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UV/Vis-spectra of ring-open isomers of 11a, 11b, 11c, and 12 were recorded in acetonitrile (Figure 31, Table 13). Besides an intense absorption in the UV originating from the aromatic thiazole moieties the structurally related compounds 11a, 11b, and 11c show a broad absorption band in the visible range between 350 – 500 nm. This band can be attributed to the charge- transfer from the electron-rich morpholinothiazoles to the strongly electron-deficient tert- butylmaleimide bridge (vide infra). Compared to the parent structure 11a, for compounds 11b and 11c the intensity of the CT-band is significantly lower and its position is shifted hypsochromically by 40 nm and 10 nm, respectively, reflecting the reduced electron-density of the CF3 substituted morpholinothiazole cores.

As proposed in the literature for analogous dithienylmaleimides,[39a] _{such a pronounced}

CT-behavior between the aryl moieties and the bridge moiety of DAEs can lead to a diminished photochemical cyclization efficiency due to the formation of a TICT (Twisted Intramolecular Charge Transfer) excited state, which is characterized by a pronounced twisting of the single bond between the electron donating and the electron accepting parts of the molecule.[40] _{In fact,}

during irradiation of a yellow solution of 11a in acetonitrile with UV light (irr = 280 nm) or

with visible light (irr= 436 nm) no changes in the UV/Vis spectrum could be observed

(Figure 31a). Furthermore, analysis of the reaction mixture by UPLC did not show any new photoproduct. Only when exposed to UV irradiation over an extended time period using a 1000 W Xe arc lamp with a broad UV bandpass filter some unspecific degradation (bleaching)

N _O O N S R1 N O S N R2 N O N S N N S N N O O O O R2 R1 R1 _{= R}2 _{= CH} 3: 11a 11b N S N N S N N O O O O R2 R1 + 2e- - 2e- - 2e- R1 _{= R}2 _{= CF} 3: R1 _{= CH} 3, R2 = CF3: 11c N S N O S N N O S N N S N N O O 12 S N N S N N O O + 2e- - 2e- - 2e- UV UV Vis Vis

took place. According to the TICT-model, the usage of cyclohexane as a non-polar solvent allowed for the photochemical cyclization reaction to take place to some extent (Figure 32, Table 14). Nevertheless, its efficiency was extremely low with a quantum yield of 0.05 for the ring-closure of 11a and a conversion of only 16% in the PSS. A similar lack of photochemical reactivity for the ring-open isomers was also observed for the CF3 substituted derivatives 11b

and 11c.

Figure 31. UV/Vis-spectra during the course of irradiation of acetonitrile solutions of a) 11a, b) 11b,

c) 11c, and d) 12 with UV light (280 nm, 1000 W Xe, interference filter). Insets show UV/Vis spectra in acetonitrile during the irradiation with visible light (436 nm, 1000 W Xe, interference filter) of the respective isolated ring-closed isomers 11a(c), 11b(c), and 12(c), which were prepared separately via oxidation of the ring-open compounds. All concentrations 5.010-5 _M.

Table 13. Photophysical properties of DAEs 11a, 11b, 11c, and 12 in acetonitrile.

comp.

λmax / nm

(ε / 104 _M-1 _cm-1_{) Φ}

AB

(280 nm)a _{(436 nm)}ΦBA a _{(280 nm)}PSS b ring-open isomer ring-closed isomer

11a 265 (2.31), 430 (0.39) 364 (1.40), 447 (1.29) < 0.001 0.13 < 1% 11b 276 (2.01); 390 (0.18) 361 (0.89); 454 (1.29) < 0.001 0.37 < 1% 11c 269 (1.89), 420 (0.25) n.d.c _{< 0.001 n.d.}c _{< 1%}

12 252 (2.11), 268 (shoulder) 408 (1.47) 0.13 0.26 36%

a) Quantum yields obtained from the initial slope of photoconversion (500 W Xe(Hg), azobenzene and Aberchrome 670 actinometry).

b) Conversion to the ring-closed isomer in the PSS reached after UV irradiation (280 nm, 1000 W Xe). c) Compound 11c(c) was not isolated.

Figure 32. UV/Vis spectra during the course of irradiation of solutions of a) 11a and b) 11b in

cyclohexane (c = 510-5 _{M) with UV-light (}

irr = 280 nm, 1000 W Xe) until reaching the PSS.

Table 14. Photophysical properties of DAEs 11a and 11b in cyclohexane.

comp.

λmax / nm

(ε / 104 _M-1 _cm-1_{) Φ}

AB

(280 nm)a _{(280 nm)}PSS b ring-open isomer ring-closed isomer

11a 266 (2.44), 425 (0.42) 356 (1.04), 443 (1.02) 0.05 16%

11b 270 (2.07), 373 (0.23) 449 (1.16) 0.05 16%

a) Quantum yields obtained from the initial slope of photoconversion (500 W Xe(Hg), azobenzene).

b) Conversion to the ring-closed isomer in the PSS reached after UV irradiation (280 nm, 1000 W Xe).

Nevertheless, the ring-closed isomers 11a(c) and 11b(c) could be isolated via oxidation of the respective ring-open isomers (see section 4.1.5). They show an intense absorption in the visible region split into two bands (insets of Figure 31a-b). Upon illumination of acetonitrile solutions of 11a(c) and 11b(c) with visible light (irr= 436 nm) both bands diminish rapidly and the

spectra of the respective ring-open compounds are obtained. Although the spectra of the ring- closed and the ring-open isomers overlap significantly at the irradiation wavelength complete conversion to the ring-open isomers is achieved, as the latter are photochemically inactive. The quantum yields for the ring-opening of 11a(c) and 11b(c) with 436 nm light were determined to be relatively high with values of 0.13 and 0.37, respectively. Notably, the ring-opening process is significantly enhanced for compound 11b(c), which may be attributed to the strong electron- withdrawing character of the CF3 groups attached on the reactive carbon atoms.[31b], 27

Contrasting this unidirectional photochemical switching behavior of DAEs 11a, 11b, and 11c, bearing a maleimide bridge, compound 12, possessing a cyclopentene as bridging unit, shows photochemical bidirectionality to some extent (Figure 31d). UV/Vis spectra as well as UPLC-traces recorded during irradiation of a colorless solution of 12(o) in acetonitrile with UV

27 _{See also section 4.5.3 for photochemical properties of α-CF}

light (irr= 280 nm) indicate the formation of the ring-closed isomer 12(c). However, only 36%

of 12(c) are formed in the PSS due to a high ring-opening quantum yield of 0.26 while the ring- closing quantum yield was determined to be 0.13. Obviously, the lack of a charge transfer between the thiazole moieties and the cyclopentene bridge, indicated by the absence of a visible absorption in the case of the ring-open isomer, restores the reversibility of the photochemical reaction typical for DAEs. An intramolecular charge transfer interaction is clearly essential to install unidirectional photochemical behavior.

To get further insight into the photochemical processes that lead to the observed lack of cyclization efficiency theoretical investigations were performed.28 _{Ground state structures were}

optimized, vertical excitation energies were calculated, and structures in the first excited state were optimized using (TD-)DFT on the CAM-B3LYP/PCM(acetonitrile)/6-31G(d) level of theory. The weak, broad band in the visible range for ring-open isomers of 11a, 11b, and 11c is reproduced by vertical excitation energies and can be attributed to a HOMO→LUMO transition. Isocontour plots of the frontier molecular orbitals (exemplarily shown for 11a in Figure 33, left) show that the HOMO and LUMO are localized on the morpholinothiazoles and the maleimide bridge, respectively. Note also that for the LUMO, which dominates the photochemical pericyclic reaction according to the Woodward-Hoffman rules, only small coefficients are found on the ring-closing carbon atoms. For compounds 11b and 11c the HOMO energies are lowered due to the electron accepting CF3 groups, reproducing the experimentally observed blue shift of

the CT band. For compound 12 no such CT transition is found and the HOMO and the LUMO are delocalized over the whole hexatriene backbone (Figure 33, right). Notably, for compounds

11a and 11c geometry optimization in the first excited state lead to stable minimum structures

showing a certain twist between the thiazole rings and the maleimide bridge, while for 12 no stable minimum could be found. Instead the calculations converged towards a conical intersection with a decreased distance between the ring-closing carbons. However, in gasphase calculations the same behavior was observed for 11a, making a clear assignment of TICT states by theory difficult.

28 _{All quantum mechanical calculations discussed in this section were performed by Manuel Utecht in the}

group of Prof. Peter Saalfrank, Department of Chemistry, Universität Potsdam. They shall be briefly discussed here, for details the reader is referenced to: M. Herder et al., Chem. Sci. 2013, 4, 1028-1040.

Figure 33. Isocontour plots of frontier molecular orbitals for compounds 11a (left) and 12 (right)

obtained on the CAM-B3LYP/PCM(acetonitrile)/6-31G(d) level of theory.