Excited State Absorption Properties of Pt(II) Terpyridyl Complexes Bearing π-Conjugated Arylacetylides

Long-lived triplet excited state in a platinum(ii) perylene monoimide complex

We report the synthesis and solution based photophysical properties of a new Pt(ii)-terpyridine complex coupled to a perylene monoimide (PMI) chromophoric unit through an acetylene linkage. This structural arrangement resulted in quantitative quenching of the highly fluorescent PMI chromophore by introducing metal character into the lowest energy singlet state, thereby leading to the formation of a long-lived PMI-ligand localized triplet excited state (τ = 8.4 μs). Even though the phosphorescence from this triplet state was not observed, highly efficient quenching of this excited state by dissolved oxygen and the observation of singlet oxygen photoluminescence in the near-IR at 1270 nm initially pointed towards triplet excited state character. Additionally, the coincidence of the excited state absorbance difference spectra from the sensitized PMI ligand using a triplet donor and the Pt-PMI complex provided strong evidence for this triplet state assignment, which was further supported by TD-DFT calculations.

Directional Self-Sorting with Cucurbit[8]uril Controlled by Allosteric π-π and Metal-Metal Interactions

To maximize Coulombic interactions, cucurbit[8]uril (CB[8]) typically forms ternary complexes that distribute the positive charges of the pair of guests (if any) over both carbonylated portals of the macrocycle. We present here the first exception to this recognition pattern. Platinum(II) acetylides flanked by 4'-substituted terpyridyl ligands (tpy) form 2:1 complexes with CB[8] in an exclusively stacked head-to-head orientation in a water/acetonitrile mixture. The host encapsulates the pair of tpy substituents, and both positive Pt centers sit on top of each other at the same CB[8] rim, leaving the other rim free of any interaction with the guests. This dramatic charge imbalance between the CB[8] rims would be electrostatically penalizing, were it not for allosteric π-π interactions between the stacked tpy ligands, and possible metal-metal interactions between both Pt centers. When both tpy and acetylides are substituted with aryl units, the metal-ligand complexes form 2:2 assemblies with CB[8] in aqueous medium, and the directionality of the assembly (head-to-head or head-to-tail) can be controlled, both kinetically and thermodynamically.

Tuning Light Absorption in Platinum(II) Terpyridyl π-Conjugated Complexes: A First-Principle Study

Platinum(II) terpyridyl complexes with a donor-acceptor (D-A) framework have long been considered as a promising component of dye-sensitized solar cells (DSSCs). To revealing the structure-property relationship of these highly modular systems, we have conducted a first-principle study at the time-dependent density functional theory (TDDFT) level on the [Pt(^tBu₃tpy)(-C≡C-Ph)_n]⁺ (^tBu₃tpy is 4,4',4″-tri-tert-butyl-2,2':6',2″-terpyridine) complexes. It was found that their visible absorbance could be improved by elongating the donor chain with n (-C≡C-Ph) units, reaching a maximum at n = 16. It is noteworthy that such a simple concatenating protocol enables a remarkable charge transfer distance as long as 5 nm, implying a promising solution for the bottleneck problem of low charge separation rate in DSSCs. Furthermore, using a A-D-A system (two Pt(^tBu₃tpy) acceptors bridged by one donor) effectively doubles the visible-harvesting ability, and twisting an benzene ring in the chain of donors to break π-conjugations can tune down light absorption in a quantitatively angular dependent manner. Finally, replacing the C≡C bond linker with C═C double bond in donor leads to comparable light absorption ability while bestowing structural flexibility. These structure-property relationships thus provide efficient knobs for molecular rational design toward high performance dye-sensitized solar cells.

Synthesis, crystal structure, photophysical property and metal ion-binding behavior of a cyclometalated platinum(ii) terpyridylacetylide with efficient π-conjugation degree

The “Donor–Phosphore–Receptor” platform as a novel building block for formation of heterotrinuclear transition metal complexes.

Effects of extending the π-conjugation of the acetylide ligand on the photophysics and reverse saturable absorption of Pt(ii) bipyridine bisacetylide complexes

Extending the acetylide ligand π-conjugation diminishes the terminal substituent effect on the lowest excited states, but expands the triplet excited-state absorption to the near-IR region.

Altering molecular photophysics by merging organic and inorganic chromophores

Photofunctional molecules and assemblies lie at the heart of many important fundamental processes in nature, and researchers have generated multitudes of artificial chromophores intended to mimic these naturally occurring systems. As dynamic spectroscopic techniques are becoming more widely available, ultrafast techniques in particular, substantial insight continues to be gleaned from the initial photon stimulation event through internal conversion, structural rearrangements, intersystem crossing, energy migration, electron transfer events, and ultimately regeneration of the ground state chromophores in both naturally occurring and inspired chromophores. This Account details research endeavors motivated by the concept that merging organic and inorganic chromophores can lead to new molecules exhibiting novel excited state properties. Moreover, these excited state properties can be fundamentally understood using combinations of static and dynamic spectroscopic tools, yielding systematic improvements to molecules poised for application in diverse research areas including light-harvesting, lifetime engineering, photocatalysis, and photochemical upconversion. Initial explorations focused on utilizing Förster energy transfer processes in Ru(II)-based metal-organic chromophores for solar light-harvesting while maintaining long excited state lifetimes. This eventually led to molecules exhibiting triplet-triplet energy transfer between energetically proximate triplet states featuring thermally activated photoluminescence from the upper charge transfer excited state with markedly extended lifetimes. Interest in systematically producing long-lived excited states with concomitant large Stokes shifts inspired the development of numerous Pt(II) bipyridyl and terpyridyl acetylide charge transfer chromophores featuring ultrafast intramolecular energy migration, high quantum yield ligand-localized phosphorescence at room temperature, and synthetically tunable excited state absorption properties. This structural motif also made it possible to access the triplet excited states of perylenediimide chromophores, permitting quantitative examination of internal conversion and intersystem crossing processes in these complex molecules. The generation of new metal-organic structures featuring unique photophysics appears limitless and simply requires the continued ingenuity of researchers.

Ultrafast intersystem-crossing in platinum containing π-conjugated polymers with tunable spin-orbit coupling

The development of efficient organic light-emitting diodes (OLED) and organic photovoltaic cells requires control over the dynamics of spin sensitive excitations. Embedding heavy metal atoms in π-conjugated polymer chains enhances the spin-orbit coupling (SOC), and thus facilitates intersystem crossing (ISC) from the singlet to triplet manifolds. Here we use various nonlinear optical spectroscopies such as two-photon absorption and electroabsorption in conjunction with electronic structure calculations, for studying the energies, emission bands and ultrafast dynamics of spin photoexcitations in two newly synthesized π-conjugated polymers that contain intrachain platinum (Pt) atoms separated by one (Pt-1) or three (Pt-3) organic spacer units. The controllable SOC in these polymers leads to a record ISC time of <~1 ps in Pt-1 and ~6 ps in Pt-3. The tunable ultrafast ISC rate modulates the intensity ratio of the phosphorescence and fluorescence emission bands, with potential applications for white OLEDs.

Femtosecond spectroscopy of the dithiolate Cu(ii) and Ni(ii) complexes

Femtosecond spectroscopy was applied to study the ultrafast dynamics for the excited states of dithiolate Cu(ii) and Ni(ii) complexes.

Tuning photophysics and nonlinear absorption of bipyridyl platinum(II) bisstilbenylacetylide complexes by auxiliary substituents

The photophysics of six bipyridyl platinum(II) bisstilbenylacetylide complexes with different auxiliary substituents are reported. These photophysical properties have been investigated in detail by UV-vis, photoluminescence (both at room temperature and at 77 K) and transient absorption (nanosecond and femtosecond) spectroscopies, as well as by linear response time-dependent density functional theory (TD-DFT) calculations. The photophysics of the complexes are found to be dominated by the singlet and triplet π,π* transitions localized at the stilbenylacetylide ligands with strong admixture of the metal-to-ligand (MLCT) and ligand-to-ligand (LLCT) charge-transfer characters. The interplay between the π,π* and MLCT/LLCT states depends on the electron-withdrawing or -donating properties of the substituents on the stilbenylacetylide ligands. All complexes exhibit remarkable reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses, with the complex that contains the NPh(2) substituent giving the strongest RSA and the complex with NO(2) substituent showing the weakest RSA.