Circularly Polarized Luminescence and Circular Dichroism of Bichromophoric Difluoroboron-β-diketonates: Inversion and Enhanced Chirality Based on Spatial Arrangements and Self-Assembly

We synthesized two bichromophoric difluoroboron-β-diketonates (DFB) connected in para and meta positions by using cyclohexane diamine as a chiral bridge ( para and meta (R/S)-CyDFB ). TD-DFT calculations revealed that the variation in connectivity of the DFB units leads to different spatial arrangements and a chirality inversion of the bichromophoric DFB. Higher g_abs values were obtained in (R/S)-CyDFB connected in para as compared to meta position. Aggregation of para (R/S)-CyDFB in mixture of solvents increase the g_lum values as compared to its monomeric form. Ultrasonication and heating induced the formation of highly ordered nano-helical wires of para (R/S)-CyDFB that increased the g_lum values to 0.015. On the other hand, meta (R/S)-CyDFB failed to form highly ordered self-assembled wires due to hindered H-binding sites. These observations indicate that the chiroptical properties of DFB bi-chromophore system can be modulated with self-assembly and spatial arrangement of the chromophores.     

Pyrene bichromophores composed of polyaminopolycarboxylate interlink: pH response of excimer emission

Fluorescent response to pH has been studied on water-soluble pyrene-based bichromophores, (edtapy)H₂ and (dtpapy)H₃, in which two pyrenyl groups are linked by an ethylenediaminetetraacetate (EDTA) and a diethylenetriaminepentaacetate (DTPA) unit, respectively, through amide linkages. The excimer emission of the EDTA derivative is strengthened sharply with increasing pH at two steps; the first step is associated with the dissociation of acidic hydrogen from amino nitrogen in partially protonated species (edtapy)H^?  and the second step is attributable to the amide group. The excitation spectra have evidenced the formation of a static excimer in the ground state of completely deprotonated species (edtapy)^2 ? . The close contact between pyrenyl groups in (edtapy)^2 ?  has been confirmed by density functional theory, which has also shown that the close contact is broken when amino nitrogen is protonated. The DTPA derivative exhibits a strong excimer emission, which shows an intensity–pH profile of an ‘off–on–off–on’ type. This rare pH response is ascribable to multiple protonation sites in the DTPA chain, as confirmed by ¹H NMR. The novel pH-sensing capabilities in specific pH regions are due to the combination of the fluorescent group with the polyaminopolycarboxylate chains whose conformations are reversibly altered by protonation–deprotonation processes.     

Communication of Bichromophore Emission upon Aggregation – Aroyl-S,N-ketene Acetals as Multifunctional Sensor Merocyanines

Aroyl-S,N-ketene acetal-based bichromophores can be readily synthesized in a consecutive three-component synthesis in good to excellent yields by condensation of aroyl chlorides and an N-(p-bromobenzyl) 2-methyl benzothiazolium salt followed by a Suzuki coupling, yielding a library of 31 bichromophoric fluorophores with substitution pattern-tunable emission properties. Varying both chromophores enables different communication pathways between the chromophores, exploiting aggregation-induced emission (AIE) and energy transfer (ET) properties, and thus, furnishing aggregation-based fluorescence switches. Possible applications range from fluorometric analysis of alcoholic beverages to pH sensors.     

Aminorhodamine (ARh): A Bichromophore with Three Emission Bands in Low Temperature Glasses

At first glance, aminorhodamine (ARh) is a typical pH responsive fluorescent, rhodamine‐type dye. However, hidden under the typical rhodamine absorption band, ARh has another electronic transition of similar energy, but polarized orthogonal to that of the rhodamine chromophore. This transition—assigned to an arylpyrylium type chromophore contained in the system—is responsible for the sensor action of the dye. ARh is non‐fluorescent, while protonation of a donor amino group turn on a strong rhodamine‐type emission. At low temperature in frozen solution emission from both electronic subsystems of ARh are observed. In order to achieve more complete understanding of the photophysical mechanisms in this type of fluorescent probes, ARh and its protonated counterpart HARh were studied by absorption and fluorescence spectroscopy, computational chemistry, and at low temperatures in solid solution. Results from fluorescence anisotropy and time‐resolved fluorescence spectra establish a bichromophore model and suggest that a remarkable weak coupling between the two nearly isoenergetic excited states in ARh enables the dual emission. All the complicated properties observed for ARh was accounted for by a bichromophore model describing the electronic system of ARh as a bichromophore constituted by a rhodamine and an arylpyrylium subsystem.     

Tetramethoxy‐Aminorhodamine (TMARh): A Bichromophore,an Improved Fluorophore,and a pH Switch

Rhodamine is one of the most widely used fluorescent dyes. Here, a new synthetic pathway to the popular dyes is reported and the effect of adding four methoxy groups to the molecular structure is investigated. Tetramethoxy‐aminorhodamine ( TMARh ) is found to show superior pH switching compared to the rhodamine without the four methoxy groups, owing to changed properties of the dark “off” state and increased fluorescence intensity in the protonated “on” state.     

Formation of a complex nonlinear response in bichromophore solutions

A theoretical investigation of the dynamics of the stimulated emission of solutions of bichoromophores consisting of a donor with a high intersystem crossing probability and an acceptor between which a radiationless inductive-resonant energy transfer is possible is carried out. The complex dynamics of the behavior of the system is demonstrated, which makes it possible to use these chromophore ensembles as multistable devices with a memory of order of the triplet state lifetime. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 353–357, May–June, 1997.