Push-Pull Effect of Terpyridine Substituted by Triphenylamine Motive—Impact of Viscosity, Polarity and Protonation on Molecular Optical Properties

Abstract

The introduction of an electron-donating triphenylamine motive into a 2,2′,6′,2′′-terpyridine (terpy) moiety, a cornerstone molecular unit in coordination chemistry, opens new ways for a rational design of photophysical properties of organic and inorganic compounds. A push-pull compound, 4′-(4-(di(4-tert-butylphenyl)amine)phenyl)-2,2′,6′,2′′-terpyridine (tBuTPAterpy), was thoroughly investigated with the use of steady-state and time-resolved spectroscopies and Density Functional Theory (DFT) calculations. Our results demonstrate that solvent parameters have an enormous influence on the optical properties of this molecule, acting as knobs for external control of its photophysics. The Intramolecular Charge Transfer (ICT) process introduces a remarkable solvent polarity effect on the emission spectra without affecting the lowest absorption band, as confirmed by DFT simulations, including solvation effects. The calculations ascribe the lowest absorption transitions to two singlet ICT excited states, S1 and S2, with S1 having several orders of magnitude higher oscillator strength than the “dark” S2 state. Temperature and viscosity investigations suggest the existence of two emitting excited states with different structural conformations. The phosphorescence emission band observed at 77 K is assigned to a localized 3terpy state. Finally, protonation studies show that tBuTPAterpy undergoes a reversible process, making it a promising probe of the pH level in the context of acidity determination.

Early bird or night owl? Controlling the ultrafast photodynamics of triphenylamine substituted 2,2′:6′,2′′-terpyridine

“…S4, panel D, ESI †). This observation, together with the presence of the ESA 1 and ESA 3 bands, is consistent with the formation of an ICT state upon conformational changes of the molecular moieties; 60 (v) The t 4 time component (210 ps) is mostly associated with the decay of the ESA 1 and SE 2 features;…”

“…Similar to chloroform, in acetonitrile the TA fit of tBuTPAterpy requires one additional component (four) with respect to the FlUC data (three), as reported in Table 1, and the longest time component matches well the decay time of the TCSPC measurements. 60 In the non-polar n-hexane, the transient signal intensity is significantly lower than that in the polar solvents under similar pump fluences. This is a consequence of the hypsochromic shift of the steady-state tBuTPAterpy absorption onset, which leads to a lower sample absorbance at 24 690 cm À1 (405 nm).…”

“…These signals reflect the formation of an S 1 excited state having a moderate ICT character and populating the lowest unoccupied molecular orbital (LUMO) centred on the terpy moiety. 60 At these time scales, the SE 1 band and the FlUC signal are blue-shifted with respect to the room temperature emission spectrum. As discussed in ref.…”

Solvent effects on the intramolecular charge transfer excited state of 3CzClIPN: a broadband transient absorption study

“…[29][30][31][32] The intricate nature of these interactions is governed by a combination of electrostatic, dispersive, inductive, and exchange-repulsive forces, depending on the particular interacting groups, molecular configuration, and charge distribution. 33,34 In this study, we investigated the spectroscopic behaviour of the ground and ICT-excited states of 3CzClIPN in both aromatic and nonaromatic solvents using steady-state absorption (SSA) and femtosecond broadband transient absorption (fs-bb-TA), respectively. The latter technique allowed us to observe the changes in absorption experienced by the ICT-excited state over timescales ranging from a few hundreds of femtoseconds (fs) to hundreds of picoseconds (ps).…”

“…29–32 The intricate nature of these interactions is governed by a combination of electrostatic, dispersive, inductive, and exchange-repulsive forces, depending on the particular interacting groups, molecular configuration, and charge distribution. 33,34…”