Temperature dependence of the conversion efficiency of photochromic perylene bisimide dithienylcyclopentene triads embedded in a polymer.

Photochromic molecules that are covalently linked to a strong fluorophore combine the requirements of external control and strong fluorescence, which will become increasingly important for super-resolution microscopy techniques based on single molecules. However, given the bulky structure of such constructs, steric hindrance might affect their photoconversion efficiencies upon immobilising them for imaging purposes. In this study the efficiencies of the photochromic conversion processes of molecular triads that are embedded in a polymer have been studied as a function of temperature. The triads consist of two perylene bisimide dye molecules that are connected via a dithienylcyclopentene photochromic bridge that undergoes a cyclization/cycloreversion reaction upon appropriate illumination. It is found that photochromic switching remains active, even at 5 K, yet with reduced but finite efficiency for the cycloreversion reaction. This might even be an advantage for the achievement of high labelling densities in super-resolution microscopy.

Deliberate Switching of Single Photochromic Triads.

Photochromic molecules can be reversibly converted between two bistable conformations by light, and are considered as promising building blocks in novel macromolecular structures for sensing and imaging techniques. We have studied individual molecular triads consisting of two strong fluorophores (perylene bisimide) that are covalently linked via a photochromic unit (dithienylcyclopentene) and distinguished between deliberate switching and spontaneous blinking. It was verified that the probability for observing deliberate light-induced switching of a single triad (rather than stochastic blinking) amounts to 0.8 ± 0.1. In a few exceptional cases this probability can exceed 0.95. These numbers are sufficiently large for application in sensitive biosensing, and super-resolution imaging. This opens the possibility to develop devices that can be controlled by an external optical stimulus on a truly molecular length scale.

Optical gating with organic building blocks. A quantitative model for the fluorescence modulation of photochromic perylene bisimide dithienylcyclopentene triads.

We investigated the capability of molecular triads, consisting of two strong fluorophores that were covalently linked to a photochromic molecule, for optical gating. Therefore we monitored the fluorescence intensity of the fluorophores as a function of the isomeric state of the photoswitch. From the analysis of our data we develop a kinetic model that allows us to predict quantitatively the degree of the fluorescence modulation as a function of the mutual intensities of the lasers that are used to induce the fluorescence and the switching of the photochromic unit. We find that the achievable contrast for the modulation of the fluorescence depends mainly on the intensity ratio of the two light beams and appears to be very robust against absolute changes of these intensities. The latter result provides valuable information for the development of all-optical circuits which would require to handle different signal strengths for the input and output levels.

A photoswitchable poly(3-hexylthiophene).

A well-defined main-chain conjugated photoswitchable poly(3-hexylthiophene) with homogeneous hydrogen end groups was synthesized. Opening and closing the photoswitch enables reversible modulation of P3HT emission, which is quenched by 70% when the switch is closed. The optical properties during switching cycles were quantified by irradiation/spectroscopy sequences.