Time and magnetic field dependent spectra of NODIPS-Tetracene

The photophysical processes of singlet fission and triplet fusion have numerous emerging applications. They respectively involve the separation of a photo-generated singlet exciton into two dark triplet excitons and the fusion of two dark triplet excitons into an emissive singlet exciton. The role of the excimer state and the nature of the triplet-pair state in these processes have been a matter of contention. Here we analyse the room temperature time-resolved emission of a neat liquid singlet fission chromophore and show that it exhibits three spectral components: two that correspond to the bright singlet and excimer states, and a third component that becomes more prominent during triplet fusion. This spectrum is enhanced by magnetic fields, confirming its origins in the recombination of weakly-coupled triplet pairs. It is thus attributed to strongly coupled triplet pair state. These observations unite the view that there is an emissive intermediate in singlet fission and triplet fusion, distinct from the broad, unstructured excimer emission.

Methods
Time-resolved photoluminescence spectroscopy (TRPL) was carried out using a SpitLight Picolo, ND:YVO4 laser system with pulse duration of 800 ps at a repetition rate of 1 kHz and wavelength of 532 nm. Photoluminescence was detected using a spectrometer (Princeton Instruments 2300i) equipped with a 300 groove-per-millimetre grating blazed at 500 nm, and an intensified time-gated camera (Princeton Instruments Pi Max-4).

Steady-state optical absorption spectroscopy was performed with a Cary 60 UV-Visible-NIR spectrometer covering a wavelength range of 190-1100 nm.

For steady-state photoluminescence measurements, samples were excited by a 532 nm continuous-wave (CW) laser. The photoluminescence was collected an imaged onto a multi-mode optical fibre by two off-axis parabolic mirrors. An Ocean Optics Flame spectrometer was used to collect the photoluminescence spectrum.