Protomer-Specific Photochemistry Investigated Using Ion Mobility

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Protomer-Specific Photochemistry Investigated Using Ion Mobility Mass Spectrometry James N Bull, Neville J. A. Coughlan, and Evan John Bieske J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05800 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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The Journal of Physical Chemistry

Protomer-Specific Photochemistry Investigated Using Ion Mobility Mass Spectrometry James N. Bull, Neville J. A. Coughlan, and Evan J. Bieske∗ School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australia E-mail: [email protected]

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Abstract The utility of tandem ion mobility mass spectrometry coupled with electronic spectroscopy to investigate protomer-specific photochemistry is demonstrated by measuring the photoisomerization response for protomers of protonated 4-dicyanomethylene-2methyl-6-para-dimethylaminostyryl-4H -pyran (DCM) molecules. The target DCMH+ species, has three protomers that are distinguished by their different collision crosssections with He, N2 , and CO2 buffer gases, trends in abundance with ion source conditions, and from their photoisomerization responses. The trans-DCMH+ protomers with the proton located either on the tertiary amine N atom or on a cyano group N atom exhibit distinct S1 ← S0 photoisomerization responses, with the maxima in their photoisomerization action spectra occurring at 420 and 625 nm, respectively, consistent with predictions from accompanying electronic structure calculations. The cis-DCMH+ protomers are not distinguishable from one another through ion mobility separation and give no discernible photoisomerization or photodissociation response, suggesting the dominance of other deactivation pathways such as fluorescence. The study demonstrates that isobaric protomers and isomers generated by an electrospray ion source can possess quite different photochemical behaviours, and emphasizes the utility of isomer and protomer selective techniques for exploring the spectroscopic and photochemical properties of protonated molecules in the gas phase.

Introduction The site at which a molecule is protonated can profoundly influence the molecule’s properties and reactivity. Protonation site is important in excited state mechanisms that stabilize biological systems to radiation damage, 1–3 tuning the photoisomerization properties of photoswitches and photopharmaceuticals, 4–6 and in determining the electronic absorption profiles of small heteroaromatics 7,8 as well as tautomers of bare nucleobases and nucleobase dimers. 2,9–12 In solution, it is difficult to control which protomer is formed when several basic

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protonation sites are available, so that elucidating protomer-specific photochemistry requires functional group modification to block certain protonation sites. Several techniques have been deployed to differentiate protomers and investigate their properties in the gas phase. Protomers of molecules such as norfloxacin have been distinguished through their distinct collision induced fragmentation patterns, 13 although this approach is limited due to protomer scrambling which typically occurs at energies required for collisional fragmentation. UV hole burning strategies have been developed to selectively remove one protomer/tautomer, allowing photodissociation spectra to be recorded for any remaining species. 10,11 In other approaches, drift tube ion mobility spectrometry and differential mobility spectrometry have been deployed to discriminate isobaric protomers, exploiting differences in the collision cross-section for protomer ions with a buffer gas. Systems investigated with ion mobility strategies include protonated aniline, 14–16 aminobenzoic acid, 17–19 norfloxacin, 13,20 and benzocaine. 16 Protomers have also been distinguished through their distinct spectroscopic signatures in infrared spectra obtained through resonance enhanced photodissociation of mass-selected ions. These studies have embraced protonated aniline, 21 fluorobenzene, 22 phenol, 23 aminobenzoic acid, 24 and flavins. 25 Protomers of protonated aminophenol and nicotinamide have been differentiated on the basis of electronic spectra measured using resonance enhanced photodissociation. 7,8 The protomers of protonated nicotinamide are distinguished not only by having different electronic spectra but also through their characteristic photofragmentation patterns. 8 Similarly, protomers of aminobenzoic acid and nucleobases have been identified from IRMPD spectra. 26–29 A promising recent approach for characterizing protomers involves coupling ion mobility separation with spectroscopic interrogation using tunable radiation. This powerful strategy has been implemented to separate and probe the protomers of aminobenzoic acid and benzocaine, 18,30 unambiguously demonstrating that the separated protomers have distinct IR spectra. Here, we deploy an alternative spectroscopic approach based on tandem drift tube mobility spectrometry, whereby the photoisomerization of mobility-selected protomers is fol-

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concentration. 45–49 Theoretical studies suggest that the absorption and fluorescence profiles of protonated DCM are dramatically shifted compared to the neutral molecules, 50 a property utilized in OLED manufacture in which dyes are co-deposited with acids to tune emission properties. 38,40,51 Ultimately, to optimize such devices it is desirable to understand the influence of protonation and protonation site on the absorption profiles and photochemistry for dyes such as DCM.

Experimental methods The DCMH+ protomers were investigated in the gas phase using a custom IMS-IMS-QMF apparatus illustrated in Figure 2. 31–34 Briefly, DCMH+ ions were produced through electrospray ionization of a ≈10 µmol L−1 solution of DCM (Exciton, USA) dissolved in acidified methanol (voltage ≈3 kV, flow rate ≈10 µL min−1 ). Electrosprayed ions were transferred via a heated capillary into a RF ion funnel (IF1), which radially gathered and confined the ions. An ion gate (IG1) at the end of IF1 injected ≈100 µs packets of ions at 20 Hz into the first IMS region (IMS1). In the IMS region the ion packets were propelled by an electric field (44 V cm−1 ) through He, N2 , or CO2 buffer gas at a pressure of ≈6 Torr. The various isomers are separated spatially and temporally because more extended ions (trans isomers) have larger collision cross-sections with the buffer gas than more compact ions (cis isomers). 52 After traversing the drift region (IMS1 + IMS2), a second ion funnel (IF2) collected the ions and introduced them into a differentially pumped octupole ion guide and quadrupole mass filter (QMF) that mass-selected the ions before they reached the ion detector. The detector was connected to a multichannel scaler that produced a histogram of ion counts against arrival time, t, corresponding to an arrival time distribution (ATD). In all presented ATDs, t=0 corresponds to the opening of IG1. The mobility resolution, t/∆t, for singlycharged ions is typically 70-80. 32 Collision cross-sections of DCMH+ with N2 buffer gas were measured relative to a series of tetraalkylammonium salts (see SI). 53

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IG2

IG1 IF1

IF2 oct QMF

ESI

detector IMS2

IMS1 OPO

Figure 2: Schematic illustration of the IMS-IMS-QMF instrument. Key: ESI - electrospray ionization; IF1 and IF2 - ion funnels; IG1 and IG2 - ion gates; IMS1 and IMS2 - ion mobility drift regions; OPO - light beam passing through the photoisomerization zone; oct - octupole ion guide; QMF - quadrupole mass filter. Total drift region length (IMS1 + IMS2) is 0.9 m, and when operated with N2 the pressure is ≈6 Torr and drift region potential gradient is 44 V cm−1 . For photoisomerization measurements, packets of ions with similar collision cross-sections were selected using a Bradbury-Nielsen ion gate after IMS1 (IG2, ≈100 µs opening time). Immediately after this, the mobility-selected ions were excited with a pulse of light from an optical parametric oscillator (OPO, EKSPLA NT342B, 2-3 mJ cm−2 pulse−1 ). Any change in the ions’ collision cross-section due to photoisomerization was manifested as a shift in arrival time following passage through a second IMS region (IMS2). The OPO was operated at 10 Hz, half the rate of ion injection, allowing accumulation of light-on and light-off ATDs. The difference between the light-on and light-off ATDs reflected the photoresponse. Photoisomerization acton (PISA) spectra were derived by plotting the photoisomer signal, normalized with respect to light pulse fluence, against excitation wavelength.

Theoretical methods Electronic structure calculations for DCMH+ species were performed using the Gaussian 09 and CFOUR software packages. 54,55 Geometrical optimizations and isomerization transition state searches were performed at the ωB97X-D/cc-pVDZ level of theory, 56 followed by single-point energy calculations with the cc-pVTZ basis set. 57 Vertical excitation ener6


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Figure 4: Arrival time distributions for DCMH+ with (a) N2 , (b) He and (c) CO2 buffer gases. In each plot, grey and black ATDs correspond to low and high RF drive voltages to IF1, respectively. Peak A is assigned to P1-trans, B to P3-trans and P2-trans, and C to P1-cis, P1-cis-2, and P3-cis (see Figure 3 for structures). In (a), Ωm are measured collision cross-sections, and t/∆t is the resolution for peaks in the black ATD. The ATDs in He buffer gas (b) were accumulated with an electric field in the drift region of 22 V cm−1 , whereas the ATDs in N2 (a) and CO2 buffer gases (c) were recorded with a 44 V cm−1 electric field. 8


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gies of DCMH+ isomers were computed at the EOM-CCSD/cc-pVTZ level of theory. 58 The correlation space was truncated to exclude virtual orbitals with binding energies exceeding 15 eV. Collision cross-sections, Ωc , were calculated using MOBCAL with the trajectory method parametrized for N2 or He buffer gas. 59,60 Input charge distributions were computed at the ωB97X-D/cc-pVTZ level of theory with the Merz-Singh-Kollman scheme constrained to reproduce the electric dipole moment. 61 Sufficient trajectories were computed to give standard deviations of ±1 Å2 for the calculated values.

Results and Discussion Isomers, protomers, and the influence of buffer gas Calculated DCMH+ structures Electronic structure calculations for DCMH+ isomers and protomers reveal the three most favourable protonation sites to be the tertiary amine N atom (Figure 1, P1), and the N atoms on the cyano groups (Figure 1, P2 and P3). In principle, each protomer can have trans and cis configurations around the central alkene (-C=C-) bond giving at least six isomers. The structures, relative energies (∆E), and calculated collision cross sections (Ωc ) of relevant protomers are summarized in Figure 3. The combination of steric hindrance and a low barrier to proton transfer between the dimethylamino and P2-cyano group in the cis geometry means that a P2-cis isomer does not represent a potential energy minimum. The calculations suggest that the P1-cis isomer is the most stable form, with its stability due to the amine-cyano hydrogen bond (length 1.78 Å). The P1-cis-2 isomer is a conformation of P1-cis in which the dimethylamine group is rotated by ≈180◦ . The P3-trans and P2-trans structures are the most stable trans isomers. Protonation on the oxygen atom in the pyran ring is unfavorable with the trans and cis isomers lying 263 and 279 kJ mol−1 above the

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P1-cis structure, respectively. These isomers are unlikely to be formed in our experiment. Calculated isomerization transition state energies are summarized in the SI. DCMH+ in N2 buffer gas ATDs of DCMH+ obtained with N2 buffer gas are shown in Figure 4a. The ATDs exhibit three peaks, labelled A, B and C, with relative intensities that depend on the RF drive voltage to IF1. High RF drive voltage causes collisional heating, favouring the most stable gas-phase isomers. Thus, the variation of ATD peak intensities with IF1 drive voltage suggest the isomers have stabilities ordered as C>B>A. Peaks A and C each have instrumentlimited widths consistent with the contribution of one predominant isomer, whereas peak B is broader in ATDs measured with low and high IF1 drive voltage, suggesting the contribution of multiple isomers with similar collision cross-sections. The peaks in the ATD recorded with N2 buffer gas (Figure 4a) can be assigned to the isomers shown in Figure 3 based on correspondences between calculated and measured collision cross-sections, and through recognition that the peak intensities should be related to the isomer’s relative energies. On this basis, peak A is assigned to the P1-trans protomer which is predicted to have the largest collision cross-section, peak B to the P3-trans and P2-trans protomers, and peak C to the three cis protomers (particularly the stable P1-cis isomer), which are predicted to have the smallest collision cross sections. As explained in the following sections, these assignments are consistent with photoisomerization measurements. The ≈4% overestimation of Ωc compared with Ωm is consistent with observations for other cations using the same computational and measurement methods. 35,53 DCMH+ in He and CO2 buffer gases We now consider ATDs for DCMH+ obtained with He and CO2 buffer gases, shown in Figure 4b and c, respectively. The ATD for DCMH+ in He buffer gas exhibits two resolved peaks with the earlier peak becoming more intense at high RF drive voltage to IF1, suggesting 10


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that it corresponds to the more stable cis isomers. The later ATD peak is broader and has a shoulder on the trailing edge that is enhanced by a high RF drive voltage to IF1, indicating contributions from several isomers. Comparing peak areas for the ATDs obtained with He and N2 buffer gas with high RF drive potential to IF1 (Figure 4a and b) suggests that the later peak in the ATD obtained with He buffer gas arises from the isomers that contribute to peaks A and B in the ATD obtained with N2 buffer gas. Calculated collision cross-sections for DCMH+ with He buffer gas support this interpretation (Ωc =110 – 115 Å2 for cis protomers and Ωc =124 Å2 for trans protomers.) The ATDs recorded for DCMH+ with CO2 and N2 buffer gases (Figure 4c and a), are similar except that the relative arrival time of peak A increases compared to peaks B and C (relative A-C peak arrival time is 1.17 with CO2 and 1.14 with N2 ). We propose that contributions to the collision cross-section from long-range intermolecular interactions are responsible for the large changes in the relative arrival times for the P1-trans protomer (peak A) going from He to N2 to CO2 buffer gases. 13–20,30 Whereas, the trans-DCMH+ protomers are predicted to have near identical hard sphere collision cross-sections for either He, N2 or CO2 buffer gases, dipole-quadrupole interaction will enhance the collision cross-sections between N2 (quadrupole moment of -4.65×10−40 C m2 , ref. 62) or CO2 (quadrupole moment of -13.4×10−40 C m2 , ref. 62) and the P1-trans protomer (peak A), which has a calculated dipole moment of 31.0 D, compared with P2-trans and P3-trans protomers (peak B) which have calculated dipole moments of 4.2 D and 5.5 D, respectively. Consequently, the P1-trans isomer’s relative arrival time should be sensitive to the buffer gas.

Photoisomerization of DCMH+ The photoisomerization responses of mobility-selected DCMH+ isomers were investigated to understand the influence of protonation site on the different isomer’s photochemical properties. Surprisingly, there was no discernible photoisomerization or photodissociation response for the cis protomers (ATD peak C) despite the large calculated oscillator strengths (see 11



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Figure 5: Photoisomerization of the DCMH+ ATD peak B (P2-trans and P3-trans): (a) Example light-off and photo-action (light-on – light-off) ATD obtained at 620 nm and (b) PISA spectra to give ATD peak C (P1-cis) and B1 (depletion shown as B2). B1 and B2 are assigned to the P2-trans and P3-trans protomers, respectively. The P3-trans S1 ←S0 vertical transition at the EOM-CCSD/cc-pVTZ level of theory is shown.

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SI), suggesting that these protomers are either efficiently deactivated through fluorescence following electronic excitation, or that photoisomerization and photodissociation following non-radiative decay are inefficient. On the other hand, the P1-trans, P2-trans and P3-trans isomers were found to undergo photoisomerization following exposure to visible and UV light. P2-trans and P3-trans Example light-off and photo-action ATDs for the P2-trans and P3-trans (Peak B) protomers are shown in Figure 5a. Because the ions were irradiated approximately halfway along the drift region (after IMS1 and IG2), the photoisomer peaks appear between the parent isomer peak and the peak for the product isomer when it is separated over both drift regions (IMS1 + IMS2). Exposure to light at 620 nm preferentially depletes the slower section of peak B (denoted B2) with an enhancement of the faster section (denoted peak B1), and generates a peak associated with ATD peak C (cis isomers). Photoisomerization action (PISA) spectra, obtained by plotting the area of each appearance peak against wavelength and normalizing with respect to light-off signal and light pulse fluence, are shown in Figure 5b. There are two clear bands, a S1 ← S0 transition extending over the 460-720 nm range with a peak at 625 nm, and a weaker S2 ← S0 transition extending from 430 nm to shorter wavelength. The wavelengths and intensities for these bands correspond to predictions from EOM-CCSD/cc-pVTZ calculations for the P2-trans and P3-trans isomers (see SI). The PISA spectra are consistent with the assignment of peak B to the P2-trans and P3-trans isomers, with both undergoing trans-cis photoisomerization around the central alkene bond to yield cis isomers (peak C) in a similar process to the photoisomerization of stilbene. 63 The photo-action ATD suggests that following photoexcitation, the P3-trans and P2-trans isomers interconvert (depletion of B2 and enhancement of B1) through torsion of the cyanomethylene group, a process that has been observed for other dyes incorporating this functional group. 64,65 The EOM-CCSD/cc-pVTZ calculations indicate that although the

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P2-trans and P3-trans protomers have similar S1 ← S0 vertical excitation energies, the P3trans protomer has a larger oscillator strength than the P2-trans (1.5 versus 1.3), consistent with the net photoconversion of the P3-trans protomer (B2) to the P2-trans protomer (B1). P1-trans The PISA spectra for the P1-trans protomer (peak A) is shown in Figure 6. Overall, the photoisomerization response was much weaker than for the peak B species (P2-trans and P3-trans protomers). The photo-action ATD (Figure 6a) shows formation of protomers associated with peak C (P1-cis) and lesser quantities of isomers associated with peak B (P2-trans and P3-trans). The PISA spectra exhibits two bands, a S1 ← S0 transition in the visible with a maximum at ≈420 nm (Figure 5), and a band with an onset at 360 nm that presumably corresponds to the S2 ← S0 band, which is predicted to have a maximum at 312 nm (see SI). Formation of the P1-cis isomer (peak C) from the P1-trans isomer again involves trans-cis photoisomerization around the central alkene bond. Generation of the P2-trans and P3-trans protomers from the P1-trans protomer is more surprising and could involve either rapid formation of the P1-cis isomer followed by ground state proton-transfer to the cyano-substituted pyran ring and isomerization back to a trans geometry before the internal energy is dissipated through collisions, or through an excited state proton transfer mechanism. Orbital analysis Protonation site has a profound influence on the S1 ← S0 vertical excitation energy of DCMH+ protomers with the maximum of the P1-trans S1 ← S0 band (Figure 6b) blue-shifted by ≈200 nm compared to the corresponding transitions of the P2-trans and P3-trans protomers (Figure 5b). These spectral differences can be explained by examining the donoracceptor character of the S0 and S1 states. Orbitals associated with the S1 ← S0 transition for trans-DCM, P1-trans-DCMH+ and P3-trans-DCMH+ protomers are shown in Figure 7. For 14


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Figure 6: Photoisomerization of DCMH+ ATD peak A (P1-trans) to give ATD peak C (cis isomers) and ATD peak B (P2-trans + P3-trans). (a) Example light-off and photo-action (light-on – light-off) ATD at 420 nm and (b) PISA spectra. Note: a change in buffer gas pressure for these measurements mean the arrival times in (a) are shifted compared with Figure 4a. The S1 ← S0 vertical transition at the EOM-CCSD/cc-pVTZ level of theory is shown.

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Conclusions DCMH+ protomers have been generated, separated and investigated in the gas phase by coupling ion mobility spectrometry and photo-excitation with tunable light. It is clear that protonation significantly modifies the intramolecular electron donor and acceptor characteristics of the neutral DCM molecule, with the electronic absorption and photoisomerization properties depending on protonation site. The trans-DCMH+ protomers, with the proton located either on the tertiary amine N atom or on the N atoms of the cyano groups exhibit distinct S1 ← S0 photoisomerization responses, with the maxima in their PISA spectra occurring at 420 and 625 nm, respectively. No photoisomerization response was observed for cis-DCMH+ protomers, even though these species should absorb visible and near-UV light, presumably because following electronic excitation, these protomers are deactivated by fluorescence rather than through non-radiative processes. Ultimately, this study demonstrates that a range of isobaric protomers and isomers can be generated using an electrospray ion source that may possess quite different photochemical behaviours, emphasising the utility of techniques that feature isomer and protomer selectivity for exploring the spectroscopic and photochemical properties of protonated molecules in the gas phase.

Acknowledgement This research was funded through the Australian Research Council Discovery Project scheme (DP150101427 and DP160100474). Computational resources were provided by the Australian National Computational Infrastructure (NCI) through Early Career Allocation ya1 to JNB.

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Supporting Information Available Collision cross-section determination in N2 buffer gas, calculated excited state energetics and oscillator strengths.

This material is available free of charge via the Internet at

http://pubs.acs.org/.

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Protomer-Specific Photochemistry Investigated Using Ion Mobility

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