Effects of meso-M(PPh3)2Cl (M = Pd, Ni) substituents on the linear and third-order nonlinear optical properties of chalcogenopyrylium-terminated heptamethines in solution and solid states
Aggregation of cyanine-like dyes can significantly affect their optical properties. Here we report the effects of bulky meso-M(PPh3)2Cl (M = Pd, Ni) substitution on the molecular and solid-state optical characteristics of chalcogenopyrylium-terminated heptamethines. Metallated dyes were synthesised by reaction of the meso-chloro dyes with Pd(PPh3)4 or Ni(PPh3)4 at room temperature. The two PPh3 ligands are trans and the plane formed by the metal atom and its ligands is approximately orthogonal to that of the polymethine π-system. Replacement of Cl by M(PPh3)2Cl leads to a large blue shift of the solution absorption maximum and a decrease in the associated transition dipole moment, these effects being slightly more pronounced for Ni than for Pd. DFT calculations and electrochemical data suggest the blue shifts can largely be attributed to destabilisation of the LUMO by the more strongly π-donating M(PPh3)2Cl groups. The magnitude of the real part of the molecular third-order polarisability, Re(γ), decreases in the order Cl > Pd(PPh3)2Cl ≫ Ni(PPh3)2Cl. Within the framework of the sum-over-states expression for Re(γ), the difference between Cl and Pd(PPh3)2Cl examples can be rationalised considering the effects of the S0 → S1 transition energy and transition dipole moment on the two-state term associated with S0 → S1. On the other hand, the magnitude of Re(γ) for a Ni(PPh3)2Cl dye is anomalously low; SAC-CI/HF/cc-pVDZ excited-state calculations reveal this is due to a two-photon-allowed S2 state at unusually low energy for a cyanine-like dye, leading to a large positive three-state contribution to γ opposing the negative two-state S1 term. Thus, despite a cyanine-like molecular structure and linear absorption spectrum, this compound does not exhibit cyanine-like nonlinear optical behavior. Turning to the effects on aggregation, molecular dynamics simulations suggest that Pd(PPh3)2Cl substitution largely suppresses H- and J-aggregate formation; indeed experimental absorption spectra for neat films of Pd(PPh3)2Cl-substituted dyes are fairly similar to corresponding solution spectra. A 50 wt% blend of a Pd(PPh3)2Cl-substituted telluropyrylium-terminated dye with amorphous polycarbonate exhibits a third-order susceptibility of −3 × 10−11 esu, a two-photon figure-of-merit in excess of 10, and linear loss of 6.3 dB cm−1, which are close to the requirements for all-optical switching applications.
- This article is part of the themed collections: 2018 Journal of Materials Chemistry C HOT Papers and Celebrating 50 years of Professor Fred Wudl’s contributions to the field of organic semiconductors