The redox behavior of unsymmetrically substituted DAEs 5a, 10c, and 5k was investigated by cyclic voltammetry (Figure 44, Table 19). Compounds 5a and 10c represent the "mismatched" and "matched" configuration, respectively, for the transduction of electronic changes appearing within the DAE scaffold to the redox active N,N-dimethylaniline group. For both compounds the first oxidation processes are fully reversible in the ring-open and ring-closed forms and can be assumed to be located at the N,N-dimethylaniline groups. Note that, similar to symmetrically
N,N-dimethylaniline substituted compound 1a, for the ring-closed isomer of 10c a two-electron
oxidation wave is observed. From the difference of the anodic and cathodic peak currents it can be derived that two one-electron oxidation processes with a potential splitting of less than 100 mV are present.
Figure 44. Cyclic voltammograms (dE/dt = 1 V s-1) of DAEs a) 5a in acetonitrile/0.1M Bu
4NPF6, b) 10c
in acetonitrile/0.1M Bu4NPF6, and c) 5k in methylene chloride/0.1M Bu4NPF6. All concentrations were
110-3 M before irradiation. For compound 5a only low conversion to the ring-closed isomer could be
achieved after extended irradiation of the electrochemical cell for 2.5 h. Note that for a) and c) solutions became more concentrated during the irradiation procedure due to long irradiation times needed in case of a) and high volatility of the solvent in case of c).
Table 19. Anodic and cathodic peak potentials of 1a, 5a, 5k and 10c, determined by cyclic voltammetry
in acetonitrile or methylene chloride. All values are reported against the ferrocene/ferrocenium redox couple as external standard (rev = reversible, qr = quasireversible, irr = irreversible).
comp. open isomer closed isomer
Epa1 / V Epa2 / V Epc1 / V Epa1 / V Epa2 / V Epc1 / V
1aa 0.13 (irr) - < -2.80 -0.37 (rev) - -2.40 (irr) 5aa 0.22 (rev) 0.64 (irr) -2.71 (irr) -0.13 (rev) 0.09 (rev) -2.02 (rev) 5kb 0.93 (irr) - -2.21 (rev) 0.11 (rev) 0.46 (rev) -1.91 (rev) 10ca 0.40 (rev) 0.95 (irr) - 0.01 (rev) - -2.23 (irr) a) in acetonitrile
b) in methylene chloride
The potential of the first oxidation process of the ring-open isomer of 10c is higher by 180 mV compared to 5a and by 270 mV compared to 1a, which is in accordance with the initial expectation (Scheme 41) that the CF3 group in the α-postion of the thiophene ring would
decrease the electron density of the aniline moiety. However, given the much larger electronic effect of α-CF3 groups on the electron density within the DAE core itself, observed by
comparing compounds 1d and 8b as well as 3d and 9b (vide supra), it can already be seen that transduction of the electronic changes to an adjacent functional unit is significantly less pronounced. Note that compared to the ring-open form of 1a the first oxidation of 5a is also shifted by 90 mV to higher potentials, although both compounds are expected to possess the same redox behavior in the first step, assuming that in the ring-open form both termini are electronically isolated from each other.
Due to the coupling of the N,N-dimethylaniline moiety with the electron-withdrawing 3,5-bis(trifluoromethyl)phenyl group in the ring-closed isomer of 5a, it was expected that its first oxidation would appear at higher potentials than the first oxidation of the ring-closed isomer of 10c, for which the aniline is decoupled from the CF3 group. In contrast, it is
experimentally found that the oxidation of 5a(c) at Epa1=-0.13 V occurs at a slightly lower
potential than that of 10c(c) at Epa1 = 0.01 V. As a result, the difference between the two DAE
derivatives in the relative potential shift upon ring-closure is relatively small. Though it is larger in the "matched" compound 10c with a value of 390 mV, the shift of 350 mV for the "mismatched" compound 5a is only slightly smaller.
A similar potential shift between the ring-open and ring-closed isomer is observed for the reversible reduction of DAE 5k, which is located at the triphenyltriazine moiety. As this compound already represents the "matched" case, the synthesis of the mismatched analogue (Scheme 41) was not attempted.
4.5.6 Summary
It was demonstrated that the HOMO and LUMO energies of DAE photochromes can be precisely tuned over a broad range by donor/acceptor substitution in the periphery, variation of the bridging moiety, substitution with CF3 groups in the α-positions of the hetaryl rings, and
exchange of thiophene with thiazole heterocycles. Structures were obtained that show remarkable photoisomerization induced shifts of the energies of electronic levels of more than 1 eV. By creating a large collection of electronically modulated DAEs and by estimating the effects of structural modifications it is possible to select specific DAE derivatives or design new structures that fulfill energetic requirements for potential applications, in particular in combination with other optoelectronically active materials to reversibly enable and disable energy transfer or charge transport between the building blocks.
It was shown that transduction of the electronic changes induced within the DAE core upon isomerization to an adjacent, conjugated functional unit is much less pronounced. For model compounds possessing easily oxidizable aniline or reducible triazine moieties potential shifts between 300 – 400 mV were observed upon ring-closure. Thereby, different patterns of substitution with acceptor groups, inducing a "matched" or "mismatched" correlation to the electronic changes within the -electronic system of the DAE, turned out to have only a small influence. In light of the desired property changes of functional units attached to DAEs, such as binding motifs or catalytically active moieties, a larger modulation of electron density is needed. Therefore, the electronic communication between the functional unit and the DAE core should be increased by restricting dihedral twisting or moving the functional unit closer to the DAE core by omitting the phenyl group or using the functional unit itself instead of one thiophene or thiazole heterocycle. Furthermore, electronic modulation can potentially be increased using -M substituents instead of CF3 groups or by placing the acceptor group at the free β-position of the
thiophene ring. Research into these directions is currently ongoing in the Hecht group.
In addition to the remarkable electronic properties of α-CF3 substituted DAEs,
with α-CF3 groups on both hetaryl rings showed a strong reduction of the quantum yield for
ring-closure, while the quantum yield for ring-opening was significantly increased compared to the methyl substituted derivatives. Importantly, installing only one CF3 group on one hetaryl
ring restores the usual photochemical behavior of DAEs. The combination of a 4-N,N-dimethylaniline donor and a CF3 acceptor placed in the two α-positions of one thiophene
ring gave a DAE showing a marked bathochromic shift of the absorbance of the ring-open form and an increased quantum yield for ring-closure. A thorough investigation of the photochemical fatigue behavior of α-CF3 substituted DAEs has not yet been conducted.