Photophysical Properties of Pyrene-Containing Biphotochromic Dyads and Corresponding Cyclobutanes Formed from Dyads in the [2+2]-Photocycloaddition Reaction

The photophysical properties of biphotochromic dyads DoX and D10, containing two identical photochromes 2-[2-(pyren-1-yl)ethenyl]quinoline (PEQ), linked together by bridging groups of different lengths, and the corresponding dipyrenylcyclobutanes CBoX and CB10, formed from the dyads in the [2+2]-photocycloaddition (PCA) reaction, have been studied by means of time-resolved emission spectroscopy (TRES). Based on TRES data, the number of emitters has been determined and their emission spectra, excited-state lifetimes, and rate constants of competing physical and chemical processes (radiation, energy transfer, reactions) have been calculated. The appearance of emitters with a significantly longer lifetime compared to the model PEQ photochrome has revealed the formation of excimers in the dyads, which are possible intermediates of the PCA reaction. In cyclobutanes, a decrease in the lifetime of pyrene substituents compared to 1-methylpyrene has been shown to indicate energy transfer from the substituents to the cyclobutane ring, which initiates the ring opening (retro-PCA) reaction via the predissociation mechanism. In addition, nonemitting conformers have been shown in CBoX. Quantum-chemical calculations using the DFT method have confirmed the possibility of forming different conformers of cyclobutane CBoX, which differ both in the relative position of the pyrenyl substituents and in the degree of their interaction with each other.