Ultrafast Excited-State Dynamics of Diketopyrrolopyrrole (DPP)-Based Materials: Static versus Diffusion-Controlled Electron Transfer Process

Abstract

Singlet-to-triplet intersystem crossing (ISC) and photoinduced electron transfer (PET) of platinum(II)-containing diketopyrrolopyrrole (DPP) oligomers in the presence and absence of tetracyanoethylene (TCNE), a strong electron-acceptor, were investigated using femtosecond and nanosecond transient absorption spectroscopy with broadband capabilities. The effect of incorporating platinum(II) in the photophysical properties of DPP molecule was evaluated by comparing the excited-state dynamics of DPP with and without Pt metal centers. Steady-state measurements reveal that platinum(II) incorporation greatly facilitates interactions between DPP-Pt(acac) and TCNE, resulting in the formation of charge transfer (CT) complexes. In the absence of TCNE, the transient absorption spectra revealed ultrafast ISC of DPP-Pt(acac) followed by a long-lived triplet state; however, in the presence of TCNE, PET from the excited DPP-Pt(acac) and from DPP to TCNE formed radical ion pairs. We measured an ultrafast PET from DPP-Pt(acac) to TCNE (i.e., a picosecond regime) that was much faster than that from DPP to TCNE (i.e., nanosecond time scale), which is a diffusion-controlled process. Our results provide clear evidence that the PET rate is eventually controlled by the platinum(II) incorporation.

Ultrafast Fluorescence Photoswitch Incorporating Diketopyrrolopyrrole and Benzo[1,3]oxazine

“…Hence, that FRET induced a decrease of DPP fluorescence due to acidochromic effect can safely be ruled out and decrease of DPP fluorescence could be rationalized by electron transfer from excited DPP to the 3 H -indolium cation. Indeed, the photoinduced electron transfer process in this case is highly exoergonic and the Gibbs free energy (Δ G 0 ) is broadly estimated to be ∼−0.8 eV considering the reduction potential of the 3 H -indolium (∼0.8 eV), oxidation potential, and excited singlet-state energy of DPP­(6) (∼0.8 eV and 2.4 eV (605 nm), respectively).…”

Heavy metal complex containing organic/polymer materials for bulk-heterojunction photovoltaic devices

“…Besides on the linear platinum acetylides comprising photovoltaic materials, heavy metal organometallic polymers containing cyclometalated Pt or Ir complexes which was attached via the C^N or O^O diketonate ligands have been attracted gradually increasing attention due to better involvement from the orbitals of Pt or Ir center and the cyclometalated organic ligand to polymer backbone in both the ground and the excited states of the material [71][72][73][74][75]. Strong spin-orbit coupling on the heave metal center allows for the mixing of singlet metal-to-ligand charge transfer state ( 1 MLCT) with the formally forbidden triplet metal-to-ligand charge transfer ( 3 MCLT) and 3 π-π* states.…”

“…Together with the close lowest unoccupied molecular orbital (LUMO) (∼4.2 eV) of the polymer to that of PCBM which lead comparatively low driving force for charge separation at the donor-acceptor interface, BHJ photovoltaic device performed a rather low PCE of 0.22%. Besides on the cycloplatinated polymers, oligomers (40)(41)(42) of cyclometalated platinum complex by introducing the widely reported high performance diketopyrrolopyrrole (DPP) or Isoindigo (IID) moiety were also reported [71,74,75]. For example, when DPP chromophores were end-functionalized with platinum containing auxochromes, photophysics study revealed that compared with the platinum acetylide units, orthometalated platinum auxochrome 40 is more likely to induce a greater extent of spin-orbit coupling.…”

“…In this review, recent progress on the development and application of heavy-metal complex containing photovoltaic donor materials for the bulk-heterojunction solar cells are presented. Low-bandgap platinum-acetylide and the cyclometalated Pt or Ir containing polymers, oligomersor small molecules [71][72][73][74][75][76][77] as donor materials for the active layer of BHJ solar cells are successively demonstrated, in which the heavy metal complex was chemically bonded to the molecular backbone. Finally, physically doping of heavy-metal complexes as additives into the BHJ solar cells to enhance the photovoltaic performance are also discussed [78][79][80][81].…”

Bane of Hydrogen-Bond Formation on the Photoinduced Charge-Transfer Process in Donor–Acceptor Systems

“…Photoinduced charge transfer remains one of the most important photochemical reactions in chemistry, biology, and materials science. − The process is usually characterized by the transfer of an electron from a photoexcited donor to an acceptor and is often driven by complex formations between the donor–acceptor moieties, especially if they carry opposite electrostatic charges. Such excited-state processes are significantly influenced by the properties of the solvents, including polarity and hydrogen-bonding interactions. − For instance, time-resolved laser spectroscopic measurements in pico- and femtosecond time domains have been used to investigate the dynamics of hydrogen bonding in the excited singlet (S 1 ) state of ketone molecules in alcoholic solvents, where evolution of the excited-state properties has been observed and attributed to repositioning of the hydrogen bonds around the carbonyl group.…”