Investigation of the Geometric and Spectroscopic Properties of Four Twisted Triphenylpyridinium Donor–Acceptor Dyes

Abstract

The geometric and spectroscopic properties of four cationic N-aryl-2,4,6-triphenylpyridinium-based donor–acceptor dyes1-[4-(9H-carbazol-9-yl)­phenyl]-2,4,6-triphenylpyridinium, 1-[4-(N,N-diphenylamino)­phenyl]-2,4,6-triphenylpyridinium, 1-(9-phenyl-9H-carbazol-3-yl)-2,4,6-triphenylpyridinium, and 1-(9-ethyl-9H-carbazol-3-yl)-2,4,6-triphenylpyridiniumare reported. The four dyes exhibited a twisted, quasi-perpendicular geometry about the central donor–acceptor bond, shown by X-ray crystallography and supported by Raman spectroscopy and DFT calculations. The electronic absorption spectra show weak charge transfer (CT) transitions at about 400 nm (ε ∼ 3000 L mol–1 cm–1). Time dependent (TD) DFT supported the nature of the CT transition, displaying an 89–97% shift in electron density from the donor to the acceptor upon electronic excitation. Excited state geometry calculations revealed significant geometry changes upon electronic excitation. Enhancement of vibrational modes attributable to this transition was also recognized in the resonance Raman spectra. Emission spectroscopies showed two distinct emission bands. The lower energy band, resulting from radiative decay of the CT excited state, exhibited large anomalous Stokes shifts of ∼9000 cm–1. Much of the Stokes shift was a consequence of geometry changes between the ground and excited states. This was confirmed by variable temperature emission studies, with Stokes shifts reducing by up to 3000 cm–1 upon cooling from 293 to 80 K. Additionally, a high energy aggregation induced emission band was present for two of the dyes, resulting from the inhibition of excited state geometry reorganization and supported by solid-state emission spectra. These phenomena exemplify the importance of geometry in short range donor–acceptor dyes such as these.

Extended Reichardt's Dye–Synthesis and Solvatochromic Properties

“…For 2D the two slopes are approximately the same, with the lines only differing by their intercepts. Such differential behaviour has been previously observed in derivatives of 1B and other D–A dyes [4,10,15,17,18] . It is typically attributed to differences in the dyes’ abilities to form hydrogen bonds compared to the 1B reference, which is dependent on the donor phenolate moiety [4,15] .…”

“…Such differential behaviour has been previously observed in derivatives of 1B and other D-A dyes. [4,10,15,17,18] It is typically attributed to differences in the dyes' abilities to form hydrogen bonds compared to the 1B reference, which is dependent on the donor phenolate moiety. [4,15] In this case, the more localised charge of the phenolate increases its ability to form hydrogen bonds.…”

Systematic Tuning of Electronic Ground and Excited States in Donor–Acceptor Dyes; Steps toward Designer Compounds for Modern Technologies

“…In all six of the compounds, the HOMO was consistently localized to the donor and linker region while the LUMO was situated on both the acceptor and linker moieties, as shown in Figure for the p -Cbz-dttp-IndO and m -Cbz-dttp-IndO dyes and in Table S9. Such confinements for the FMOs involved in CT transitions are typical in organic donor–acceptor dyes. ,− Spatial localization was also observed in the HOMO – 1 and LUMO + 1 orbitals in each dye, and these orbitals were likely involved in additional CT* transitions shown in emission spectroscopy ( vide infra ). The LUMO + 1 MOs were spatially contained over the acceptor units for all dyes.…”

“…Conversely, the CT* transition exhibited blue shifting upon cooling to 80 K, which possibly resulted from the hindrance of geometric relaxation in the electronic excited state upon cooling, as seen in previous investigations. 16,62,63 Additionally, blue shifting of this band caused overlapping with the nonthermochromic π−π* transitions, making them indistinguishable.…”

“…In these transitions, electron density is displaced from an electron-rich donor unit to an electron-poor acceptor unit in response to photoexcitation. This electron displacement results from the localization of the frontier molecular orbitals (FMOs), whereby, to a first approximation, the highest occupied molecular orbital (HOMO) is typically based on donor units and the lowest unoccupied molecular orbital (LUMO) is confined to acceptor groups. − Thus, the energy of the CT transitions can be tuned by altering the donor and acceptor moieties. ,,− Additionally, π-linking groups also provide an avenue for the tuning of the CT. By altering the length or level of conjugation, the HOMO–LUMO band gap can be adjusted. ,− A systematic approach to tuning these features is to alter the substitutions within each of these respective units, allowing for finer control of the FMO energies and hence the band gap of key CT transitions. − Larsen et al showed that the localization of the FMOs in these systems also lent predictability to the effects of substitutions made to each unit. Substitutions on the donor moiety led to tuning of the HOMO, while those on the acceptor tuned the LUMO.…”