The Coumarin‐Dimer Spring—The Struggle between Charge Transfer and Steric Interactions

Abstract

The synthesis of a weakly coupled, strongly polarized coumarin dimer has been achieved for the first time. The three-step strategy comprises the Skattebøl formylation followed by the Knoevenagel reaction and the formation of a tertiary amide by using a peptide-type procedure. The molecule consists of two different coumarin moieties: One is a classical donor-acceptor system and the second one possesses a weaker amide donor at the 7-position. The polarized coumarin dimer can form an electronically conjugated structure possessing an electric dipole larger than that of 7-(dimethylamino)coumarin-3-carboxylic acid. The limited flexibility of the inter-coumarin connection results in stable conformers of different electric dipole moments and complex photophysics. In the solid state, this compound has a strongly bent conformation with the two coumarin units forming an angle of around 74°. In solution, two conformers are in equilibrium. The existence of the molecule as two conformers in the ground state has been confirmed by optical studies, and further corroborated by molecular calculations. The fluorescence spectra possess a unique feature: A charge-transfer band (ca. 550 nm) is visible only in nonpolar or weakly polar solvents. Optical spectroscopy studies coupled with molecular calculations allowed us to rationalize this phenomenon: The large amplitude of intramolecular motions is responsible for the conformational isomerization as well as producing a conical intersection between the potential energy surfaces of the excited singlet state and the ground state, which opens an internal conversion channel that effectively competes with the fluorescence of the conformers.

Quadrupolar Dyes Based on Highly Polarized Coumarins

“… 6 A particularly important class of coumarin-based emitters are donor–acceptor systems possessing an electron-donating substituent at position 7 and an electron-withdrawing one at position 3. 7 , 8 It is well-known that 7-aminocoumarins exhibit excellent emissive properties. 9 Intriguingly, switching the position of an electron-donating substituent from 7 to 6 leads to marked differences in optoelectronic properties.…”

Polarized, V-Shaped, and Conjoined Biscoumarins: From Lack of Dipole Moment Alignment to High Brightness

“…The emission intensity is smaller, however, and it decreases sharply while moving to solvents with larger ε, although one can notice that dye 16 has a less sharp decrease in Φ F in a polar solvent, which resembles earlier observations by Ahn 36 as well as by Gryko and Sobolewski showing that coumarins possessing an ethylamino group at position 7 are fluorescent in polar environments. 37 …”

“…Results of the calculations presented in Table 2 reveal that all conjoined biscoumarins are highly polarized in the ground state and their dipole moments (≈14 D) are much larger than that of benzo[ g ]coumarin 9 (7 D) 33 or CoumMono (≈10 D) 37 structurally analogous to coumarins used for laser dyes and other purposes. 41 In the excited state, the dipole moment increases to values around 20 D or larger (again larger than CoumMono ( 37 ) and comparable to dye 9 , 33 Table 2 ) and the optical transition is fully allowed in absorption and in emission, suggesting possible large fluorescence quantum yield, which is indeed observed ( Table 1 ). Interestingly, the observed solvatochromic effects are rather moderate considering the high dipole moment.…”

Highly Polarized Coumarin Derivatives Revisited: Solvent‐Controlled Competition Between Proton‐Coupled Electron Transfer and Twisted Intramolecular Charge Transfer

“…There is a growing family of polarized molecular systems in which optical excitation to the 1 LE state can significantly increase the dipole moment of the system . Among these, a class of highly polarized molecular dyads composed of two coumarin moieties linked in a (donor–acceptor)–linker–(donor–acceptor) (DA‐L‐DA) orientation, recently studied by us, is particularly interesting. However, flexibility of the amide linker used in these studies results in very complex photophysical behavior, related to both thermo‐ and photochemical populations of numerous conformers.…”