A Modern Look at Spiropyrans: From Single Molecules to Smart Materials

Photochromic compounds of the spiropyran family have two main isomers capable of inter-switching with UV or visible light. In the current review, we discuss recent advances in the synthesis, investigation of properties, and applications of spiropyran derivatives. Spiropyrans of the indoline series are in focus as the most promising representatives of multi-sensitive spirocyclic compounds, which can be switched by a number of external stimuli, including light, temperature, pH, presence of metal ions, and mechanical stress. Particular attention is paid to the structural features of molecules, their influence on photochromic properties, and the reactions taking place during isomerization, as the understanding of the structure-property relationships will rationalize the synthesis of compounds with predetermined characteristics. The main prospects for applications of spiropyrans in such fields as smart material production, molecular electronics and nanomachinery, sensing of environmental and biological molecules, and photopharmacology are also discussed.

Spiropyran/Merocyanine Amphiphile in Various Solvents: A Joint Experimental–Theoretical Approach to Photophysical Properties and Self-Assembly

This joint experimental-theoretical work focuses on molecular and photophysical properties of the spiropyran-containing amphiphilic molecule in organic and aqueous solutions. Being dissolved in tested organic solvents, the system demonstrates positive photochromism, i.e., upon UV stimulus the colorless spiropyran form is transformed into colorful merocyanine isomer. However, the aqueous solution of the amphiphile possesses a negative photochromism: the orange-red merocyanine form becomes thermodynamically more stable in water, and both UV and vis stimuli lead to the partial or complete photobleaching of the solution. The explanation of this phenomenon is given on the basis of density functional theory calculations and classical modeling including thermodynamic integration. The simulations reveal that stabilization of merocyanine in water proceeds with the energy of ca. 70 kJ mol−1, and that the Helmholtz free energy of hydration of merocyanine form is 100 kJ mol−1 lower as compared to the behavior of SP isomer in water. The explanation of such a difference lies in the molecular properties of the merocyanine: after ring-opening reaction this molecule transforms into a zwitterionic form, as evidenced by the electrostatic potential plotted around the opened form. The presence of three charged groups on the periphery of a flat conjugated backbone stimulates the self-assembly of merocyanine molecules in water, ending up with the formation of elongated associates with stack-like building blocks, as shown in molecular dynamics simulations of the aqueous solution with the concentration above critical micelle concentration. Our quantitative evaluation of the hydrophilicity switching in spiropyran/merocyanine containing surfactants may prompt the search for new systems, including colloidal and polymeric ones, aiming at remote tuning of their morphology, which could give new promising shapes and patterns for the needs of modern nanotechnology.

An easily accessible isospiropyran switch

In the presence of SiCl4, three molecules of 5'-bromo-2'-hydroxyacetophenone underwent an unexpected tandem aldol condensation to give a novel isospiropyran switch (69%), with X-ray crystallography confirming its structure. The strong Brønsted acid CH3SO3H turned the colorless isospiropyran into its protonated and open form possessing red color. This process was reversed using the Et3N base, with the acid/base toggling repeatable for at least six times (UV-Vis). When printed on a silica plate, however, the isospiropyran formed a blue-colored product due to, as posited, its stabilization by hydrogen bonding (HB) to silica. An exposure to HB-competing ethyl acetate temporarily "erased" the print only to be brought back by subjecting the plate to a higher temperature for evaporating the solvent. The isospiropyran described here is an easily accessible, chromic, modular and switchable compound that one can incorporate into dynamic materials or use for building chemosensors, molecular machines and organic electronic devices.

Directional and regioselective hole injection of spiropyran photoswitches intercalated into A/T-duplex DNA

The hole electron transfer of UV excited spiropyran intercalated in dsDNA is directional, asymmetric and regioselective, as shown by quantitative multiscale computations.

Tuning the photoreactivity of Z-hexatriene photoswitches by substituents - a non-adiabatic molecular dynamics study

To understand how substituents can be used to increase the quantum yield of photochemical electrocyclic ring-closing of the Z-hexa-1,3,5-triene (HT) photoswitch forming cyclohexadiene (CHD), we investigate the S1 photo dynamics of HT and its derivatives 2,5-dimethyl-HT (DMHT), 2-isopropyl-5-methyl-HT (2,5-IMHT), 1-isopropyl-4-methyl-HT (1,4-IMHT), and 2,5-diisopropyl-HT (DIHT) using time-dependent density functional theory surface hopping dynamics. We report detailed photoproduct distributions, formation mechanisms, branching ratios, and wavelength-dependent product quantum yields. Most products have been confirmed experimentally and include all-trans HT derivatives, cyclopropanes, cyclobutenes, cyclopentene, cyclohexadienes, and bicyclic compounds. Regarding CHD formation, we find that for the 2,5-substituted derivatives DMHT, 2,5-IMHT, and DIHT, the branching ratios increase with increasing size of the substituents. In contrast the branching ratios of the E/Z-isomerization decrease with increasing size of the substituents. Due to steric interactions, increasing the size of the substituents increases the amount of gZg rotamers in the ground state, which are prone to CHD formation and have lower E/Z-isomerization probability. Furthermore, we find [1,4], [1,5], and [1,6]-sigmatropic hydrogen shift reactions occurring at large percentages (5% to 15%); for sterical reasons these reactions stem from tZg conformers. DIHT shows the lowest percentage of side product formation among the 2,5-substituted molecules and highest CHD branching ratio; its CHD quantum yield can be increased up to more than 64%, by excitation of DIHT on the red tail of its absorption spectrum, whereas the Z/E-isomerization is reduced below 5% and side reactions practically vanish. This makes DIHT the best candidate for applications in molecular switches.

Substituent Effects and Mechanism in a Mechanochemical Reaction

We report the effect of substituents on the force-induced reactivity of a spiropyran mechanophore. Using single molecule force spectroscopy, force-rate behavior was determined for a series of spiropyran derivatives substituted with H, Br, or NO₂ para to the breaking spirocyclic C-O bond. The force required to achieve the rate constants of ∼10 s^-1 necessary to observe transitions in the force spectroscopy experiments depends on the substituent, with the more electron withdrawing substituent requiring less force. Rate constants at 375 pN were determined for all three derivatives, and the force-coupled rate dependence on substituent identity is well explained by a Hammett linear free energy relationship with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic, dissociative character. The methodology paves the way for further application of linear free energy relationships and physical organic methodologies to mechanochemical reactions, and the characterization of new force probes should enable additional, quantitative studies of force-coupled molecular behavior in polymeric materials.

First-Principles Prediction of Wavelength-Dependent Product Quantum Yields

We present a method to predict wavelength-dependent product quantum yields (PQYs) for photochemical reactions and applied it to Z/E-isomerization and several ring-closing reactions of Z-2,5-dimethyl-1,3,5-hexatriene and truncated previtamin D. Using branching ratios from surface hopping molecular dynamics, individual trajectories are correlated with the absorption spectra of their initial structures. The wavelength-dependent PQYs are computed by dividing the average spectrum of the initial structures of the product-forming trajectories by the average spectrum of all initial structures. Accurate absorption spectra are calculated using the correlated ADC(2) method with an implicit solvent. Calculations reproduce the experimentally found trend of increasing six-ring formation and decreasing Z/E-isomerization on the red side of the spectrum. Over all seven reactions studied, the mean absolute error (MAE) between experimental and calculated PQYs (MAE) amounts to 8.1%, and the largest MAE is 18.6%. For four reactions, predicted values agree quantitatively with experiments within 5.6%.

How useful is molecular modelling in combination with ion mobility mass spectrometry for 'small molecule' ion mobility collision cross-sections?

Ion mobility mass spectrometry is used to measure the drift-time of an ion. The drift-time of an ion can be used to calculate the collision cross-section (CCS) in travelling wave ion mobility (e.g. Waters Synapt and Vion instruments) or directly determine the experimental CCS (e.g. Agilent 6560 instrument and many drift-tube instruments). A comparison of the experimental CCS and theoretical CCS values obtained from trajectory method He(g) parameterised MOBCAL and N2(g) parameterised MOBCAL software, for a range of 20 'small molecules' is presented. This study utilises density functional theory B3LYP methods and the 6-31G+(d,p) basis set to calculate theoretical CCS values. This study seeks to assess the accuracy of a common procedure using CCS calibration with poly-(d/l)-alanine derived from drift-cell measurements and the original release of MOBCAL software and compare it with recent improvements with a drug-like molecule calibration set and a revision of MOBCAL parameterised for N2(g) drift gas. This study represents one of the first quantitative evaluations of the agreement between theoretical CCS and experimental CCS values for a range of small pharmaceutically relevant molecules using travelling wave ion mobility mass spectrometry. Accurate theoretical CCS may allow optimisation of ion mobility separations in silico, provide CCS databases that can confirm structures without the need for alternative analytical tools such as nuclear magnetic resonance spectroscopy (NMR) and assignment of unknowns and positional isomers without requiring reference materials.

One- and two-photon absorption of a spiropyran-merocyanine system: experimental and theoretical studies

We report on the nonlinear optical properties measurements and quantum-chemical calculations of a well-known photochromic system consisting of spiropyran and the merocyanine photoproduct. The study of nonlinear absorption and refraction properties of the molecules dissolved in chloroform were performed with the Z-scan technique, using femtosecond pulses in a wide range of wavelengths. Maxima in the two-photon absorption spectrum at 700 and 1050 nm were found for the merocyanine form, and the corresponding two-photon absorption cross section is 80 GM and 20 GM, respectively. The latter feature does not vanish completely in the nonlinear spectrum of the spiropyran form, possibly because of the existence of some photoconversion caused by the laser beam during the measurements. A nonlinear absorption peak at 900 nm is found in the spiropyran form with an effective cross section of about 20 GM; it is likely due to three-photon absorption or to absorption by some intermediate species. The experimental data are supported by calculations performed with the use of a hybrid quantum mechanics-molecular mechanics approach.

Photoisomerization action spectroscopy: flicking the protonated merocyanine–spiropyran switch in the gas phase

Molecular ions drifting through buffer gas are exposed to tunable laser light causing a detectable change in their collision cross section and arrival time at the ion detector. The effect can be exploited to obtain action electronic spectra for selected molecular isomers in the gas phase.

Retinal shows its true colours: photoisomerization action spectra of mobility-selected isomers of the retinal protonated Schiff base

Isomers of the retinal protonated Schiff base are separated and probed using laser radiation in a tandem ion mobility spectrometer yielding isomer-specific electronic spectra.

Colorimetric and ratiometric pH responses by the protonation of phenolate within hemicyanine

The indolium–phenol based tetramethylene hemicyanine has colorimetric and ratiometric optical responses under acidic and basic conditions.