Chiral exciplex dyes showing circularly polarized luminescence: extension of the excimer chirality rule

A series of axially chiral binaphthyls and quaternaphthyls possessing two kinds of aromatic fluorophores, such as pyrenyl, perylenyl, and 4-(dimethylamino)phenyl groups, arranged alternately were synthesized by a divergent method. In the excited state, the fluorophores selectively formed a unidirectionally twisted exciplex (excited heterodimer) by a cumulative steric effect and exhibited circularly polarized luminescence (CPL). They are the first examples of a monomolecular exciplex CPL dye. This versatile method for producing exciplex CPL dyes also improved fluorescence intensity, and the CPL properties were not very sensitive to the solvent or to the temperature owing to the conformationally rigid exciplex. This systematic study allowed us to confirm that the excimer chirality rule can be applied to the exciplex dyes: left- and right-handed exciplexes with a twist angle of less than 90° exhibit (-)- and (+)-CPL, respectively.

Donor-acceptor complex formation in tetra-n-butylammonium chloride: n-decanoic acid deep eutectic solvent

Complex formation between pyrene (Py) and N,N-dimethylaniline (DMA) is presented in a deep eutectic solvent constituting of tetra-n-butylammonium chloride (TBAC) and n-decanoic acid (DA) in a 1:2 mol ratio, respectively, named TBAC:DA. The addition of DMA to a Py solution of TBAC:DA results in the formation of a fluorescent Py-DMA charge-transfer complex, which is manifested via a broad structureless bathochromically shifted band centered at 550(±2) nm. The solvatochromic nature of the Py-DMA fluorescent complex indicates the solvent polarity of TBAC:DA to be higher than that of methanol. The absence of a negative pre-exponential factor in the intensity decay at 550 nm combined with the excitation scans implies the presence of weak interaction between Py and DMA in the ground-state, leading to the rapid formation of a Py-DMA complex possibly at a sub-nanosecond time scale. The Stern-Volmer quenching constant (K_SV) varies from 53(±2) to 96(±1) M^-1, and the bimolecular quenching rate constant (k_q) varies from 3.0(±0.4) × 10⁸ to 8.8(±1.3) × 10⁸ M^-1 s^-1 by increasing the temperature (T) from 283.15 to 313.15 K, implying efficient deactivation of electron-acceptor Py in the excited-state induced effectively by the electron-donor DMA within TBAC:DA. ln k_q varies linearly with 1/T with an activation energy (E_a) of 26.4(±0.4) kJ mol^-1. The linear behavior between k_q and 1/η suggests conformity to the Stokes-Einstein relationship within TBAC:DA. The Py-DMA complex formation efficiency increases with an increase in T and reaches maxima at 298.15 K before decreasing with a further increase in T. The initial reduction in η favors Py-DMA complex formation; this effect is overcome by preferential thermal deactivation of the Py-DMA fluorescent complex as compared to that of pyrene.

Folded short azapeptide for conformation switching-based fluorescence sensing

Dipeptide-based amidothiourea (azapeptides) bearing folded β-turn structures were employed to build beacon-like fluorescent sensors signalled by pyrene exciplex or excimer dual emission to allow highly sensitive ratiometric fluorescence sensing of an anion able to bind to the structural moiety placed within the turn structure.

Enhancement of fluorescence quenching and exciplex formation in DNA major groove by double incorporation of modified fluorescent deoxyuridines

5-(1-Naphthalenylethynyl)-2'-deoxyuridine ((N)U) and 5-[(4-cyano-1-naphthalenyl)ethynyl]-2'-deoxyuridine ((CN)U) were synthesized and incorporated into oligodeoxynucleotides. Fluorescence emissions of modified duplexes containing double (N)U were efficiently quenched depending upon the sequence pattern of the naphthalenes in DNA major groove, as compared to the duplex possessing single (N)U. When one of the naphthalene moieties has a cyano substituent, the exciplex emission from the chromophores in DNA major groove was observed at longer wavelength.

SNP detection for cytochrome P450 alleles by target-assembled tandem oligonucleotide systems based on exciplexes

We report the first use of exciplex-based split-probes for detection of the wild type and *3 mutant alleles of human cytochrome P450 2C9. A tandem 8-mer split DNA oligonucleotide probe system was designed that allows detection of the complementary target DNA sequence. This exciplex-based fluorescence detector system operates by means of a contiguous hybridization of two oligonucleotide exciplex split-probes to a complementary target nucleic acid target. Each probe oligonucleotide is chemically modified at one of its termini by a potential exciplex-forming partner, each of which is fluorescently silent at the wavelength of detection. Under conditions that ensure correct three-dimensional assembly, the chemical moieties on suitable photoexcitation form an exciplex that fluoresces with a large Stokes shift (in this case 130 nm). Preliminary proof-of-concept studies used two 8-mer probe oligonucleotides, but in order to give better specificity for genomic applications, probe length was extended to give coverage of 24 bases. Eight pairs of tandem 12-mer oligonucleotide probes spanning the 2C9*3 region were designed and tested to find the best set of probes. Target sequences tested were in the form of (i) synthetic oligonucleotides, (ii) embedded in short PCR products (150 bp), or (iii) inserted into plasmid DNA (approximately 3 Kbp). The exciplex system was able to differentiate wild type and human cytochrome P450 2C9 *3 SNP (1075 A-->C) alleles, based on fluorescence emission spectra and DNA melting curves, indicating promise for future applications in genetic testing and molecular diagnostics.

Detection of nucleic acids in situ: novel oligonucleotide analogues for target-assembled DNA-mounted exciplexes

This research describes the effects of structural variation and medium effects for the novel split-oligonucleotide (tandem) probe systems for exciplex-based fluorescence detection of DNA. In this approach the detection system is split at a molecular level into signal-silent components, which must be assembled correctly into a specific 3-dimensional structure to ensure close proximity of the exciplex partners and the consequent exciplex fluorescence emission on excitation. The model system consists of two 8-mer oligonucleotides, complementary to adjacent sites of a 16-mer DNA target. Each probe oligonucleotide is equipped with functions able to form an exciplex on correct, contiguous hybridization. This study investigates the influence of a number of structural aspects (i.e. chemical structure and composition of exciplex partners, length and structure of linker groups, locations of exciplex partner attachment, as well as effects of media) on the performance of DNA-mounted exciplex systems. The extremely rigorous structural demands for exciplex formation and emission required careful structural design of linkers and partners for exciplex formation, which are here described. Certain organic solvents (especially trifluoroethanol) specifically favour emission of the DNA-mounted exciplexes, probably the net result of the particular duplex structure and specific solvation of the exciplex partners. The exciplexes formed emitted at approximately 480 nm with large Stokes shifts ( approximately 130-140 nm). Comparative studies with pyrene excimer systems were also carried out.