Mechanistic Insights into the Triplet Sensitized Photochromism of Diarylethenes

Abstract Operating photoswitchable molecules repetitively and reliably is crucial for most of their applications, in particular in (opto)electronic devices, and related to reversibility and fatigue resistance, which both critically depend on the photoisomerization mechanism defined by the substitution pattern. Two diarylethene photoswitches bearing biacetyl triplet sensitizers either at the periphery or at the core were investigated using both stationary as well as transient UV/Vis absorption spectroscopy ranging from the femtosecond to the microsecond time scale. The diarylethene with two biacetyl moieties at the periphery is switching predominantly from the triplet excited state, giving rise to an enhanced fatigue resistance. In contrast, the diarylethene bearing one diketone at the photoreactive inner carbon atom cyclizes from the singlet excited state and shows significantly higher quantum yields for both cyclization and cycloreversion.


Introduction
In order to be incorporated into devices, photoactive materials need to exhibit ap recisea nd typically repeatable response to (localized) light stimuli. In this context, photoswitches have been heavily explored over the past decades and among them diarylethenes (DAEs) [1] have proven exceptionallyu seful due to their thermalb istability and ability to modulate fluorescence [2] and electronic properties. [3] This has led to the development of opticalt ransistors [4] as well as storage media [5] based on optical writing ande rasing of DAE-based materials. Although this class of photochromic compounds has been structurally optimized to improvei ts performance, in particular with regard to excitation wavelengths, [6] quantum yields, [7] and HOMO-LUMO modulation, [8] fatigue due to photochemical byproduct formation [9] remains an issue, in solution but also in the solids tate. To prevent UV-induced byproduct formation,s everal approaches have been followed, for example by stericallyb locking and suppressing radical reactivity [10] or by the incorporation of electron-withdrawing moieties. [4b] Clearly,i no rder to develop an extraordinarily robust DAE, it is essential to fully understand the mechanism of the desired ring closure and opening as well as the competing photoreactions. Several theoretical [11] as well as time-resolveds pectroscopy studies [12] have been carried out to determine the involved excited states and their role in the switching process for classical DAEs. Depending on the actual structure, switching has been reported to occur primarily from ah ot vibrationally singlet excited state (Franck-Condon (FC) state) as well as from the relaxed singlet state buta lso from tripleta nd higher excited states. [13] We have recently demonstrated that combining at riplet pathway with visible light excitation resultsi nt he remarkably stable DAE 1 (Scheme 1). [6b] Our approach took advantage of introducing two biacetyl moieties at the periphery of the DAE via a p-conjugated vinylene bridge to lead to efficient population of the triplet excited state upon blue-light excitation (405 nm) of the bathochromically shifted diketone absorption band (l max = 390 nm) with am uch increased molar extinction coefficient (e(390 nm) = 27,000 m À1 cm À1 )a sc ompared to parentb iacetyl. As ar esult, DAE 1 underwent efficient ring closure upon visible light excitation (in the absence of oxygen, which acts as triplet quencher) and displayed unique longterm photostability, which we have been ascribing to the fact that switching primarily occurs on the triplet manifold. [6b] The enhanced fatigue resistance of DAE 1 was, however,a ccompanied by reducedq uantum yields of ring opening, which requires population of the reactive singlet excited state. [14] In this article, we report on the excited state dynamics of two DAE photoswitches carrying triplet sensitizing moieties. In addition to DAE 1,w ea re describing DAE 2 as an alternative structure, in which one biacetyl unit is introduced directly at the central hexatriene systemv ia av inylene spacert oa ssure for p-conjugation in the open isomer yet not in the closed isomer.I nt he latter,w ea re expecting al ess efficient intersystem crossing (ISC) and thus improved ring openingq uantum yield as compared to DAE 1.W ea re discussing the isomerization pathways of both DAEs, whichh ave been investigated via transienta bsorption spectroscopy to reveal the involved excited states and comparet heir reactivities, in particularw ith regard to the point of attaching the sensitizing moiety.

Results and Discussion
The "ideal" DAE shouldc ombine severalp ropertiest oe xhibit optimal switching behavior:( i) The absorption of the open isomer should be shifted from the UV to the visible range to utilize excitation with lower energiest oa ffect ring closure; (ii)ring closures hould result from the triplet excited state to avoid potentials ide reactions originating from the singlet excited state;( iii)the absorption of the closed isomer should be in the red region of the visible spectrumt oa llow for its selective excitationa nd ring openinga tl ow excitation energies; and (iv) ring openings houldo ccur with reasonable cycloreversion quantum yield, which requires sufficient population of the singlet excited state andt hus limited ISC. DAE 2 was designed to fulfill these requirements by attaching the triplet sensitizing diketone unit to the inner 2-position of one of the thiophene moieties (Scheme 1). On the one hand, the absorption of the open isomer 2o should be extended towards the visible range because of the increased length of the p-system similarly to DAE 1.M ore importantly,e lectronic excitation of 2o,i nw hich p-conjugation between the DAE core and the sensitizing diketone unit is assured, should lead to efficient ISC and predominant population of the triplet excited state. On the other hand, upon ring closure to the closed isomer 2c the diketone sensitizer is decoupled from the DAE p-system since the resulting sp 3 -hybridized thiopheneC -2 atom impedes electronic communication. Consequently,l ow energy,v isible light excitation of the DAE should lead to increased population of the singlet excited state due to less efficient ISC and thus enhancedr ing openingw hen compared to 1c.I na ddition, the absorption of 2c is expectedt ob eh ypsochromically shifted when compared to 1c andi ti sa lso expected to display hypochromic behavior due to the shorter p-system and non-symmetrical substitution pattern. AD AE bearing diketone motifs at both inner 2-positions of the two thiophene moieties in as ymmetric fashion was not considered due to anticipated sterich indrance during ring closure.
An acetonitrile solution of 2 shows the characteristic absorption pattern of the open form of anon-symmetrically substituted DAE derivative( Figure 1). The intense absorbance band at l max = 289 nm corresponds to the overlap of the absorption of both electronically decoupled aryl moieties. The shoulder at l max = 357 nm is attributed to the half of the DAE with longer p-conjugated system bearing the diketone moiety. This lowest energy transition of 2o is slightly blue-shifted andl ess intense when compared to 1o,w hich can be explained by the interaction between the thiophene donor and diketone acceptor units that extends over al onger p-system in 1o including the intermediate phenylmoiety.
Upon irradiation with blue light (l irr = 405 nm), the build-up of ar ed-shifted broad band with am aximum in the visible range (l max = 582 nm) can be observed. This can be assigned to the formation of the closed isomer and is also accompanied by achange in the spectral signature in the UV-region showing new maxima at 273, 314,a nd 384 nm. As expected, the long wavelength absorbance of 2c is blue-shifted when compared to the one of 1c,y et it is essentially separated from the ab-sorptionof2o.
Conversion to 2c at 405 nm is quantitative (see Table 1), a result that was also observed for 1o!1c andthat is important for further applicationso ft his molecule. The quantum yield of the ring closure (F o!c )w as determinedu nder degassed conditions since oxygen can act as triplets tate quencher. [15] This process is not only lowering the efficiency (and therefore the quantum yield) of the cyclization, but the thus-generated singlet oxygen can irreversibly oxidize the photoswitch,l eading to considerable fatigue. Aq uantum yield of F o!c = 0.55 was Scheme1.Investigated diarylethenes bearing abiacetyl sensitizer at the periphery (1,red) [6b] or in the central position (2,blue). determined for the ring closure, which is significantly higher than for 1 and also slightly exceeds the typicallyc onsidered maximum of 0.5 for commonD AEs, which is based on an equal distribution of the photoreactive antiparallel and unreactive parallel conformation,i na ddition to an efficient ultrafast cyclization from singlet excited state. [16] In the case of 2o,t he bulky internal vinylene diketone substituent presumably destabilizes the parallel form, thus leadingt oalarger fraction of the antiparallel conformer in solution. [17] In addition, there appears to be as ignificant rotational barrierb etween both conformers as indicated by two distinct signals observed in the room temperature 1 HNMR of 2o with ar elative integral ratio of 52:48 provingt he unequal distribution of both conformers (see Figure S21 in the Supporting Information). In view of the different solvents (CH 3 CN for quantum yield determination vs. CDCl 3 for NMR spectroscopy) as well as measurement errors, the quantum yield and conformational equilibrium values are in reasonable agreement.
Ring closure of 1o via the triplet exciteds tate was initially provenb yq uenching experiments,o fw hich the easiest one is the measurement of the quantum yield under non-degassed conditions. In an aerated solution, oxygen is present in am uch higher concentrationt han the investigated DAE (ca. 2 10 À3 m as compared to 4 10 À5 m in acetonitrile) [18] and able to quencht riplet excited states and photochemical pathways originating from them. Surprisingly,t he ring closure quantum yield at 405 nm of 2o in non-degassed acetonitrile was determined to be stillq uite high with F = 0.41. This contradicts our initial hypothesis of as olely operating triplet cyclization pathway and could result in significant fatigue upon irradiation. To investigate this, 2o was exposed to continuous irradiation with light of l irr = 405 nm and the development of the absorbance of the closed isomer was monitored (Figure2)a si ndication of side reactions, usually bleaching the compound and forming a hypsochromically absorbing byproduct.
Comparing the fatigue behavior of 1 and 2,i tb ecomes clear that 1 is not showing any fatigue over several hours of intense irradiation, whereas 2 shows ac onsiderable decrease of the band of the closed isomer during the same durationa nd under identicalc onditions. As expected from the destructive effect of high-energyi rradiation, the fatigue behavior of 1 and 2 is highly wavelength-dependent,s howings ignificantd ecompositionu pon exposure to UV-light with aw avelength of 313 nm. [9a] The stabilityo f1 decreases as wellu nder atmos-pheric conditions, that is, withoutf lushing the sample with argon, and oxidative side-reactions originating from singlet oxygen are supposed to be responsible for this degradation. However,s uch ad ependence on the presence of oxygen was not found for DAE 2.
These results indicate that ring closure does not exclusively occur from the triplet state for 2o in contrast to 1o.T of urther investigate this behavior,t ransienta bsorption spectroscopy was used to get insight into the excitation pathways. Whereas diarylethene cyclization from the singlet state is known to be very fast, occurring in the hundreds of femtoseconds range, [12c,d] triplet excited states are typically long-livedi nt he microsecondt ime window.F or this reason, first the behavior of 1 and 2 was investigated using nanosecond flashphotolysis, that is, nano-to microsecond UV/Vis transientabsorption spectroscopy.
The exact set-up has been detailed elsewhere, [19] but in this work for instance: An initial nanosecondl aser pulse of 1mJ with aw avelength of l = 355 nm was applied to samples of the DAEs 1 or 2,r espectively,d issolved in acetonitrile with a concentration adjusted to an absorbance of A = 1a ti rradiation wavelength (5-7 10 À5 m). The absorbance difference( with laser minus withoutl aser) evolution for different wavelengths was monitored. The initial pulse had awidth of 8nanoseconds, and only triplet excited state and photo-product evolution are observed in this time scale.
As olution of 1o was prepared under degassed conditions by flushing with argon and wasm easured accordingly.T he initial formation of ab road positive transient band at around l max = 680 nm could be observed as well as an egative absorption that can be ascribed to the bleaching of the stationary band of the open isomer at around 390 nm (Figure 3, top left). Both bands decay with atimeconstant of t = 2.9 msthat is typical for the triplet excited state of the parallel conformer (see also Figure S4 in the Supporting Information). [12a] In accordance with the stationarye xperiments,a fter 50 mst he initial spectrum is not fully recovered, but an ew band appearsa ta round 580 nm corresponding to the formation of the closed isomer 1c.O nly one time constant is found for the decay of the antiparallela nd parallel triplet excited state. Assuminga nequal quantum yield for the formation of the ISC for both conformers, the depopulationb and recovery is characteristico fr elaxation to the open form (without cyclization) from the parallel triplet excited state, cyclizationa nd relaxation to the open form from the antiparallel triplet excited state. About 30 %o f the depopulationr emains that is in agreementw ith the cyclization quantum yield of compound 1 (Table1). Noteworthy, the transient profile can be fitted accurately by only one decay time constant andn od eviating behavioro ft he parallel and antiparallel conformers was observed. The spectrum of the closed isomero btained from the transient data of 1 can be reproduced well by the differences pectrum of 1c and 1o from the stationary experiment, verifying the correct fitting (Figure 3, top right).
As econd sample of 1o was now measured under air in non-degassed conditions. The presence of oxygen as quencher should have asignificant effect on the lifetimeofthe triplet excited state and indeed, the spectral pattern obtained for the non-degassed solution is decaying with t = 200 ns and thus much faster than under argon (see Figure S3 in the Supporting Information). The complete recovery of the depopulation band as well as the absence of an ewly formed band at the end of the recorded time (5 ms) are in agreementw ith the low quantum yield under non-degassed conditions. Samples of 2o were prepared analogously as for 1o and measured under degassed conditions. Upon excitation with l = 355 nm, at ransient absorption spectrum with ad epopulation band and ap ositive band at 580 nm waso bserved (Figure 3, bottom left). The positiveb and hast he same maximum as the absorption spectrumo ft he closed form but has a shoulder at 480 nm. The shoulder feature and depopulation decay with atime constant of t = 3.8 msand is assignedt ot riplet relaxation of the parallel form. [12] After its decay,t he difference absorption spectrum of the closed form remains. It shouldb es tressed here that the absorbance at 580 nm is constant and in agreement with the quantum yield measurements. These results suggest ring closure to occur directly from the singlet exciteds tate in case of 2o.Furthermore, upon irradiation under non-degassed conditions, the spectral features of 2c appear immediately with the initial pulse while no intermediate long-lived transient absorption band indicating the presence of at riplet excited state could be observed( compare FigureS14 in the Supporting Information). Similart ot he literature, the triplet transient band observedi nt he degassed sample could be ascribed mainly to the parallel conformation in solution, which is not able to undergo ring closure.
Formation of the triplet excited state and closed form of 2 occur on the sub nanosecond times cale. Femtosecond transient absorption experiments were thus conducted to have a more detailed insight into their formation.
The data obtained for ad egassed sample of 1o excited with a1 50 fs pulse at 320 nm are summarized in Figure 4( top left). The initial transient absorption spectrum at 0psi sc haracterized by ab road positive band with am aximum at around 560 nm assigned to the FC state (a hot, vibrationallysinglet excited state) and the depopulationb and. It is relaxing in 160 fs to the cold singlet excited state comprising as ignature with a maximum at 500 nm. This vibrational relaxation is much faster than the subsequentI SC with ac haracteristic time of 5.9 ps towards the triplet state with ab road maximum at around 670 nm and the presence of an isosbestic point at 590 nm, sig-  nature of asequentialmechanism.Arather short time constant like this was observed before for organic triplet sensitizers such as benzophenone. [20] The data can be accurately fitted by sequential dynamics. It means that the parallel and antiparallel forms have quite similard ecay times and spectral separation is not possible. The picoseconds time constant for the decay of the singlet excited state is in agreement with the absence of cyclization from this state. (Scheme 2,top).
In contrast, compound 2o shows distinct decay times for its conformers during the relaxation process in the femtosecond range ( Figure 4). First, the two FC states evolve simultaneously with the initial pulse to as uperposition band located roughly at 500 nm. The transient absorption spectrums howed ab road positive absorption band that is growingi na bout 650 femtoseconds with three maximaa t4 30, 460 and 500 nm with at ail > 600 nm (0.5 ps spectrum). This is characteristic for the formation of both S 1 AP (antiparallel)a nd S 1 P (parallel) relaxeds tates from FC states. At the same time, the closed isomer is formed from the antiparallel Franck-Condon state. Theb and at 460 nm disappearsand is thus assigned to the decay of S 1 AP towards the ground state. Then, in few tens of picoseconds,t he bands at 430 and 500 nm decay and ab road transient absorption remains equal to the first spectra measured at 100 ns in the other setup. Therefore, the bands located at 430 nm and 500 nm are assigned to the S 1 P state. The singlet excited state of the parallel conformer is slowly undergoing ISC to the triplet excited state with at ime constant of t = 48 ps. These findings are in agreement with the resultso btained from the nanosecond measurements and confirmt hat ring closure of 2o,i n contrast to 1o,o ccurs via the singlet excited state.
The ring openinge vent was also analyzed as well using femtosecond to millisecond transienta bsorption spectroscopies.
As expected, no reactive triplet excited state was found in case of 1c upon excitation with femtosecond 640 nm light (see Figure S10 in the Supporting Information). As the closed isomer does not comprise different conformers or other simultaneously excited isomers, no other long-lived species corresponding to any triplet state were observed. As imilar behavior was found for compound 2c (see Figure S13 in the Supporting Information). Upon initial femtosecondl aser pulse excitation with 640 nm, aF Ca bsorption band was formed and relaxed fast (t = 170 fs) towards the singlet and also rather fast towards the ground state and some open isomer with ad ecay time of 1.5 ps. No contribution, neither from long-lived bands nor from the electronically separated biacetyl moiety,w as observed. The isolated biacetyl presumably keeps its original spectralf eatures with af orbidden transition at the blue edge of the visible range. The quantitative conversiont ot he closed isomer 2c at 405 nm despite the reasonable quantum yield of ring-opening at 577 nm (Table1)m ight be ar esult of the different nature for orbitals excited upon irradiating into another absorption band of 2c at 405 nm.
The overall excitation pathways derived from the transient absorption measurementso fD AEs 1 and 2 are summarized in Scheme 2. Exceptf or the specific decay times, the main difference between these two derivatives arises from their major cyclization pathways, which occurs via the triplet manifold for 1o carrying the terminally attachedb iacetyl units but via the singlet manifold for 2o with an inner biacetylu nit. The reason for this might be the rotation of the inner biacetyl unit out of the plane of the thiophenem oiety in 2o due to steric hindrance and al ower energetic coupling of the orbitals similarly to the reason mentioned for the blue-shifted absorption as compared to 1o.T herefore, irradiationi sl eading to al ocalized excitation at the hexatriene scaffold,t he ISC is suppressed at least in the antiparallel conformation, and ring-closure occurs from the singlet manifold as usual for DAEs.

Conclusions
The spectroscopicf eatureso ft wo DAE photoswitches bearing biacetyl triplet sensitizers were investigated in detail using stationary and transient spectroscopy in the nanosecond to microseconda sw ell as the femtosecond to picosecond range. We could show that establishing p-conjugation between the photoswitchable hexatriene core and the biacetyl triplet sensitizer does not necessarily lead to as witching via the triplete xcited state, which is known to preventb yproduct formation. On the contrary,t he decoupling of the sensitizer by converting the open to the closed isomer can drastically increase the efficiency of the ring opening, which is originating from the singlet excited state in both investigated switches.

Experimental Section
Synthetic procedures and characterization data including stationary and transient photochemistry are detailed in the Supporting Information.