Positional Effects from σ-Bonded Platinum(II) on Intersystem Crossing Rates in Perylenediimide Complexes: Synthesis, Structures, and Photophysical Properties

Abstract

In this investigation, the synthesis and photophysical properties of a series of new chromophores featuring Pt­(II) σ-bonded to perylenediimide (PDI) cores are reported. A Pt­(PPh3)2X (X = Cl, Br) moiety was attached to PDI in either the ortho or the bay position (2- or 1-positions respectively) or a Pt­(PPh3)2 subunit was used to bridge two bay positions (1- and 12-positions) forming a Pt­(II) cyclometalate. Through a combination of steady-state and transient absorption and photoluminescence spectroscopy, the excited-state dynamics of these molecules were revealed, indicating that the Pt atom location on the PDI has a substantial impact on observed intersystem crossing (ISC) rates. The ISC time constants for the bay-substituted and cyclometalated PDIs are between 2.67 and 1.29 ns, respectively, determined by the singlet fluorescence decays from the initially populated singlet excited states. In the case of the ortho-substituted PDI, ISC to the triplet state occurs on the ultrafast time scale with a time constant of 345 fs, determined through ultrafast transient absorption spectroscopy. In all instances, the measured PDI-based fluorescence quantum yields quantitatively correlate with the measured ISC rates leaving little doubt that variation in the Pt­(II) substitution position(s) markedly influence the resultant photophysics. Electronic structure calculations suggest differing amounts of metal-based contributions in the lowest excited states as a function of substitution position, supporting the trend of the observed ISC rates.

Fluorescent perylenylpyridine complexes: an experimental and theoretical study

“…Substitution of Pd by Pt in the five-membered [M(PerPy)(acac)] complexes (7 vs. 9) diminishes 23-fold the quantum yield (Ф fl = 0.011 for the Pt complex 9) due to the influence of spin-orbit coupling associated to the heavy atom effect of Pt, inducing a faster S to T intersystem crossing where the level of radiationless d-d transition is lower than that of emissive energy in triplet state, so that T1 is relaxed through thermally non-radiative d-d deactivation processes. 19 A similar result was reported for perylene dimide, where the quantum yields dropped only from 100 to 37 after -coordination of a PtCl(PPh 3 ) 2 fragment (compare with the case of our silver complex), but much more sharply, to 5, upon metalation to a planar 5-membered platinacycle. 19 Unexpectedly, a significant 20-fold enhancement in fluorescence was observed in the platinum(IV) compound 10 (Ф fl = 0.2), which suggests an attenuation of the quenching process in the platinum(II) complexes.…”

“…19 A similar result was reported for perylene dimide, where the quantum yields dropped only from 100 to 37 after -coordination of a PtCl(PPh 3 ) 2 fragment (compare with the case of our silver complex), but much more sharply, to 5, upon metalation to a planar 5-membered platinacycle. 19 Unexpectedly, a significant 20-fold enhancement in fluorescence was observed in the platinum(IV) compound 10 (Ф fl = 0.2), which suggests an attenuation of the quenching process in the platinum(II) complexes. In summary, quantum yield exhibits a remarkably complex dependence on the different metals, the ancillary ligands, the oxidation state, and the cycle size.…”

“…18 However, Pt II complexes -bonded to perylene diimides are able to produce partial or total fluorescence quench by facilitating the intersystem crossing to populate the triplet excited state. 19 The combination of both strategies was achieved using a coordinating ligand able to induce cyclometallation (perylene imine). This afforded cyclopalladated or cycloplatinated complexes with very modest quantum yields (in the range 0.01-0.13 for Pd II , and < 0.01 for Pt II ).…”

Perylene Bisimide Dyes with up to Five Independently Introduced Substituents: Controlling the Functionalization Pattern and Photophysical Properties Using Regiospecific Bay Substitution

“…In the case of perylene dyes, exact positioning of all substituents is essential considering the fact that their optoelectronic properties and morphology are strongly influenced by the position of the substituents. These pronounced positional effects are evident in photoinduced charge transfer, ,, intersystem crossing, molecular probes, and also molecular packing in the solid phase. − Consequently, better control over the substitution pattern around the perylene core is highly relevant.…”

On the intersystem crossing rate in a Platinum(ii) donor–bridge–acceptor triad

“…ISC is often rapid in such systems, with lifetimes in the region of tens of femtoseconds to picoseconds. [2][3][4][5][6][7][8] Numerous observations of ISC in metal complexes have been reported in the past two decades, containing, for example Cr, 9,10 Fe, [11][12][13][14][15][16][17] Cu, 18 Zn, 19 Ni, 20 Ru, [21][22][23][24][25][26] Re, 27 Os, 28 Pt, 20,[29][30][31][32][33][34][35][36] or Pd 20 , or Mo 37 . ISC has been reported to occur in metal complexes over a large range of timescales, from < 30 fs 11,38 to hundreds of picoseconds, but it is not clear yet what mechanisms govern the rates of ISC, 3 which can be vastly different even in compounds of similar structure.…”