Article pubs.acs.org/JPCA
Photoisomerization Action Spectroscopy of the Carbocyanine Dye DTC+ in the Gas Phase Brian D. Adamson,† Neville J. A. Coughlan,† Gabriel da Silva,‡ and Evan J. Bieske*,† †
School of Chemistry and ‡Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia ABSTRACT: Molecular photoisomerization plays a crucial role in diverse biological and technological contexts. Here, we combine ion mobility spectrometry and laser spectroscopy to characterize the photoisomerization of molecular cations in the gas phase. The target molecular ions, polymethine dye cations 3,3′-diethylthiacarbocyanine (DTC+), are propelled through helium buffer gas by an electric field and are photoisomerized by light from a tunable laser. Photoexcitation over the 450− 570 nm range converts trans-DTC+ to cis-DTC+, noticeably modifying the ions’ arrival time distribution. The photoisomerization action spectrum, which has a maximum at 535 nm, resembles the absorption spectrum of DTC+ in solution but is shifted 25 nm to shorter wavelength. Comparisons between measured and calculated mobilities suggest that the photoisomer involves a twist about the second C−C bond in the methine chain (8,9-cis isomer) rather than a twist about the first methine C−C bond (2,8-cis isomer). It is postulated that the excited gas-phase ions internally convert from the S1 Franck− Condon region to the S0 manifold and explore the conformational landscape as they cool through He buffer gas collisions. Master equation simulations of the relaxation process in the S0 manifold suggest that the 8,9-cis isomer is preferred over the 2,8cis isomer because it lies lower in energy and because it is separated from the trans isomer by a substantially higher barrier. The study demonstrates that the photoisomerization of molecular ions can be probed selectively in the gas phase, providing insights into photoisomerization mechanisms and information on the solvent-free absorption spectrum.
1. INTRODUCTION One of the fundamental ways in which molecules respond to light is through conformational change, with perhaps the most familiar example being the photoisomerization of 11-cis-retinal, the primary light-sensing event in visual photoreceptors.1 Photoisomerizing molecules are also at the heart of light-driven molecular machines and switches with photoactive molecular elements that include fulgides, azobenzenes, azulenes, alkylidene cycloalkanones, spiropyrans, diarylethenes, and overcrowded alkenes.2−4 Molecular photoisomerization has been investigated extensively in solution using steady-state and transient optical absorption, ultrafast spectroscopy, Raman spectroscopy, and NMR spectroscopy.1,5 A critical issue in condensed-phase photoisomerization studies is the influence of solvent interactions on the dynamics, which inevitably complicate theoretical treatments of the process. In contrast, gas-phase investigations allow more facile comparisons between theory and experiment. The drawback is that the techniques commonly employed to monitor isomer populations in solution (direct absorption, fluorescence, NMR) are difficult to apply to gaseous species due to low molecular densities. This is particularly true for photoactive molecular ions. One possibility for charged species is to probe photoinduced conformational rearrangement using ion mobility spectrometry (IMS), exploiting the sensitivity of a charged molecule’s drift mobility to its conformation. © 2013 American Chemical Society
In our approach, packets of ions are periodically injected into a drift tube filled with helium buffer gas through which the ions are propelled by an electric field toward an ion detector. Alternate packets of ions are exposed to tunable laser radiation at the beginning of the drift tube. Photoisomerization alters the molecular ions’ effective size and collision cross section with the helium buffer gas so that the parent ions and photoisomer ions arrive at the detector at slightly different times. One can record a photoisomerization action spectrum by measuring the ions’ arrival time distribution (ATD) as a function of laser wavelength. The method, which we label photoisomerization action (PISA) spectroscopy, is an extension of conventional IMS, which has been deployed to probe the isomeric landscapes of organic molecules,6 clusters,7,8 peptides, and proteins.9−11 The current study focuses on photoisomerization of the 3,3′diethylthiacarbocyanine cation (DTC+), a symmetric carbocyanine dye consisting of two heterocycles linked by a conjugated chain of three methine carbon atoms. NMR studies show that the trans-DTC+ isomer (Figure 1) predominates in soluSpecial Issue: Terry A. Miller Festschrift Received: June 10, 2013 Revised: August 19, 2013 Published: August 21, 2013 13319
dx.doi.org/10.1021/jp405747q | J. Phys. Chem. A 2013, 117, 13319−13325
The Journal of Physical Chemistry A
Article
or by selecting the ions according to their drift mobility prior to spectroscopic interrogation.18 This latter approach has delivered conformer-specific electronic spectra of the peptide bradykinin in its doubly protonated charge state with resolution of individual vibronic transitions.18 In contrast, our studies are directed toward directly probing the photoisomerization response of gas-phase ions; by monitoring the photoisomer yield as a function of wavelength, we record a PISA spectrum that represents the molecule’s absorption spectrum multiplied by the photoisomerization probability (which, depending on the particular molecular system, may depend on the wavelength). Figure 1. The trans, 8,9-cis, and 2,8-cis forms of DTC+. Relative energies are from B3LYP/6-31G(d) DFT calculations.
2. EXPERIMENTAL DETAILS The experimental approach has been described previously and involves irradiating DTC+ cations in a purpose-built ion mobility apparatus.16 Briefly, electrosprayed DTC+ cations, produced from a 10−4 M solution of DTC iodide in methanol (electrospray voltage ≈ 3 kV; flow rate ≈ 5 μL/min) are accumulated in an ion funnel before being launched in a short, 100 μs pulse into a 0.9 m drift tube filled with helium buffer gas (10 Torr). The electric field in the drift tube (19 V/cm) is sustained by a series of ring electrodes. At the end of the drift tube, the ions are gathered radially by a second ion funnel before passing through a 0.3 mm orifice into an octopole ion guide from where they exit through a second 3 mm orifice into a quadrupole mass filter. Mass-selected DTC+ ions are sensed by a channeltron detector connected to a discriminator and a multichannel scaler. Typically, the apparatus is run at 20 Hz and the ions’ ATD is built up as a histogram of ion counts versus time. The mobility resolution for the DTC+ ions is td/ Δtd = 60, which can be compared to a maximum resolution under the prevailing conditions of 75.16 Under typical operating conditions with a drift voltage of 2000 V and He buffer gas pressure of 10 Torr, the effective temperature of the ions is predicted to be 340 K.19 For the photoisomerization studies, alternate ion packets were irradiated immediately after exiting the first ion funnel with the 10 Hz output of a pulsed, tunable optical parametric oscillator (OPO, λ = 400−710 nm, 10 ns pulse width, 5 cm−1 bandwidth, 5−10 mJ/pulse, ∼5 mm beam diameter). The laserinduced change is reflected in the difference between laser-on and laser-off ATD histograms.
tion.12,13 Following S1 ← S0 electronic excitation over the 460− 580 nm range, trans-DTC+ isomerizes about one of the chain’s C−C bonds to form either the 8,9-cis isomer or 2,8-cis isomer (Figure 1). On the basis of calculated absorption cross sections and transition energies14 and NMR spectra of cooled, irradiated samples,13 it has been proposed that the 2,8-cis isomer is the main photoproduct, although the evidence might be considered equivocal given the much stronger evidence for the formation of the 8,9-cis photoisomer of DOC+ (DTC+ with S atoms replaced by O atoms).13 Once formed, the photoisomer backisomerizes over a barrier on the S0 surface to form the trans isomer, with a temperature-dependent rate described by an Arrhenius expression with an activation energy and preexponential factor that depend on the solvent viscosity and that, in the case of DTC+, range from 58 to 71 kJ/mol and 1012.3 to 1014.3, respectively.15 Here, we explore the photodynamics of DTC+ in the gas phase using a combination of IMS and laser photoisomerization, obtaining information on the photoisomer branching ratios and measuring a gas-phase PISA spectrum. The experimental results are interpreted with the aid of quantum chemical calculations of energies and structures, from which collision cross sections and mobilities are predicted. Branching ratios for formation of the energetically accessible 8,9-cis isomer or 2,8-cis isomer photoisomers are predicted using master equation modeling of coupled chemical and vibrational relaxation on the S0 manifold. Previously, we deployed ion mobility mass spectrometry to study the photoisomerization of the polymethine dye HITC+, which has seven methine carbons linking the two heterocycles.16 Surprisingly, we found that the electrosprayed ion population contained at least two isomers, including the most stable trans isomer, and that different photoisomerization processes prevail at short and long wavelengths. Although it was clear that the main process involved trans → cis photoisomerization, it was difficult to decide which of the several possible cis isomers was the main photoproduct. The difficulties were exacerbated because the relevant HITC+ isomers have similar drift mobilities and were difficult to resolve. The situation is much clearer for the smaller DTC+ ion as there are fewer accessible photoisomers, enabling us to identify clearly the main photoisomer and elucidate the likely photoisomerization mechanism through comparisons with the master equation simulations. The current investigation complements recent spectroscopic studies of cryogenically cooled biomolecule cations in which impressive isomer-specific photodissociation spectra are obtained either through IR−UV double-resonance schemes17
3. RESULTS AND DISCUSSION The measured ATD for DTC+ cations without laser irradiation is shown in Figure 2. The ATD is fitted extremely well by the sum of two Gaussian curves, evidence that the ion population is composed of just two significant isomers. The most intense peak (td = 20.44 ms) can be assigned to the trans-DTC+ isomer, whereas the smaller, earlier peak (td =19.93 ms) is presumably due to either the 2,8-cis or 8,9-cis isomers (see Figure 1). To help identify the relevant DTC+ isomers, the energies and geometries for the more stable structures and transition states were determined using density functional theory (DFT) [B3LYP/6-31G(d)] utilizing the Gaussian 09 package.20 The three lowest energy structures are shown in Figure 1. Reduced mobilities were estimated using the exact hard sphere (EHS) scattering model as implemented in the MOBCAL program.21−23 Measured and calculated data for the lowest-energy isomers of DTC+ are summarized in Table 1. The calculations predict that 8,9-cis-DTC+ lies 13.7 kJ/mol higher in energy than 13320
dx.doi.org/10.1021/jp405747q | J. Phys. Chem. A 2013, 117, 13319−13325
The Journal of Physical Chemistry A
Article
later when considering the photoisomerization mechanism in section 3.3. 3.1. Photoisomerization of DTC+ . The effect of irradiating the DTC+ cations at 535 nm is shown in Figure 2c. Depletion of the trans-DTC+ peak by ∼5% is matched by a corresponding enhancement of the 8,9-cis-DTC+ peak. The laser-on − laser-off difference ATD (Figure 2c) is fitted with the function 2
fλ (t ) = A(λ) × [e[−(t − ta)
/2σw 2]
2
− e[−(t − tb)
/2σw 2]
]
(1)
assuming that the only photophysical process is photoisomerization from trans-DTC+ to 8,9-cis-DTC+. Here, ta and tb are the mean arrival times of the 8,9-cis and trans isomers, σw describes the peak width, and A(λ) is the wavelengthdependent amplitude of the photoresponse. Although the fit to the experimental data indicates that a minor fraction of the DTC+ cations (
