Photoinduced electron transfer in host-guest interactions of a viologen derivative with a didansyl-pillar[5]arene

A Multiresponsive Calix[6]arene Pseudorotaxane Empowered by Fluorophoric Dansyl Groups

“…The major one was assigned to an interwoven structure in which TDA assumes a partial cone (pC) conformation P[TDA(pC)�DOV]2OTs with an inversion point associated with a ring bearing a methoxy group (Figure 2b). This geometry was confirmed by its typical NMR pattern, which includes: i) a down-field shift of the methoxy groups that gave rise to two signals in a 1 : 2 ratio (£* + $*); ii) the split of the two doublets (ax + eq) A/A' of the methylene bridge in TDA into six doublets, with geminal coupling, in a 1 : 1 ratio; iii) a significant shift of the 13 C resonances for the α/α' couple to δ = 35.6 ppm (Figure S13 in Supporting Information). [15] Further NMR investigation allowed us to confirm the geometry of the second minor species as the cone (C) conformer P[TDA(C)�DOV]2OTs (Figures S16 and S17).…”

“…Upon titration of TDA with DOV ⋅ 2OTs the absorption spectrum shows small changes on the dansyl band (Figure 5a): the expected charge transfer band arising from the interaction between the DOV guest and the calixarene cavity, [5,18] is hidden beneath the absorption band of the dansyl moieties, which undergoes a slight red shift. The luminescence is quenched (Figure 5b), most likely because of the photoinduced electron transfer from the dansyl units to the bipyridinium guest [13,19] …”

“…The luminescence is quenched (Figure 5b), most likely because of the photoinduced electron transfer from the dansyl units to the bipyridinium guest. [13,19] Upon fitting the data with a 1 : 1 model, an association constant as large as 8 × 10 6 M À 1 is obtained (Figure 5b and Table 2), in analogy with parent triphenylureido calix [6]arene. [5] On the other hand, upon titration of the protonated TDA with DOV • 2OTs the appearance of the charge transfer band can be detected just below 400 nm (Figure 5c).…”

“…From the photophysical and electrochemical experiments reported above, two features of the system must be highlighted: i) the emission of the dansyl moieties in the pseudorotaxane complex is quenched, most likely by virtue of photoinduced electron transfer from the excited dansyl groups to the bipyridinium unit; [13,19] ii) reduction of bipyridinium results in disassembly of the pseudorotaxane complex. Taken together, these results prompted us to verify the possibility to control the de-threading reaction by means of photoinduced electron transfer.…”

“…Furthermore, the amine units of the dansyl group can undergo protonation reactions, [12] which can be exploited to alter the complexation properties of the macrocycle. As it is well known that pseudorotaxanes based on the calixarene‐bipyridinium recognition motif can be destabilised by reduction of the guest component, the disassembly of the supramolecular complex could be obtained both by electrochemical reduction [5] and photoinduced electron transfer from the dansyl units [13] . Overall, our aim is to exploit the versatility of the newly devised wheel in a multiresponsive pseudorotaxane system [2] operated by three different, orthogonal, and reversible stimuli, acting on both the ring and the axle (Figure 1).…”