Fine tuning the optical properties and Frontier orbital energy in pyrrolylenones as an example of D–π–A sensitizers via functionalization of the molecular backbone

Abstract

Using pyrrolylenones as examples of D–π–A push–pull molecules with intramolecular charge transfer, the common principles of tuning the HOMO and LUMO energy levels and the HOMO–LUMO gap via the functionalization of the molecular backbone are established. The attachment of the electron-donating groups to the pyrrole moiety as a D unit, and the joining of the electron-withdrawing groups to the bridging moiety and/or to the carbonyl group as an A unit, results in a considerable lowering of the HOMO and LUMO energy levels and a reduction in the HOMO–LUMO gap. In addition, a bathochromic shift of the λ_max absorption wavelength from the near UV to the red zone of visible light is stimulated. The functional dependencies of the values of HOMO and LUMO energies, the HOMO–LUMO gap and λ_max absorption wavelength on the σ_p Hammett constants of the substituents at the pyrrolylenone backbone were found. The strength of the push–pull effect in the pyrrolylenone π-scaffold was quantitatively characterized via the molecular tailoring approach. The effects of lowering the HOMO and LUMO energy levels, the contraction of the HOMO–LUMO gap and the bathochromic shift of the λ_max absorption wavelength were shown to be associated with a strengthening in the push–pull effect in pyrrolylenones. Guided by the observed effects of pyrrolylenone backbone functionalization on the Frontier orbital energy characteristics and optical properties, the photoelectric performance and light harvesting efficiency of organic sensitizers can be improved. In addition, the dependence of photovoltaic performance on the width of the Δ(HOMO–LUMO) gap and the λ_max absorption wavelength for a series of pyrrolylenones was studied. It is recognized that the Δ(HOMO–LUMO) gap can be contracted by ca. 1 eV while the λ_max absorption band can be red shifted by ca. 90 nm without a critical loss of the photovoltaic performance of the sensitizer. Thus, the acceptable boundaries of the Δ(HOMO–LUMO) gap narrowing and the bathochromic shift of the λ_max wavelength due to the functionalization of the backbone should be maintained in order to avoid a critical loss of the sensitizer photovoltaic performance.