Thiophenylazobenzene: An Alternative Photoisomerization Controlled by Lone‐Pair⋅⋅⋅π Interaction

Abstract Azoheteroarene photoswitches have attracted attention due to their unique properties. We present the stationary photochromism and ultrafast photoisomerization mechanism of thiophenylazobenzene (TphAB). It demonstrates impressive fatigue resistance and photoisomerization efficiency, and shows favorably separated (E)‐ and (Z)‐isomer absorption bands, allowing for highly selective photoconversion. The (Z)‐isomer of TphAB adopts an unusual orthogonal geometry where the thiophenyl group is perfectly perpendicular to the phenyl group. This geometry is stabilized by a rare lone‐pair⋅⋅⋅π interaction between the S atom and the phenyl group. The photoisomerization of TphAB occurs on the sub‐ps to ps timescale and is governed by this interaction. Therefore, the adoption and disruption of the orthogonal geometry requires significant movement along the inversion reaction coordinates (CNN and NNC angles). Our results establish TphAB as an excellent photoswitch with versatile properties that expand the application possibilities of AB derivatives.


Introduction
Theoperation of molecular photoswitches is based on the reversible transformation of the switching molecule between states of different physicochemical properties (for example, geometrical structure,d ipole moment, absorption spectrum, or redox potential). [1] Theu tilization of light as at rigger allows easy manipulation, which, combined with the instantaneous property change of the molecule,makes photoswitches extremely attractive for applications in chemical science and technology. [2] Azobenzenes (AB) are ap rominent group of widely utilized photoswitches.T hey are highly fatigue-resistant and relatively easy to synthesize. [3] Thephotochromism of ABs is based on the EQZ photoisomerization of the central N=N bond, which results in substantial geometric and spectral changes. [4] Spectrally,A Bs are characterized by as trong p!p*t ransition band (S 2 ), typically located in the 300-350 nm range for the (E)-isomer and the 250-290 nm range for the (Z)-isomer.T hey also show aw eakly allowed n!p* transition band (S 1 )i nt he 400-450 nm range for both isomers.T he photoisomerization quantum yield (QY) of ABs is excitation-wavelength-dependent:Q Y E(p!p*)!Z % 13 %, QY E(n!p*)!Z % 29 %, QY Z(p!p*)!E % 24-36 %, QY Z(n!p*)!E % 47-51 %. [4,5] The E!Z photoisomerization of ABs proceeds on the sub-ps to ps timescale after both n!p*a nd p!p*e xcitations.After S 2 excitation, ultrafast ( % 100 fs) relaxation to the S 1 state is observed. [6] Thedecay of the S 1 state is described by two lifetimes ( % 400 fs and % 2ps), originally attributed to direct and diffusive motion from the Franck-Condon region to the conical intersection with the ground state. [6b,c,7] Relaxation lifetimes after S 1 excitation are slightly different, indicating that the relaxation pathways are differently populated. [6b,d] Recently,o nly the longer ( % 2ps) S 1 decay component was associated with E!Z isomerization, while the shorter one ( % 400 fs) was assigned to an onreactive relaxation pathway from ar egion on the S 1 potential energy surface (PES) that is accessible only after S 2 excitation. [6f] While this result designates an important tendency, it does not yet explain the presence of as imilar ( % 400 fs) lifetime component after S 1 excitation. Also,additional pathways may be involved, making ac lear distinction between the timescales of the relaxation pathways in the ultrafast data hard. [8] In fact, lifetime-distribution analysis has shown that the two S 1 relaxation lifetimes discussed above belong to ab road % 700 fs lifetime distribution. [6e] This indicates as ignificant overlap of the timescales of the reactive and the nonreactive pathways. [8] The Z!E photoisomerization is ultrafast with amain lifetime of % 150 fs and aminor contribution of % 1ps lifetime. [6b,d,7a] In both isomerization directions,r elaxation to the ground state is followed by vibrational cooling (10-20 ps lifetime). [6b,e,f,7a, 9] Thep hotoisomerization mechanism of ABs has been analyzed extensively by quantum-chemical calculations. [4] In the latest works,aconsensus arises that inversion-assisted rotation is the dominant photoisomerization mechanism, particularly in the condensed phase. [6f, 8, 10] Thel ower isomerization QY after p!p*( S 2 )e xcitation was attributed to an onreactive internal-conversion channel from the S 1 to the ground state accessible only after S 2 !S 1 relaxation [6f, 10f] or to anonreactive channel due to crossing of the S 2 and S 3 PESs. [11] In contrast to the conventional, well-studied ABs,t he photochemical properties of azoheteroarenes remain largely unexplored. Recent reports reveal that azoheteroarenes,like arylazopyrazoles and arylazopyrroles,c an adopt different ground-state conformations stabilizing or destabilizing the (Z)-isomer,w hich allows for ar emarkable tunability of the thermal relaxation rate from seconds to % 1000 days. [12] In other azoheteroarenes,i mpressively high thermal-relaxation rates on the microsecond [13] and even the nanosecond [14] timescale have been achieved. Furthermore,d ue to distortions because of steric effects,t he spectral properties of azoheteroarenes are altered. This often results in afavorable separation of the (E)-and the (Z)-isomer bands that permits > 98 %c onversion in each direction. [12a,b,d] Evidently,c hoice, position, and orientation of the heteroaryl and its substituents represent an ew tuning dimension for ABs.M oreover,t he presence of heteroatoms permits new functional designs previously unavailable in conventional ABs.T herefore, azoheteroarenes offer an untapped potential for further optimization and expansion of the capabilities of AB photoswitches.
In this work, we explore ad ifferent azoheteroarene design, where one of the phenyl groups of aconventional AB is substituted by athiophenyl group.W epresent its synthesis along with the theoretical and experimental investigation of the photoisomerization of this thiophenylazobenzene (TphAB) photoswitch.

Results and Discussion
Synthesis Thei nvestigated TphAB 2 was synthesized based on the coupling of aryldiazonium salts with aryllithium compounds [15] (Scheme 1). Herein, thiophene (1)w as readily lithiated at the 2-position and subsequently added to a4methylphenyldiazonium tetrafluoroborate suspension at low temperature to obtain TphAB 2 in 67 %yield (see Supporting Information).

Photochromic Properties
All spectroscopic experiments with TphAB were performed in acetonitrile.T he absorption spectrum of the thermodynamically stable (E)-isomer of TphAB shows ad ominant p!p*a bsorption band at 365 nm and aw eaker n!p*b and at % 450 nm ( Figure 1A,B). Irradiation of the (E)-isomer with 365 nm leads to E!Z photoisomerization of the central N = Nb ond ( Figure 1C). The( Z)-isomer is characterized by two main absorption bands:i )a p!p* absorption band at % 285 nm, which has about half the intensity of the (E)-isomer p!p*b and (365 nm);a nd ii)an n!p*band at % 450 nm, which is only about as intense as the one of the (E)-isomer.D ue to the identical spectral location and intensity of the n!p*b ands of the (E)-and the (Z)isomers,the ZQE photoconversion of TphAB is achieved by irradiation in the corresponding p!p*absorption bands.
The E!Z conversion under 365 nm irradiation is extremely efficient, resulting in less than 3% (E)-isomer in the photostationary state PSS 365 ( Figure S5, Supporting Information). This unusually high photoconversion level is due to the favorable separation of the (E)-isomer p!p*band from the (Z)-isomer absorption band in TphAB.T he (Z)-isomer content in PSS 285 is also very low ( % 13 %). TheQ Y determination (see Supporting Information and ref. [16]) for the two photoisomerization reactions of TphAB after p!p* excitation reveals some of the highest ever reported QYs for an AB system, with an impressive QY E(p!p*)!Z of % 44 %and QY Z(n!p*)!E of % 65 %. These QYs are even significantly higher than the ones typically reported for n!p*e xcitation of the conventional ABs. [4] TphAB shows very high fatigue resistance to repeated photoswitching.A fter 50 photocycles equivalent to % 9h of high-intensity light exposure, % 3% degradation at most is detected ( Figure S1).
The Z!E thermal-relaxation time of TphAB is considerably shorter than that of AB,w ith ah alf-life of % 120 min (20 8 8C). Thet emperature dependence of the thermal relaxation rate was determined at four different temperatures   Figure S5 for the pure (Z)-isomer spectrum);C)T phAB isomerization;D)Eyring plot for the thermal Z!E relaxation (see also  Table S1 for the used parameters). between 10 8 8Ca nd 40 8 8Ct oo btain the thermodynamic parameters for the corresponding transition state (Figure 1D).

Quantum-Chemical Calculations
Theoretical calculations using the second-order algebraic diagrammatic construction scheme for excitation energies (ADC(2)) [17] and linear-response time-dependent density functional theory (TDDFT; [18] see Supporting Information) were performed to gain insight into the molecular properties of the studied TphAB.Aground-state geometry optimization of TphAB yielded two stable geometries for both the (E)-and the (Z)-isomer with different rotational orientations of the thiophenyl ring with respect to the azophenyl group-TphAB-1 and TphAB-2 (see Figure 2, left). Similar to AB, the (E)-isomers of TphAB are planar. However, the (Z)isomers of TphAB adopt ar ather unusual geometry where the thiophenyl ring lies in plane with the CNNC moiety,while the phenyl ring is either perfectly orthogonal to this plane (TphAB-1) or slightly twisted away from it (TphAB-2; Figure 2, left). This is in stark contrast to AB [4] and the related azothiophene, [19] where both rings are twisted away from the CNNC plane.Similar unusual geometries of the (Z)isomer have been reported for some nitrogen-based azoheteroarenes, [12a,b,d] but not for the conventional AB.I nterestingly,i tw as found that in these compounds,t he orthogonal geometry is disfavored when abulky substituent is present in ortho position to the CNNC group. [12d] Alternatively,t he orthogonal geometry is adopted when an H-atom is present in ortho position due to af avorable CÀH···p interaction. [12d] Remarkably,T phAB adopts an orthogonal geometry only when the S-atom, which is in ortho position, faces the phenyl ring (TphAB-1, Figure 2l eft), while the twisted geometry is realized when the S-atom is facing away from the phenyl group (TphAB-2, Figure 2l eft). Thed ifferences in the conformations adopted by TphAB and by nitrogen-based azoheteroarenes [12d] can be explained by differences in the attractive and repulsive interactions.I nT phAB-1, the orthogonal structure of the (Z)-isomer is stabilized by af avor-able interaction between the lone pair of the S-atom and the p-system of the phenyl ring (lone-pair···p interaction [20] ). Based on the results from azoheteroarenes, [12d] one would also expect the adoption of an orthogonal structure in the TphAB-2(Z)-isomer,where the ortho H-atom on the thiophenyl ring is involved in aC ÀH···p interaction. However,i nt his configuration (TphAB-2), where the S-atom is facing away from the phenyl ring, the lone pair of the S-atom comes into close contact with the lone pair of the N-atom of the azo group,which results in arepulsive interaction and the ensuing twisting of the thiophenyl ring.
Although the geometry optimization of TphAB yields two stable geometries,t he twisted and the orthogonal conformation of the (Z)-isomer,their Boltzmann distribution indicates that at room temperature,the orthogonal structure (TphAB-1) represents % 99.5 %o ft he population. Interestingly,t he Boltzmann distribution of the (E)-isomers also shows that the TphAB-1 configuration is dominant ( % 97.2 %). Thee xcitation energies for all isomers obtained from the theoretical calculations ( Figures S3 and S4, using the ADC(2) and BHLYP methods) and the Boltzmann distributions were used to simulate the theoretical absorption spectra of the isomer mixtures (Figure 2, right, and Figure S5). These calculated spectra reproduce the experimental spectra of the (E)-and (Z)-isomers of TphAB in acetonitrile ( Figure 1B)v ery well given an energy blue-shift of % 0.61 eV.F or the (E)-isomer, as trong transition is present at % 320 nm, which has p!p* character (see Figure S2 for attachment/detachment densities). Then!p*transition is only weakly allowed in the (E)isomer and therefore the intensity in the > 400 nm range is small. Thec alculated spectrum of the (Z)-isomer of TphAB shows astrong contribution in the 250-300 nm region due to two p!p*t ransitions (Figure 2, right). Thea ttachment/ detachment densities for the dominant TphAB-1 configuration ( Figure S2) show av ery interesting character for these p!p*t ransitions:T he lower-energy transition is located entirely on the azothiophenyl group,while the higher-energy transition shows charge-transfer character from the phenyl to the thiophenyl group.Concerning the n!p*transition of the (Z)-isomer,i ta ppears that in the orthogonal geometry,t he transition is very weak and has negligible contribution to the absorption spectrum (Figure 2, right) in contrast to AB.T his result is in agreement with the experiential absorption spectrum ( Figure 1B), where the intensity in the > 400 nm region is very similar for both the (E)-and the (Z)-isomer. Noteworthy,i nt he twisted geometry of the (Z)-isomer, the n!p*t ransition is stronger ( Figure S3 and S4) due to the smaller angle between the plane of the nonbonding orbitals of the azo group and the azothiophenyl plane.Nevertheless,the contribution of the twisted geometry to the experimental spectrum is negligible as this configuration is essentially not present at room temperature (see above).

Ultrafast E!ZP hotoisomerization
The E!Z photoisomerization of TphAB after 355 nm excitation in the p!p*a bsorption band of the (E)-isomer was studied by ultrafast transient absorption spectroscopy Figure 2. Left:Geometry-optimized conformations adopted by TphAB. Right:a bsorption spectra calculated using the ADC(2) method (see Table S1 and Figures S3 and S4), taking into account the distribution of the conformations.
(see Supporting Information and ref. [16] for adescription of the pump-probe setup). Theu ltrafast dynamics (Figures 3A and S6 A) resemble that of the conventional AB. [6b,e] However,t he main excited-state absorption (ESA) bands appear shifted to the red by % 30 nm (to 510 nm and 420 nm). On the sub-250 fs timescale,t he ESA located at % 510 nm decays concomitantly with the rise of the ESA at 420 nm ( Figure S6). In turn, the decay of the 420 nm ESA proceeds on the early ps timescale but is not associated with acomplete recovery of the ground-state bleach (GSB) band at 365 nm. Instead, ah ot ground-state band contribution is observed on the lowerenergy side of the GSB,which decays on the 10 ps time scale. No product-band formation is visible in the transient absorption data in the detected wavelength range.This is due to the dominant absorption of the (E)-isomer in this spectral window,w hich obscures the formation of the (Z)-isomer. Nevertheless,t he formation of the (Z)-isomer can be indirectly deduced by the strong nondecaying GSB signal on the nanosecond timescale,w hich is indicative for the depopulation of the initial (E)-isomer.
Thee xperimental data were analyzed by lifetime-distribution analysis (see Supporting Information and ref. [21]) and the corresponding lifetime-distribution map is presented in Figure 3B.T he decay of the 510 nm ESA is described by a % 100-200 fs lifetime distribution (positive amplitude) and can be assigned to the decay of the initially excited p!p* state (S 2 )ofthe (E)-isomer into the n!p*state (S 1 ;negative amplitudes). Thelifetime distributions for this process are not fully resolved due to the limited time resolution ( % 100 fs) of the experiments.Based on similar studies of the conventional AB with higher time resolution, [6d,f] it can be expected that the lifetime of this relaxation is even shorter.T herefore,i no ur experiments,t his lifetime possibly overlaps with the lifetime describing the relaxation on the S 1 surface.T he decay of the 420 nm ESA, ascribed to the n!p*state (S 1 ), is characterized by ar elatively broad lifetime distribution centered at 950 fs (positive amplitude). Asimilar albeit slightly shorter lifetime distribution was found in AB. [6e] Therefore,t he decay of TphAB from the excited state S 1 to the ground state appears to be slightly slower.T ypically,this decay dynamics is fitted by two lifetime components [6b,f] via conventional global lifetime analysis. [21] However,l ifetime-distribution analysis indicates that those are artificially assigned to describe the rather broad distribution of relaxation processes.N evertheless,i tw as proposed that the slower S 1 relaxation pathways are reactive, while the fast ones are nonreactive. [6f, 8] In this respect, the shift of the corresponding lifetime distribution in TphAB towards longer lifetimes (dominance of the reactive pathways) may potentially explain the much higher isomerization QY (44 %, see above) as compared to AB ( % 10 %). [4,5] After the relaxation of the S 1 state,apair of an egative (365 nm) and ap ositive (405 nm) 10-20 ps elongated and tilted lifetime distributions are observed that describe the nonexponential cooling dynamics in the ground state.

Ultrafast Z!EPhotoisomerization
Theu ltrafast Z!E photoisomerization of TphAB was investigated after 455 nm excitation in the n!p*( S 1 ) absorption band of the (Z)-isomer. On the sub-250 fs timescale,t he transient-absorption data ( Figures 4A and S6 B) shows abroad ESA signal over the complete detection range.
This ESA is interrupted only by the GSB at % 450 nm (in the range of the n!p*absorption of the (Z)-isomer). This early ESA undergoes an ultrafast blue-shift, which results in complete ESA/GSB signal compensation and an ESA rise in the 365 nm range.T herefore,a fter % 200 fs,o nly positive transient absorption is present in the detected spectral range. TheE SA then decays on the sub-ps to the low ps timescale. This decay is most obvious above 430 nm, while below 430 nm, the ESA transforms into the product-absorption signature (330-420 nm). Theproduct absorption continues to grow strongly on the ps timescale and results in an intense long-lived product band associated with the efficient Z!E isomerization. Initially,the product band appears broader on the red ( % 400-450 nm) spectral side.However,this red side  undergoes ab lue-shift (from % 420 nm to 365 nm) on the 10 ps timescale due to the cooling of the hot ground-state product ( Figure 4A).
Theultrafast dynamics of the (Z)-isomer is exceptionally nonexponential, as illustrated by all lifetime distributions below 1-2 ps ( Figure 4B). Above 425 nm, apair of apositive and negative,t ilted and elongated lifetime distributions stretches from < 100 fs to % 1ps. They account for the ultrafast, nonexponential excited-state relaxation dynamics that proceeds from the S 1 Franck-Condon region towards the S 1 minimum and through the conical intersection with the ground state.S imilarly,i nt he 350-430 nm range,astrong, elongated, and tilted (from short to long wavelengths) negative lifetime distribution is present that also stretches from < 100 fs to 1-2 ps.T he shorter,b lue side of this distribution is associated with the early ESA-shift dynamics due to the relaxation on the S 1 PES,while the longer red side is associated with the S 1 to S 0 transition and the formation of the hot ground-state product band. Again, due to the strong nonexponentiality of the dynamics,t he relaxation on the S 1 and to the ground state cannot be observed as separate lifetime distributions.T he cooling dynamics of the hot ground-state photoproduct is described by apair of apositive and an egative distribution at % 8-20 ps around 350-430 nm ( Figure 4B).

Photoisomerization Mechanism
It is generally accepted that the torsion of the central CNNC moiety plays ad ominant role in the isomerization of ABs. [6f, 8, 10] Therefore,weperformed relaxed PES scans along the torsion coordinate of TphAB in the S 1 (n!p*) state ( Figure S7). Ther esulting PESs for the orthogonal (TphAB-1) and the twisted (TphAB-2) geometries resemble the shape of those in AB,w ith ac onical intersection with the ground state at aCNNC angle of % 908 8.
However,given the unconventional geometry adopted by the (Z)-isomer (Figure 2), we decided to further investigate the contribution of other reactive coordinates to the isomerization mechanism operating in TphAB.Inprinciple,nucleardynamics simulations would be required to obtain adynamical picture of the reaction mechanism, which are not feasible at present. Instead, we performed unconstrained geometry optimizations for both the (E)-and (Z)-TphAB-1 (orthogonal geometry) starting from the corresponding Franck-Condon geometries in S 1 (see the Supporting Information for details). These calculations mimic the structural relaxation processes and identify the relevant reaction coordinates.T he shape of the PESs obtained along theses optimizations allows for conclusions with respect to the dynamic mechanism. The optimizations bring the molecule from the Franck-Condon region to the conical intersection with the ground state.A t this point, we switched the unconstrained optimization to the S 0 PES to obtain the complete isomerization pathway.T he optimizations of (E)-and (Z)-TphAB-1 unveil contributions of further coordinates beyond the typical CNNC torsion. For clarity,w en ame the angle on the thiophenyl side of the molecule CNN,w hile NNC corresponds to the angle on the phenyl side and examine the changes along these two coordinates ( Figures 5, 6, S8 and S9;T ables S2 and S3).

Isomerization Mechanism of (E)-TphAB-1
Theunconstrained optimization of (E)-TphAB-1 in the S 1 state shows ar elatively large increase ( % 158 8)i nb oth the CNN and the NNC angles,while the CNNC angle remains at % 1808 8 (Figures 5and S8). This is followed by alarge decrease of the CNNC angle to 1168 8 and, simultaneously,aminor decrease in the CNN and the NNC angles to % 1258 8 ( Figure 5, point 3, and Figure S8). At this point, the unconstrained optimization converges to alocal minimum at aCNNC angle of 908 8 due to the presence of asmall barrier on the S 1 PES on the way to the conical intersection with the ground state. From this point, we performed constrained geometry optimizations with fixed CNNC angles between 1168 8 and 908 8 to reach the conical intersection. During this torsion of the CNNC angle,the NNC angle increases back to % 1328 8,while the CNN angle decreases further to % 1208 8.AtaCNNC angle of 908 8,wecontinued the unconstrained optimization on the S 0 PES to obtain the complete isomerization pathway.T he optimization shows agradual decrease of the CNNC angle to 08 8,a nd of the NNC angle to 1228 8 (Figures 5a nd S8 A). In contrast, the NNC angle first increases from % 1208 8 to % 1308 8 to finally equilibrate at 1278 8 ( Figure 5). Thec hanges in the CNN and NNC angles in the ground-state optimization indicate that the molecule initially attempts to adopt atwisted conformation, which apparently is not accessible,a nd thus it moves towards the orthogonal conformation, which is stabilized by the lone-pair···p interaction.
Overall, the E!Z photoisomerization from S 1 is dominated by the CNNC-torsional reaction coordinate.However, this torsional motion appears to be assisted by significant changes along the inversion-reaction coordinate (D > 158 8 for ]CNN and ]NNC). These changes essentially adjust the relative position of the phenyl and the thiophenyl rings,a nd result in the adoption of the orthogonal configuration by the (Z)-isomer.Furthermore,this additional degree of freedom is most likely the reason for the observed ( Figure 3B)s lightly longer S 1 lifetime of (E)-TPhAB compared with (E)-AB [6e] (lifetime distributions centered at 950 fs and 700 fs,r espectively).

Isomerization Mechanism of (Z)-TphAB-1
Theu nconstrained relaxation of (Z)-TphAB-1 from the Franck-Condon region in S 1 is initially dominated by alarge opening motion of the NNC angle,which increases by % 358 8 to % 1588 8 (Figures 6a nd S9). During the last 58 8 of this opening, the torsion of the CNNC moiety is activated and the CNNC angle quickly reaches 308 8.T he initial changes of the CNN angles are minor (< 58 8). Thef ollowing geometric changes towards the conical intersection with the ground state are governed by the torsion of the CNNC moiety,w hich brings the CNNC angle from 308 8 to 908 8.T his torsional motion is accompanied by ad ecrease in the NNC angle to 1308 8.F rom the conical intersection, the unconstrained optimization proceeds on the ground state towards the (E)-isomer. This relaxation is associated with adecrease of both the CNN and the NNC angles to 1158 8 and ac oncomitant increase of the CNNC angle to 1808 8.Similar to the E!Zphotoisomerization direction, the Z!E direction is also dominated by the torsional motion about the CNNC moiety.H owever,h ere the inversion-reaction coordinate plays an even more important role,aswedetected much larger changes in the CNN and NNC angles (D = 258 8 and D = 428 8,r espectively) than during the transformation of the (E)-to the (Z)-isomer.
This photoisomerization mechanism of (Z)-TphAB-1 can be straightforwardly explained considering the orthogonal geometry of the isomer and the lone-pair···p interaction (see above). Essentially,t oi nitiate the isomerization, first ad isruption of the lone-pair···p interaction is required. Such adisruption would then effectively free the torsional reaction coordinate.T he disruption of the lone-pair···p interaction is achieved via the opening of the NNC angle,which pulls the Satom of the thiophenyl ring away from the plane of the phenyl ring. Therefore,the initial motion on the S 1 PES is led by the NNC opening. Apparently,above an NNC angle of 1508 8,the strength of the lone-pair···p interaction is sufficiently reduced and the CNNC torsion is activated. From this point, the mechanism is governed by the torsional reaction coordinate, while the CNN and the NNC moieties work towards planarization of the TphAB molecule.T he complex changes that (Z)-TphAB-1 undergoes along the S 1 PES also explain the strongly nonexponential dynamics observed in the transient absorption data (sub-250 fs timescale) discussed above.S uch nonexponential dynamics is not present in the relaxation of (Z)-AB. [6e] Conclusion Theu ncommon properties of azoheteroarenes have drawn significant attention as an alternative design to the popular AB. [22] However,t hose studies have focused mostly on the investigation of nitrogen-based azoheteroarenes.Here we present ad etailed study on the photochromism of ad ifferent TphAB photoswitch. We show that the TphAB Figure 6. Top: Example geometries from the unconstrained optimization of (Z)-TphAB-1 illustratingthe isomerization mechanism on the S 1 PES. Bottom:P ES obtained from the unconstrained optimization of (Z)-TphAB-1 started in the Franck-Condon region on S 1 .Note:C NN is the angle on the thiophenyl side of the molecule, while NNC is the angle on the phenyl side.
photoswitch has outstandingly high photoisomerization QYs (QY E(p!p*)!Z = 44 %, QY Z(n!p*)!E = 65 %.), photoconversion levels (PSS 365 contains only % 3% (E)-isomer,w hile PSS 285 contains only % 13 %(Z)-isomer), and fatigue resistance.Our theoretical calculations demonstrate that the (Z)-isomer of TphAB adopts ag eometry where the thiophene ring is perfectly orthogonal to the phenyl ring, with the S-atom facing the phenyl ring. This orthogonal geometry is stabilized by arare lone-pair···p interaction between the S-atom and the phenyl ring. We reveal that while the ultrafast photoisomerization of TphAB occurs on atimescale similar to that of AB, the corresponding dynamics is remarkably rich. Thetorsional motion about the CNNC moiety is the dominant reaction coordinate.H owever, the formation and disruption of the unusual orthogonal geometry of the (Z)-isomer requires significantly larger changes along the inversion coordinate (]CNN and ]NNC) than those discussed in AB. [10h] Therefore,t he presence of the lone-pair···p interaction and the ensuing orthogonal geometry add an additional degree of freedom compared to AB,w hich evidently alters the photochemistry of TPhAB.
Our work delivers important insight into the molecular basis of the photoisomerization mechanism operating in TphAB,w hich is relevant not only to azoheteroarenes but also to conventional ABs.W ea lso establish TphAB as an excellent photoswitch with versatile properties,inmany ways better than the once widely utilized conventional AB and many of its derivatives.