Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

Abstract Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas‐phase studies, which involve exposing mobility‐selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers. The Z and E isomers have distinct photoisomerization response spectra with maxima separated by 40–80 nm, consistent with theoretical predictions for their absorption spectra. Solvation of the HI molecules in acetonitrile displaces the absorption bands to lower energy. Together, gas‐phase action spectroscopy and solution NMR and UV/Vis absorption spectroscopy represent a powerful approach for studying the intrinsic photochemical properties of HI molecular switches.


Theoretical methods and calculated conformer structures and energies
To assess the potential contribution of different conformations of the alkyl chain and aniline/julolidine moiety to each ATD peak, a non-exhaustive conformer search was performed using the Force Field tool in Avogadro [5]. Conformations with relative energies <10 kcal/mol were re-optimized at the DFT wB97X-D/cc-pVDZ level of theory using the Gaussian16 package [6]. Cartesian coordinates for these structures are given in the SI Appendix. These geometries were then used to calculate collision cross section using a version of the MOBCAL package parametrized for N2 buffer gas [7]. Vertical excitation wavelengths were determined for the conformers were determined at the df-CC2/aug-cc-pVDZ (augcc-pVDZ-RI auxiliary basis set) level of theory using the MRCC program [8]. Figures S1, S2 and S3 show the optimized three-dimensional structures of Z and E conformers of hemiindigos 1, 2 and 3, respectively. Calculated energies, vertical excitation wavelengths and collision cross-sections for these conformers are presented in Table S1.

Figure S1
Optimized structures for Z (upper) and E (lower) conformers of HI 1, computed at the wB97X-D/cc-pVDZ level of theory.

Figure S2
Optimized structures for Z (upper) and E (lower) conformers of HI 2, computed at the wB97X-D/cc-pVDZ level of theory.

Figure S3
Optimized structures for Z (upper) and E (lower) conformers of HI 3, computed at the DFT wB97X-D/cc-pVDZ level of theory.

Table S1
Optimized ground state energies, wavelengths and oscillator strengths for vertical S1ßS0 and S2ßS0 transitions, and calculated collision cross sections in N2 buffer gas for a series of Z/E conformers of hemiindigos 1-3 shown in Figure S1-3. The energies were computed at the DFT ωB97X-D/cc-pVDZ level of theory. Collision cross sections were calculated using the MOBCAL program with appropriate parameter for N2 buffer gas [7]. Vertical excitation wavelengths for S1ßS0 and S2ßS0 transitions were calculated at the df-CC2/aug-cc-pVDZ (augcc-pVDZ-RI auxiliary basis set) level of theory using the MRCC program, with the italic numbers indicated in brackets to the corresponding oscillator strengths obtained from the CIS wavefunctions [8]. Hemiindigo 11.2 518 (0.9) || 397 (0.0) 247 † oscillator strengths given in parentheses S16

Gas phase photoisomerization experiments
The photoisomerization of the isolated charge-tagged hemiindigos 1-3 was investigated using a homebuilt tandem ion-mobility mass spectrometer (IMS) [4]. The principle of ion-mobility spectrometry rests on the spatial and temporal separation of charged molecular isomers due to differences in their drift velocity (vd) as they travel through buffer gas under propelled by an electric field (E).

vd=K.E
The mobility K can be expressed by the Mason-Schamp equation (eq. 2): Here, z is the ion's charge number, e the electron charge, N the density of the buffer gas, µ the reduced mass of the collision partners, kb the Boltzmann constant, l is the length of the drift region, td is the drift time, and V is total voltage applied across the drift region. W is the integral collision cross section, which depends on the interaction between ion and buffer gas molecule, and is therefore influenced by the structure of the molecular ion. Bulky, unfolded molecular ions have larger collision cross sections and therefore drift more slowly (larger td) than more compact molecular ions.

Figure S4
Tandem ion mobility mass spectrometer. Further details can be found in ref. [4]. (eq. 1) S17 could either be held open or opened momentarily to select target isomer ions which successively passed through the second ion mobility stage, through an ion funnel, an octupole ion guide, and a quadrupole mass filter before being sensed by a Channeltron ion detector connected to a multichannel scaler. Arrival time distributions (ATDs) were obtained by plotting ion count against arrival time.
Example ATDs for hemiindigos 1-3 are shown in Figure S5. Two baseline resolved peaks were obtained for hemiindigo 2 in N2, whereas only one broad peak was observed for compounds 1 and 3 with N2 buffer gas (see Figure S5, upper row). Better separation was achieved by seeding the N2 buffer gas with ≈1% 2-propanol ( Figure S5, lower row). This allowed separation of the E and Z isomers for hemiindigos 1 and 3, allowing individual isomers to be isolated and irradiated.

Ion Intensity
Ion Intensity

ATD peak assignments and determination of isomer yields upon irradiation in solution
To assign the ATD peaks to specific isomers, a series of experiments were carried out in which the hemiindigo solutions in the syringe connected to the electrospray source were irradiated by visible light.
ATDs were monitored after exposure of the sample in the syringe to the output of either a blue 39.5mW,, green (Thorlabs CPS533, 4.5mW, 532nm) or red <15mW,632.8nm) CW laser for 5-10 minutes that served to establish a photostationary state (PSS). These ATDs are compared to ATDs obtained using solutions protected from light (see Figure 2 in manuscript). The effects of irradiating the samples on ATD peak intensities are apparent in Figure S6, where there is clear evidence for the interconversion of Z and E isomers with the relative isomer abundances depending on wavelength. The measured ATDs were fitted by the sum of two Gaussian functions having equal widths to estimate the relative isomer abundances.

Figure S6
Fitted arrival time distributions (ATDs) for HI 1-3 ions obtained using electrosprayed solutions exposed to light of different wavelengths. The left column shows ATDs obtained after 5 minutes exposure of the respective sample to blue light (430-480 nm) prior to electrospray ionization, whereas the right column shows ATDs obtained following exposure of the solution to green light (532 nm) or red light (632.8 nm). The fitted contributions of each isomer are given.
The isomer PSS abundances derived from the ATDs are consistent with abundances measured in solution through analysis of UV-vis spectra (see Table S2).

Photoisomerization action spectroscopy experiments
To investigate the photoisomerization of the hemiindigo ions in the gas phase, a particular isomer was selected using a pulsed Bradbury-Nielsen ion gate (IG2) situated midway along the drift region which was opened for 120 μs at an appropriate delay with respect to IG1. As shown in Figure S4, shortly after passing through the gate, the ions were exposed to a light pulse from a tuneable optical parametric oscillator (OPO, EKSPLA NT342B, 20 Hz, 5 ns pulse width). The photoisomers were separated from the parent isomers in the second stage of the drift region and were then guided through a second ion funnel (IF2) followed by a differentially pumped octupole ion guide, a quadrupole for mass selection and a Channeltron detector. The OPO operated at 20 Hz and overlapped alternate ion packets allowing 'laser on' and 'laser off' ATDs to be collected, the difference between which reflects the effect of light on the parent cation. Thus, a given photo-isomer appeared as a separate peak in the 'laser on' ATD.

Power dependence of the photoisomerization yield
To evaluate the effect of light intensity on the photoisomerization yield, the Z isomer of hemiindigos 1-3 were exposed to blue light (430-480 nm) over a range of fluences. Resulting power dependence plots are shown in Figure S7. The photoisomer yield is directly proportional to light fluence for hemiindigos 1 and 2, consistent with a single-photon isomerization. For hemiindigo 3, the linear dependence is not followed at fluences exceeding 1 mJ/pulse/cm 2 , indicating the onset of saturation/multiphoton processes.
Although power dependence measurements were not performed for E→Z photoisomerization, we expect similar power dependences to the Z→ E processes.

Figure S7
Normalized yield of E photoisomer as a function of light fluence. The experiments were performed at 450nm (hemiindigos 1 and 3) and 430nm (hemiindigo 2), respectively.

Solution photoisomerization experiments Determination of the UV-vis absorption spectra of Z and E isomers
The UV-vis absorption spectra of pure Z and E isomers of 1 to 3 in acetonitrile were obtained by subtraction of one E/Z-mix spectrum with known isomer composition (previously determined by integration from 1 H NMR spectroscopy) from another E/Z-mix spectrum with different but also known isomer composition and subsequent multiplication with a weighting factor. Weighting was done by multiplying the first E/Z-mix spectrum with the Z (or E) isomer percentages of the second E/Z-mixture (determined via 1 H NMR spectroscopy) and vice versa. The obtained absorption spectrum of the respective pure isomer was multiplied by compensation factors to match the previously determined absorption values at isosbestic points.
This method of spectra determination relies on the following conditions: -The system must consist of two chromophores, which interconvert without side reactions or decomposition -The total chromophore concentration is constant during determination of isosbestic points, thus absorption spectra of mixtures result solely from the addition/subtraction of pure E and Z isomer spectra -Distinct isosbestic points must be observed Eq. 2 defines the spectrum S of the e.g. E isomer (E) as a matrix of colligated numeric values: S( ) = S( >?,5…, , >?,5… ) (eq. 2) with wEi (wavelength in nm) as fixed experimental parameters value and aEi (absorption in a.u.) as experimental observables value representing the absorption spectrum. Eq. 3/4 define the measured E/Zenriched mixture spectrum, Smix(E+/Z+), as a composite of pure S(E) and S(Z) spectra: with f1,… being factors to account for the concentrations of each isomer in the mixture, which were determined by NMR measurements and the corresponding magnitudes of the absorption spectra. Solving the system of linear equations for S(E) and S(Z) results in eq. 5/6: Factors f1 -f4 were obtained from integrated indicative signals in the 1 H NMR spectrum (percentage divided by 100) for the E or Z isomer in the E or Z enriched mixture according to the following matrix: The hereby determined spectra consist of 100% E isomer S(E) and 100% Z isomer S(Z), respectively. The isomeric yields obtained in the photostationary state (PSS) at different irradiation wavelengths were determined by irradiation in NMR tubes with subsequent analysis of the isomer composition by 1 H NMR spectroscopy or by irradiation in 10 mm quartz cuvettes followed by UV-vis measurements. In the latter case, isomer abundances were determined by first scaling obtained UV-vis spectra at the PSS to the absolute positions of previously obtained isosbestic points and then calculating the Z/E-ratio in the PSS from the known extinctions of pure isomers at distinct wavelengths aberrant to isosbestic points. Isomer abundances obtained in the PSS at different irradiation wavelengths are given in Table S2.

Figure S9
PSS UV-Vis spectra at different irradiation wavelengths for 2 Z/E in acetonitrile.

Figure S10
PSS UV-Vis spectra at different irradiation wavelengths for 3 Z/E in acetonitrile.

Appendix: Cartesian coordinates for calculated structures
Cartesian coordinates for lower energy conformations of HI 1, HI 2 and HI 3. Structures were optimised at the DFT wB97X-D/cc-pVDZ level of theory using the Gaussian16 package [6] and are depicted in Figures S1, S2