Modulation of Intersystem Crossing Rate by Minor Ligand Modifications in Cyclometalated Platinum(II) Complexes

Abstract

Photophysical properties of four new platinum(II) complexes comprising extended ppy (Hppy = 2-phenylpyridine) and thpy (Hthpy = 2-(2'-thienyl)pyridine) cyclometalated ligands and acetylacetonate (acac) are reported. Substitution of the benzene ring of Pt-ppy complexes 1 and 2 with a more electron-rich thiophene of Pt-thpy complexes 3 and 4 leads to narrowing of the HOMO-LUMO gap and thus to a red shift of the lowest energy absorption band and phosphorescence band, as expected for low-energy excited states of the intraligand/metal-to-ligand charge transfer character. However, in addition to these conventional spectral shifts, another, at first unexpected, substitution effect occurs. Pt-thpy complexes 3 and 4 are dual emissive showing fluorescence about 6000 cm(-1) (∼0.75 eV) higher in energy relative to the phosphorescence band, while for Pt-ppy complexes 1 and 2 only phosphorescence is observed. For dual-emissive complexes 3 and 4, ISC rates kISC are estimated to be in order of 10(9)-10(10) s(-1), while kISC of Pt-ppy complexes 1 and 2 is much faster amounting to 10(12) s(-1) or more. The relative intensities of the fluorescence and phosphorescence signals of Pt-thpy complexes 3 and 4 depend on the excitation wavelength, showing that hyper-intersystem crossing (HISC) in these complexes is observably significant.

Synergistic Effects in PtII–Porphyrinoid Dyes as Candidates for a Dual‐Action Anticancer Therapy: A Theoretical Exploration

“…The ISC mechanism that involves these higher states could be sufficiently faster so as to compete with internal conversion within the singlet manifold. Such a mechanism was previously suggested for Pt II compounds and other metal complexes . As expected, no high SOC values were computed between higher states in the metal‐free dyes, which is due to the lack of any metal contribution in the involved orbitals.…”

“…Indeed,f or all the platinated conjugates,w ef ound larger values for the couplings that involved higher excited states than those obtained for S 1 ,S 2 ,a nd the lower triplet states, which can rationalize the high 1 O 2 quantum yield that was experimentally determined at 401 nm (see the Supporting Information, Figures S6-S9). [4] Interestingly,c ompound 2c displayedv ery high SOC values (up to 580 cm À1 )b etween S 3 and S 4 states as well as the energetically close triplet states T 9 -T 12 .I nc ontrast, neither bacteriochlorin nor porphyrin derivatives displayed comparable values, or such an umber of deactivation channels, like in the case of compound 2c.T he ISC mechanism that involves these higher states could be sufficiently faster so as to compete with internal conversion within the singlet manifold.S uch am echanism was previously suggested for Pt II compounds [23] and other metal complexes. [15] As expected, no high SOC values were computed between higher states in the metal-free dyes, which is due to the lack of anym etal contributioni nt he involved orbitals.…”

Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs

“…of the Supporting Information. It can be seen that corresponding platinum-ligating atom bond lengths are the same (within about 2 standard deviations) for the two metal centres, and in all cases similar to those in related mononuclear Pt(N^C) 2 (O^O) complexes that have been structurally characterised 35,49. As in such mononuclear cases, the Pt-C bonds are slightly shorter than the Pt-N bonds, whilst the Pt-O bonds that are trans to the cyclometallated carbon are elongated relative to those that are trans to the pyridines, reflecting the high trans influence associated with…”

Harnessing White‐Light Luminescence via Tunable Singlet‐and Triplet‐Derived Emissions Based on Gold(III) Complexes *

“…The dual emission experimentally observed for transition‐metal complexes is usually a consequence of the relatively slow rate of the S 1 →T 1 intersystem crossing (ISC) due to the low contribution of the metal center in the involved π–π* ligand‐centered excited states allowing the fluorescence S 1 →S 0 to compete with the intersystem crossing. Previous theoretical studies on cyclometalated platinum(II) complexes that display typical phosphorescence and dual emission reported the intersystem crossing rate k ISC (S 1 →T n ) expressed in terms of the spin‐orbit coupling operator and the energy difference Δ E (S 1 –T n ) between the excited states at their equilibrium geometries, suggesting that ISC between two states S 1 and T n is expected to be faster for molecules with smaller Δ E (S 1 –T n ) gap than in molecules with a larger gap . TD‐DFT calculations performed on complexes 1 – 4 revealed values of the S 1 –T 1 energy gap in the narrow range 0.97–1.10 eV, large values and not significantly different (Table S2) to be the key point of the mechanism operating in these gold(III) systems.…”