Rotations in Excited ICT States – Fluorescence and its Microenvironmental Sensitivity

Abstract

Fluorescence spectroscopy as an analytical tool for various applications as well as fluorescence probes showing a sensitive behavior towards their microenvironmental properties have experienced an increasing interest in research in the last decades. Recently our group has contributed to this field of interest by introducing a novel mechanism where a planarization in the excited state occurs leading to an advantageous allowed emission. Here, we provide an overview of intramolecular charge transfer states where a rotation in the excited state causes a high sensitivity for microenvironmental changes. Furthermore, effects which influence the surroundings of the fluorophores are described in detail. Recent results where twisted intramolecular charge transfer (TICT) and aggregation‐induced emission (AIE) effects were combined are described and a promising outlook for further research progress is provided.

Control of Excited‐State Conformations in B,N‐Acenes

“…The classic example in this respect is dimethylaminobenzonitrile (DMABN; Figure ): In the excited state, the amine rotates around the carbon–nitrogen bond thereby maximizing charge transfer, which leads to dual emission . This phenomenon has been termed twisted intramolecular charge transfer (TICT), and has also been observed in other nitrogen‐containing compounds …”

“…However, it has been shown early on that the B−N rotational barrier (29.3 kcal mol −1 ) of aminoborane is much smaller than that of the C=C bond in ethylene (62.5 kcal mol −1 ) . Interestingly, analogous behavior has been observed for aminoborane theoretically as well as experimentally . In the excited state, the conformation resulting from a 90° rotation around the B−N bond is not the lowest in energy, though.…”

Control of Excited‐State Conformations in B,N‐Acenes

“…[2,4] This phenomenon has been termed twisted intramolecular charge transfer (TICT), and has also been observed in other nitrogencontaining compounds. [3,[5][6][7][8] Similar considerations can be made for one of the smallest possible nitrogen-containing compounds with such aproperty, aminoborane (H 2 NÀBH 2 ;F igure 1). TheB ÀNs caffold has been discussed as C=Cs urrogate because of its isoelectronic character.…”

“…[3,15] Interestingly,a nalogous behavior has been observed for aminoborane theoretically [4,16] as well as experimentally. [5][6][7][8]15] In the excited state,t he conformation resulting from a908 8 rotation around the B À Nbond is not the lowest in energy,though. Further relaxation by pyramidalization is more favored owing to negative hyperconjugation [2,[9][10][11][12][13][14]17] resulting in at wisted out-of-plane arrangement of the amine.…”

Regioisomeric BODIPY Benzodithiophene Dyads and Triads with Tunable Red Emission as Ratiometric Temperature and Viscosity Sensors

“…Moreover, fluorescent molecular temperature/viscosity sensors are advantageous for biological applications because of their noninvasive nature . Among the various design strategies for small organic molecule based ratiometric temperature‐ and viscosity‐probes, donor‐acceptor (D‐A) rotor molecules that show twisted intramolecular charge transfer (TICT) are of particular interest . In recent years, multifunctional donor‐acceptor TICT rotors with large Stokes shift, pronounced solvatochromism, aggregate induced emission, temperature and viscosity sensing have been reported .…”