Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers

Abstract

We report the synthesis of a novel polythiophene-based host-guest copolymer incorporating a Pt-porphyrin complex (TTP-Pt) into the backbone for efficient singlet to triplet polymer exciton sensitization. We elucidated the exciton dynamics in thin films of the material by means of Transient Absorption Spectrosopcy (TAS) on multiple time scales and investigated the mechanism of triplet exciton formation. During sensitization, singlet exciton diffusion is followed by exciton transfer from the polymer backbone to the complex where it undergoes intersystem crossing to the triplet state of the complex. We directly monitored the triplet exciton back transfer from the Pt-porphyrin to the polymer and found that 60% of the complex triplet excitons were transferred with a time constant of 1087 ps. We propose an equilibrium between polymer and porphyrin triplet states as a result of the low triplet diffusion length in the polymer backbone and hence an increased local triplet population resulting in increased triplet-triplet annihilation. This novel system has significant implications for the design of novel materials for triplet sensitized solar cells and upconversion layers.

Synthesis and Exciton Dynamics of Donor-Orthogonal Acceptor Conjugated Polymers: Reducing the Singlet–Triplet Energy Gap

“…It is interesting to note that, in contrast to standard small-molecule TADF emitters, ASFCN exhibits no clear delineation between 'prompt' and 'delayed' emission. 6,16,19 Moreover, comparison with transient absorption data below reveals a strong correlation between substantial 3 (π-π)* character in the ASFCN excited state and the emission on all timescales, suggesting a more complex TADF mechanism than is typically proposed. Photoluminescence quantum yields are discussed in the supplementary information page S30.…”

“…This can be assigned to the 3 (π-π*) state of ASFCN. However, the distinction between the final states is less sharp: no 3 (π-π*) PIA growth is detected during CT decay, and indeed some 3 (π-π*) features can be detected throughout the CT lifetime and vice versa. We consider that the smaller ΔEST here results in some dynamic equilibrium or mixing between 3 (π-π*) and emissive CT states, analogous to signatures of singlet-mixed triplet-pair states observed in certain small molecule systems.…”

“…In both cases, there is clearly a much richer set of excited states in the D-oA polymers to facilitate orbital mixing. Therefore, the weakly donating SFCN polymer retains the 3 (π-π*) as the lowest energy excited state meaning that the singlet-triplet energy gap is reduced to the energetic difference between the newly created 1 CT state and the previously existing 3 (π-π*). In the case of the stronger donating ASFCN the newly created singlet and triplet CT states are considerably below the 3 (π-π*), and due to the highly orthogonal nature of the spirocyclic charge transfer states we can assume ΔEST to be very small (<0.1 eV) as has been observed for small molecule spirocyclic TADF materials.…”

“…However, the localised and overlapping wavefunctions result in a large triplet-stabilising exchange energy ΔEST of ≈ 0.7 eV. [1][2][3] The low energy of the non-emissive triplet state means it forms a trap and loss mechanism in virtually all organic optoelectronic devices (OPV, OLED, OLET, OLASER). [4][5][6] This could be avoided were it possible to reduce ΔEST through material design.…”

Energy-Transfer Pathways and Triplet Lifetime Manipulation in a Zinc Porphyrin/F8BT Hybrid Polymer

“…Previous work on a different porphyrin/polymer hybrid co-polymer revealed the presence of triplet sensitisation, singlet energy transfer, and a thermal equilibrium between the polymer and porphyrin triplet populations. 19 One motivation behind this work was to establish the generality of these findings, and also to explore the new hybrid polymer's potential for photon upconversion. A zinc porphyrin derivative was used on the basis of its narrow singlet-triplet gap, high triplet yields, and long triplet lifetimes.…”