No abstract

A series of π-expanded coumarins comprising of 4–5 conjugated rings were designed and synthesized. The strategic placement of two dialkylamino groups containing long alkyl chains attached to the peripheral ends of bis-coumarins resulted in dyes with superb solubility. As α,β-unsaturated ketones, these compounds display properties of donor–acceptor–donor (D–A–D)-type chromophores. Photophysical studies of the new functional dyes revealed a combination of favorable properties: strong absorption of blue and green light, weak fluorescence, reasonable two-photon absorption (2PA) cross-section, and complete solubility in nonpolar solvents. The fluorescence lifetimes of coumarin-derived α,β-unsaturated ketones were measured for the first time. The placement of two amine groups at peripheral positions of the dyes produced two-photon absorption cross-section values at the level of 150–400 GM around 800 nm, which generated two-photon photoinitiation. The highest 2PA cross-section was approximately 400 GM for the derivative of 4-methylcyclohexanone. Directly using these compounds as sensitizer or initiator, two (2D)- and three-dimensional (3D) nanopatterns were successfully fabricated by two-photon initiated polymerization. 3,3′-Carbonyl-biscoumarin, which contains two dihexylamino substituents at positions 7 and 7′ possesses the largest fabrication window. MC3T3-E1 preosteoblastic cells exhibited strong adherence to all π-expanded coumarins and the same spindle-shaped morphology as the tissue culture treated polystyrene control surface. Additionally, our results showed an increase in cell proliferation after 3 and 7 days in culture, as well as a high cell viability of approximately 100% on all materials compared to the control surface. These findings confirm that D–A–D-type ketocoumarin derivatives used as potential photoinitiators are noncytotoxic and can be used in the fabrication of biomaterial scaffolds for tissue engineering applications.

Suppressing the charge recombination (CR) that follows an efficient charge separation (CS) is of key importance for energy, electronics, and photonics applications. We focus on the role of dynamic gating for impeding CR in a molecular rotor, comprising an electron donor and acceptor directly linked via a single bond. The media viscosity has an unusual dual effect on the dynamics of CS and CR in this dyad. For solvents with intermediate viscosity, CR is 1.5-3 times slower than CS. Lowering the viscosity below ∼0.6 mPa s or increasing it above ∼10 mPa s makes CR 10-30 times slower than CS. Ring rotation around the donor-acceptor bond can account only for the trends observed for nonviscous solvents. Media viscosity, however, affects not only torsional but also vibrational modes. Suppressing predominantly slow vibrational modes by viscous solvents can impact the rates of CS and CR to a different extent. That is, an increase in the viscosity can plausibly suppress modes that are involved in the transition from the charge-transfer (CT) to the ground state, i.e., CR, but at the same time are not important for the transition from the locally excited to the CT state, i.e., CS. These results provide a unique example of synergy between torsional and vibronic modes and their drastic effects on charge-transfer dynamics, thus setting paradigms for controlling CS and CR.

A set of thioxanthone derivatives bearing strongly electrondonating dialkylamino groups have been synthesized efficiently via the Buchwald−Hartwig, Sonogashira, and Heck reactions. Compounds possessing both one and two donors have been prepared. Their optical properties proved to be intriguing. The presence of the amino group alone increased the fluorescence quantum yield 100 times. Dyes possessing arylethylene and arylethynyl linkages absorbed UV radiation and violet light while emitting (weakly) red light, as a consequence of different geometries at ground and excited states. Our design led to an increase in two-photon absorption cross sections at the wavelength of interest (800 nm) to the highest values ever recorded for thioxanthones, while maintaining excellent solubility due to the fact that all new photoinitiators are highly viscous oils at room temperature. Some of the prepared D−π−A−π−D systems also proved to be initiators in two-photon polymerization.