cambridge, U.K.) 2013, 49, 6063− 6065. (25) Brouwer, A. M. Standards for Photoluminescence Quantum Yield Measurements in Solution (IUPAC Technical Report). Pure Appl. Chem. 2011, 83, 2213−2228. (26) Lippert, E.; Luder, W. Photochemical Cis-Trans Isomerization of p-Dimethylaminocinnamic Acid Nitrile. J. Phys. Chem. 1962, 66, 2430−2434. (27) Głowacki, E. D.; Apaydin, D. H.; Bozkurt, Z.; Monkowius, U.; Demirak, K.; Tordin, E.; Himmelsbach, M.; Schwarzinger, C.; Burian, M.; Lechner, R. T.; et al. Air-Stable Organic Semiconductors Based on 6,6′-Dithienylindigo and Polymers Thereof. J. Mater. Chem. C 2014, 2, 8089−8097. (28) Uno, H.; Moriyama, K.; Ishikawa, T.; Ono, N.; Yahiro, H. Thioindigo Precursor: Control of Polymorph of Thioindigo. Tetrahedron Lett. 2004, 45, 9083−9086. (29) Klessinger, M.; Lüttke, W. Theoretische Und Spektroskopische Untersuchungen an Indigofarbstoffen, III. Der Einfluss Zwischenmolekularer Wasserstoff Briicken Auf Die Spektren von Indigo Im Festen Zustand. Chem. Ber. 1966, 99, 2136−2145. (30) Lüttke, W.; Hermann, H.; Klessinger, M. Theoretically and Experimentally Determined Properties of the Fundamental Indigo Chromophore. Angew. Chem., Int. Ed. 1966, 5, 598−599. (31) Sadler, P. W. Absorption Spectra of Indigoid Dyes. J. Org. Chem. 1956, 21, 316−318. (32) Sadler, P. W.; Warren, R. L. Synthesis and Absorption Spectra of the Symmetrical Chloroindigos. J. Am. Chem. Soc. 1956, 78, 1251− 1255. (33) McGlynn, S. P.; Azumi, T.; Kasha, M. External Heavy-Atom SpinOrbital Coupling Effect. V. Absorption Studies of Triplet States. J. Chem. Phys. 1964, 40, 507−515. (34) Yang, C.; Trinh, Q. T.; Wang, X.; Tang, Y.; Wang, K.; Huang, S.; Chen, X.; Mushrif, S. H.; Wang, M. Crystallization-Induced Red Emission of a Facilely Synthesized Biodegradable Indigo Derivative. Chem. Commun. (Cambridge, U.K.) 2015, 51, 3375−3378. (35) Wyman, M.; Carolina, N. The Interaction of Excited Thioindigo with Hydroxylic Compounds and Its Implications on the Photostability of Indigo. Chem. Commun. (Cambridge, U.K.) 1971, 1332− 1334. (36) Wyman, G. M.; Zarnegar, B. M. Excited State Chemistry of Indigoid Dyes. II. The Interaction of Thio- and Selenoindigo Dyes with Hydroxylic Compounds and Its Implications on the Photostability of Indigo. J. Phys. Chem. 1973, 77, 1204−1207. (37) Schanze, K. S.; Lee, L. Y. C.; Giann, C.; Whitten, D. G. Photoreduction of Indigo Dyes by Electron Donors. One- and TwoElectron-Transfer Reactions as a Consequence of Excited-State Quenching. J. Am. Chem. Soc. 1986, 108, 2646−2655. (38) Seixas de Melo, J.; Moura, A. P.; Melo, M. J. Photophysical and Spectroscopic Studies of Indigo Derivatives in Their Keto and Leuco Forms. J. Phys. Chem. A 2004, 108, 6975−6981. (39) Moreno, M.; Ortiz-Sánchez, J. M.; Gelabert, R.; Lluch, J. M. A Theoretical Study of the Photochemistry of Indigo in Its Neutral and Dianionic (leucoindigo) Forms. Phys. Chem. Chem. Phys. 2013, 15, 20236−20246. (40) Setsune, J.; Matsuura, T.; Fujiwara, T.; Kitao, T. One-Electron Photoreduction of N,N′-1,3-Proano-Bridged Indigo. Chem. Lett. 1984, 1755−1758. (41) Pouliquen, J.; Wintgens, V.; Toscanot, V. Evidence for Electron Transfer of Excited Indigo Dyes with Both Electron-Acceptors and Electron-Donors. Dyes Pigm. 1985, 6, 163−175. (42) Langhals, H.; Karolin, J.; B-A, L. Spectroscopic Properties of New and Convenient Standards for Measuring Fluorescence Quantum Yields. J. Chem. Soc., Faraday Trans. 1998, 8, 2919−2922.

circuits”. We thank Elisa Tordin and Gundula Voss for synthesis support.

■

REFERENCES

(1) Seefelder, M. Indigo in Culture, Science, and Technology, 2nd ed.; Ecomed: Landsberg, Germany, 1994. (2) Zollinger, H. Color Chemistry. Syntheses, Properties and Applications of Organic Dyes and Pigments, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2003. (3) Seixas de Melo, J.; Rondão, R.; Burrows, H. D.; Melo, M. J.; Navaratnam, S.; Edge, R.; Voss, G. Photophysics of an Indigo Derivative (Keto and Leuco Structures) with Singular Properties. J. Phys. Chem. A 2006, 110, 13653−13661. (4) Yamazaki, S.; Sobolewski, L.; Domcke, W. Molecular Mechanisms of the Photostability of Indigo. Phys. Chem. Chem. Phys. 2011, 13, 1618−1628. (5) Dittmann, M.; Graupner, F. F.; Maerz, B.; Oesterling, S.; de Vivie-Riedle, R.; Zinth, W.; Engelhard, M.; Lüttke, W. Photostability of 4,4′-Dihydroxythioindigo, a Mimetic of Indigo. Angew. Chem., Int. Ed. 2014, 53, 591−594. (6) Wyman, G. M. The Cis-Trans Isomerization of Conjugated Compounds. Chem. Rev. (Washington, DC, U.S.) 1955, 55, 625−657. (7) Wyman, G. M.; Zenhausern, A. F. Spectroscopic Studies on Dyes. V. Derivatives of Cis-Indigo. J. Org. Chem. 1965, 30, 2348−2352. (8) Weinstein, J.; Wyman, G. M. Spectroscopic Studies on Dyes. II. The Structure of N,N′-Dimethylindigo. J. Org. Chem. 1956, 78, 4007− 4010. (9) Ross, D.; Blanc, J.; Matticoli, F. J. Photochromic Indigoids. II. Absorption Spectra and Quantum Yields for Photoisomerization of Selenoindigo. J. Am. Chem. Soc. 1970, 92, 5750−5752. (10) Omote, Y.; Imada, S.; Matuzaki, R.; Fujiki, K.; Nishio, T.; Kashima, C. The Stereoisomerization of N,N′-Diacyl Derivatives of Indigo. Bull. Chem. Soc. Jpn. 1979, 52, 3397−3399. (11) Blanc, J.; Ross, D. L. A Procedure for Determining the Absorption Spectra of Mixed Photochromic Isomers Not Requiring Their Separation. J. Phys. Chem. 1968, 72, 2817−2824. (12) Ross, D. L. Photochromic Indigoids. III: A Photochromic Element Based on the Cis-Trans Photoisomerization of a Thioindigo Dye. Appl. Opt. 1971, 10, 571−576. (13) Haucke, G.; Paetzold, R. Spectroscopy and photochemistry of indigoid compounds. VI. Direct Trans → cis Photoisomerization of Indigoid Dyes. J. Prakt. Chem. (Leipzig) 1979, 321, 978−986. (14) Ikeda, T.; Tsutsumi, O. Optical Switching and Image Storage by Means of Azobenzene Liquid-Crystal Films. Science 1995, 268, 1873− 1875. (15) Głowacki, E.; Horovitz, K.; Tang, C. W.; Marshall, K. L. Photoswitchable Gas Permeation Membranes Based on Liquid Crystals. Adv. Funct. Mater. 2010, 20, 2778−2785. (16) Berkovic, G.; Krongauz, V.; Weiss, V. Spiropyrans and Spirooxazines for Memories and Switches. Chem. Rev. (Washington, DC, U.S.) 2000, 100, 1741−1753. (17) Yokoyama, Y. Fulgides for Memories and Switches. Chem. Rev. (Washington, DC, U.S.) 2000, 100, 1717−1740. (18) Irie, M. Diarylethenes for Memories and Switches. Chem. Rev. (Washington, DC, U.S.) 2000, 100, 1685−1716. (19) Giuliano, C. R.; Hess, L. D.; Margerum, J. D. Cis-Trans Isomerization and Pulsed Laser Studies of Substituted Indigo Dyes. J. Am. Chem. Soc. 1968, 90, 587−594. (20) Scharf, H.-D.; Flesichhauer, J.; Leismann, H.; Ressler, I.; Schleker, W.; Weitz, R. Criteria for the Efficiency, Stability, and Capacity of Abiotic Photo-Chemical Solar Energy Storage Systems. Angew. Chem., Int. Ed. 1979, 18, 652−662. (21) Rondão, R.; Seixas de Melo, J.; Melo, M. J.; Parola, A. J. ExcitedState Isomerization of Leuco Indigo. J. Phys. Chem. A 2012, 116, 2826−2832. (22) Głowacki, E. D.; Voss, G.; Sariciftci, N. S. 25th Anniversary Article: Progress in Chemistry and Applications of Functional Indigos for Organic Electronics. Adv. Mater. (Weinheim, Ger.) 2013, 25, 6783− 6800. 3568

DOI: 10.1021/jp512346z J. Phys. Chem. A 2015, 119, 3563−3568