Redox-driven porphyrin based systems for new luminescent molecular switches

Abstract

In this work, we explore the possibility of tuning the fluorescence intensity of two porphyrin systems through the electrochemical oxidation of an appended ruthenium acetylide bridge. Two electrochemically switchable systems, a dyad (ZnP-Ru, 3) and a triad (ZnP-Ru-P2H, 5), were prepared and investigated. In the ZnP-Ru dyad, the fluorescence of the zinc porphyrin was switched reversibly between the ON and OFF state upon the oxidation of the ruthenium unit, the most probable quenching process involved after oxidation being the electron transfer from the singlet excited state of ZnP to the oxidized ruthenium center. In the ZnP-Ru-P2H triad, we show that both porphyrins' fluorescence are highly quenched independent of the redox state of the ruthenium bridge owing to the efficient photoinduced charge transfer within the ruthenium complex.

Fluorenylporphyrins functionalized by electrochromic ruthenium units as redox-triggered fluorescence switches

“…The first oxidation step (from o. c. to 1 V) clearly involves the oxidation of the Ru center as discussed above. Similarly to dyad C, 7 oxidizing the Ru center does not enhance the fluorescence as in previously reported systems, 3,4 but conversely leads to further quenching.…”

“…Furthermore comparison between the emission spectra of 3 and 6 and the absorption spectra of 4 and 5 (see Figure S20a-b) reveals that singlet energy transfer from the porphyrin toward the Ru(II) endgroup is not relevant. 30,43 In contrast, the available CV data suggests that hole-transfer to a ruthenium-based ΜΟ leading to a π* Por ←d Ru singlet MLCT state will constitute a quite likely process, as previously stated for the closely related dyad C. 7 Interestingly, the TD-DFT calculations on 1 5 and 2 fail to identify this charge-separated (CS) state among the first ones computed, in line with a forbidden or poorly allowed vertical Di-cations. From our SEC data ( Figure S13), 5 both the porphyrin core and the ruthenium alkynyl endgroup appear to be mono-oxidized in the dications 1 2+ and 2 2+ .…”

“…Based on that statement, we believe that both neutral and mono- In the studies of related dyes in which a redox-active (organometallic) endgroup is connected to a porphyrin, electron-and energy-transfer processes are often put forward to rationalize the changes in luminescence relative to the unsubstituted reference porphyrin. 3,4,7,30 To investigate further the mechanism involved in the electrofluorochromic behavior of 1, UV-vis absorption spectroelectrochemistry was performed in a thin layer cell and the spectral changes were plotted on a differential scale to better evidence them (ESI; Figure S19). As already stated, 5 Ru(II) oxidation leads to very weak changes.…”

“…These sub-units can therefore be considered as presenting the same spectral signatures as the chromophores/luminophores constituting the dyad but taken Under such conditions the similarity of the porphyrin-based emission for a given dyad in its first two redox states (1 n+ or 2 n+ , n = 0,1) is not surprising. 3,7 Luminescence quenching processes in 1 and 2…”

“…6 While this study was underway, one of us independently verified that the redox-modulation of luminescence can actually be effected using porphyrins functionalized by electroactive [trans-Ru(dppe) 2 Cl(C≡C-)] or A c c e p t e d m a n u s c r i p t [trans-Ru(dppe) 2 (C≡CAr)(C≡C-)] units with dyad C, even when they present lower luminescence quantum yield in their than 1 Ru(II) state (Ф lum ≈ 0.4 % for C). 7 In these systems, the redox switching of luminescence operates in a reverse way to that reported for the dyads A and B. 8 Based on our experience with tetrafluorenylporphyrins, [9][10][11][12] the new dyad 2 was then targeted, in which the 1,4-phenylene linker is replaced by a 2,7-fluorenylene linker, with the expectation that this extended linker would slow down the rate of the photo-induced electron transfer at the origin of the low luminescence quantum yield in the Ru(II) state, 12 and would therefore show a more contrasted redox-switching compared to 1 (and C).…”