Z Photoisomerization

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Molecular Switching via MultiplicityExclusive E/Z Photoisomerization Pathways Jiawang Zhou, Xin Guo, Howard E. Katz, and Arthur E Bragg J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 10 Aug 2015 Downloaded from http://pubs.acs.org on August 10, 2015

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Journal of the American Chemical Society

Molecular Switching via Multiplicity-Exclusive E/Z Photoisomerization Pathways Jiawang Zhou,† Xin Guo,‡ Howard E. Katz,†‡ and Arthur, E. Bragg†* Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA ‡ Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218, USA †

Abstract: Mutual exclusivity in the nature of forward and reserve isomerization pathways holds promise for predictably controlling responses of photoswitchable materials according to molecular structure or external stimuli. Herein we have characterized the E/Z photoisomerization mechanisms of the visible-light-triggered switch 1,2-dithienyl-1,2dicyanoethene (4TCE) in chlorobenzene with ultrafast transient absorption spectroscopy. We observe that switching mechanisms occur exclusively by relaxation through electronic manifolds of different spin multiplicity: trans-to-cis isomerization only occurs via electronic relaxation within the singlet manifold on a timescale of 40 ps; in contrast, cis-to-trans isomerization is not observed above 440 nm, but occurs via two rapid ISC processes into and out of the triplet manifold on timescales of ~2 ps and 0.4 ns, respectively, when excited at higher energies (e.g. 420 nm). Observation of ultrafast ISC in cis-4TCE is consistent with photoinduced dynamics of related thiophene-based oligomers. Interpretation of the photophysical pathways underlying these isomerization reactions is supported by the observation that cis-to-trans isomerization occurs efficiently via triplet-sensitized energy transfer, whereas trans-to-cis isomerization does not. Quantum-chemical calculations reveal that the T1 potential energy surface is barrierless along the coordinate of the central ethylene dihedral angle (θ) from the cis Franck-Condon region (θ = 175°) to geometries that are within the region of the trans ground-state well; furthermore, the T1 and S1 surfaces cross with a substantial spin-orbital coupling. In total, we demonstrate that E/Z photoswitching of 4TCE operates by multiplicity-exclusive pathways, enabling additional means for tailoring switch performance by manipulating spin-orbit couplings through variations in molecular structure or physical environment.

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1. INTRODUCTION. Photoswitchable molecules and materials that exhibit substantial changes in structure or functional properties upon wavelength-selective excitation are of considerable interest for numerous applications,1-3 including photochromism,4 memory storage,5 logic devices,6 molecular motors,7,8 mechanical manipulation,9-13 and light-triggered chemical sensitization14 and conductivity.15 Photoswitches that operate via large-scale structural changes, such as E/Z isomerization, are particularly attractive for manipulating the distance between chemical or biochemical moieties to interrogate the nature of their interactions.9-13,16-19 Photoswitching also has potential for controlling or patterning morphologies of aggregated organic materials.20,21 Desirable photoswitch characteristics include high conversion efficiency,22,23 robust fatigue resistance,24 and the feasibility for isomerization at red excitation wavelengths25-27 that can transmit through materials such as biological tissue.9 In addition, mutual exclusivity in the nature of the excited-state relaxation pathways that drive forward and reserve isomerization reactions can provide a handle for controlling the reversibility of a switch as desired for a particular application.28,29 On all counts, an understanding of photophysical dynamics and how they may be manipulated with structure are of prime importance for the synthetic design of photoresponsive materials. Previously we reported a new photochromic molecule, a 1,2-dithienyl-1,2-dicyanoethene (4TCE), that can be induced to isomerize via excitation at visible wavelengths and that exhibits relatively high photostationary yields for photochromic conversion upon irradiation.27 The UVVis spectra and structures for both cis and trans isomers are shown in Figure 1. The trans isomer of 4TCE is most stable energetically when dissolved in solution at room temperature, but is readily converted to the more photostable cis isomer in 100% yield upon exposure to room lights.

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In contrast, the reverse process can be accomplished either through illumination with blue light (80 oC, 100% conversion). All isomerization processes are complete in the course of minutes. Importantly, the low reactivity of the β carbon sites on the thiophene rings eliminates the possibility of a competing cyclization pathway at low energies, such that 4TCE is a two-state E/Z switch when used in conjunction with visible light. Additionally, 4TCE possesses high photostability, with no detectable photodegradation after several isomerization cycles. A particularly attractive feature of the 4TCE switch design is that oligothienyl groups could be lengthened, affecting both photoswitchable mechanical separation of its two ends and the spectral properties of the switch; notably, longer pendant oligothienyl groups would give rise to increased molecular conjugation and a redder absorption spectrum.

Figure 1. UV-Vis absorption spectra of 4TCE isomers in chlorobenzene (CB) (2.4 × 10!! M). The structures of trans and cis isomers of 4TCE are shown in the inset. We previously ascribed the photoswitching characteristics of 4TCE to the large separation in maximum absorption wavelength (λmax) and differences in molar extinction coefficient (ε) between the trans and cis isomers (Figure 1): λmax for the trans and cis isomers are located at 520 and 470 nm in toluene, and the ε of trans is 2.5 times larger than that of cis at their 3



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respective λmax. However, a complete characterization of the relative photoselectivity for each of these pathways and their corresponding photostationary yields must also take into consideration the energy-dependent quantum yields for isomerization, which are determined by the nature of underlying photophysical processes. As small thiophene-based systems are known to exhibit large spin-orbit couplings (SOCs) and efficient intersystem crossing (ISC) rates in their excited states due to the presence of sulfur atoms,30-35 the E-Z and Z-E isomerization reactions of 4TCE also might be expected to occur via electronic relaxation through either or both of the singlet and triplet manifolds; in contrast, E/Z switches such as stilbenes and azobenzenes only occur by way of singlet relaxation pathways upon direct photoexcitation.12,36,37 Manifold-exclusive relaxation pathways could enable control of the relative significance of one pathway over the other by tailored molecular design, energy-transfer sensitization, or the application of external stimuli (e.g. electric fields). Thus, an understanding of photoswitching behavior and how to manipulate switch properties for improved performance or extended spectral utilization requires greater understanding of how molecular structure impacts the course of excited-state dynamics in either direction. Towards this end, we have used ultrafast transient absorption (TA) spectroscopy to interrogate the relaxation pathways associated with the energy-dependent isomerization of 4TCE. We find that trans-to-cis isomerization occurs via relaxation only through singlet electronic states following low-energy excitation (530 nm); low-energy excitation of the cis isomer likewise results in electronic relaxation only via singlets, but with negligible isomerization. In contrast, transient spectral dynamics measured at higher excitation energies (420 nm) reveal that excited-state relaxation of both species and isomerization of the cis isomer, in particular, involves ISC into and out of the triplet manifold. As cis relaxation is driven by fast (~2 ps) and

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efficient ISC from the optically active singlet level, we hypothesize that structural modifications that enhance or disrupt spin-orbit couplings between manifolds could be used to manipulate cisto-trans photostationary yields. We further demonstrate that triplet sensitization may provide an alternate method for highly pathway-selective E/Z isomerization of 4TCE and its derivatives at red wavelengths with high photostationary yield.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Sample preparation and characterization. The synthesis of 4TCE has been described previously.27 Solutions of 4TCE in chlorobenzene (CB, Fisher Scientific, >99% purity) were prepared at concentrations of ~2.4×10-5 mol/L. Sample solutions were circulated through a 1 mm path-length quartz flow cell for optical measurements. The optical density of the sample solutions were ~0.8 and ~0.3 for trans and cis at their peak absorption wavelengths, respectively. All components of the flow circuit are chemically resistant to sample solutions. Exposure to room light can drive the isomerization from trans to cis in solution to completion, and therefore spectroscopic measurements of trans-to-cis isomerization were conducted in the absence of room light. In contrast, the pure trans form of 4TCE can be prepared by heating a cis or mixed cis/trans sample to 80 OC and then cooling it to room temperature in the dark. These processes were used to purify samples before laser experiments. A diode-array spectrometer fiber-optically coupled to tungsten and incandescent deuterium light sources (Stellarnet) was used to measure steady-state UV-Vis spectra of sample solutions before and after time-resolved measurements. For measurements of trans to cis isomerization, no noticeable change of the trans ground state absorption was observed, indicating that the amount of cis generated from photoinduced isomerization during a course of

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two hours of experiments was negligible. Identical steady-state spectra of both trans and cis were measured with samples before and after experiments, implying that photoexcitation of 4TCE did not produce any additional photoproducts. 2.2. Transient Absorption (TA). Our experimental set-up has been described in detail elsewhere;38,39 here we briefly note important features of the set-up that are critical for the experiments described in this work. Ultrafast light pulses used in these measurements were generated with a regeneratively amplified Ti:sapphire laser system (Coherent Legend Elite, 1 kHz rep. rate, 35-fs pulse duration, 4.0 mJ/pulse). Tunable excitation pulses were generated with an optical parametric amplifier (OPA, Coherent Operasolo), either through fourth-harmonic generation of the OPA signal (350-380 nm), fourth harmonic generation of the idler (420-460 nm) or sum-frequency of the signal (470-530 nm); 400-nm excitation pulses were generated via second-harmonic generation of the laser fundamental. Broadband probing continua (400-740 nm) were obtained by white-light generation in a 2-mm calcium fluoride (CaF2) plate. Pulse energies for all actinic pump wavelengths were attenuated to about 1 µJ/pulse for all data presented. In order to eliminate signatures of time-dependent polarization anisotropy from the measured dynamics, the polarization of the white-light probe was set at the magic angle relative to the polarization of the actinic pump using a thin broadband wire-grid polarizer (Thorlabs) placed immediately before the sample. Probe pulses were focused to a spot size at the sample of 50 microns using an off-axis parabolic reflector; the pump beam was focused to a size of

Z Photoisomerization

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