Excited state intramolecular double-proton transfer dynamics of [2,2′-bipyridyl]-3,3′-diol inside mesoporous silica nanochannels

The modulation of kinetics and pathways in the ESIPT process of proton transfer probes holds significant potential for advancing applications in bio-imaging, drug delivery, and OLEDs. One effective approach for achieving this modulation is altering the H-bonding donating capability of the surrounding medium. To investigate this, we conducted a comprehensive study on the excited state intramolecular double proton transfer process of [2,2′-bipyridyl]-3,3′-diol (BP(OH)2) within the confined spaces of silica nanochannels, namely, MCM-41. MCM-41, known for its versatile properties, has emerged as a promising host in various fields, such as drug delivery and heterogeneous catalysis. Upon encapsulation within the MCM-41, the double proton transfer process of BP(OH)2 is significantly modulated, which is reflected in both steady-state and time-resolved photophysical experiments. We have observed an almost 100 times increment in emission intensity and a 30 nm blue-shift in the emission maxima when the probe gets encapsulated inside the silica nanopores. Most importantly, the femtosecond up-conversion profile exhibits an interesting feature. The rise component of 10 ps, which was attributed to MK→DK conversion in bulk acetonitrile (MeCN), is not observed when the probe resides inside the MCM-41, suggesting the proton transfer is concerted rather than sequential, like in the case of bulk MeCN. This anomalous proton transfer mechanism inside the nanochannel was attributed to the weak H-bonding donating ability of the silanol groups, which could not stabilize the MK form, and thus favoured the concerted pathway over sequential. Moreover, DFT calculations corroborate the concerted pathway observed in the MCM-41 with the gas-phase calculations and the sequential mechanism observed in bulk MeCN with the solution-phase calculations.