Distinguishing between Triplet-Pair State and Excimer Emission in Singlet Fission Chromophores Using Mixed Thin Films

Abstract

In the photoluminescence spectra of thin films made of singlet fission (SF) materials emission features that are red-shifted from the free exciton emission are of particular interest. They can be fingerprints of the correlated triplet-pair state and as such offer insights into the mechanisms of the multistep SF process. However, excimer formation or trap-state population can also cause such features and a clear disentanglement of the various contributions can be challenging. Here, we use blends of anthradithiophene (ADT) and weakly interacting organic semiconductors to control the polarizability of the molecular environment and, thus, to distinguish between excimer emission and emission from the correlated triplet-pair state. Using time-resolved photoluminescence spectroscopy measurements, we clarify the relation between excimer formation and SF in ADT and find that excimer formation constitutes a parallel relaxation channel for the exciton and neither mediates nor hinders SF.

Spectroscopic analysis of vibrational coupling in multi-molecular excited states

“…Nevertheless, there exist strategies to reduce the free parameter space and to control these uncertainties for certain cases. For example in the above presented analysis of the ZnPc excimer, the experimental data allows for an estimation of the X-dimer's zero point energy by means of the semi-classical approximation of the FWHM (14) which can be either used as a reliable start value for the fitting procedure, as demonstrated for the quantum mechanical approach, or even eliminate the parameter at all by assuming it to be a fixed value, as shown for the semi-classical approach. Furthermore, as demonstrated by Birks et al [23] the low temperature spectra allow to infer the ground state potential parameter R 0 and the X-state displacement q e , because the low temperature emission spectra are dominated by transitions from the X-dimer's vibrational ground state to ground state phonons.…”

“…For this purpose equation ( 9) was fitted to each emission spectrum with R 0 , q e and D e as free parameters by a least square method of the SciPy package [82]. Again, to minimize the number of correlated free parameters, we assume the X-state potential as constant with temperature and set it to the fixed value determined from the FWHM using equation (14). This leads to a slight overestimation of the vibrational energy compared to the results of the quantum-mechanical fit, but, as will be shown below, still yields reasonable results in the reproduction of the emission lineshape and hence gives an opportunity to test the semi-classical approach without any input from the full quantum mechanical model.…”

“…Emission from excited states which emerge from an inter-molecular geometry relaxation, e.g. excimers or exciplexes (vide infra), are common in molecular single crystals as well as polycrystalline thin films [2,[12][13][14] and are often found in organic light emitting diodes (OLEDs) [15][16][17][18] and solar cells [19,20]. Usually, the emission from such states is characterized by a broad and unstructured spectrum which is considerably red shifted compared to the single molecule emission [2,21,22].…”

Spectroscopic analysis of vibrational coupling in multi-molecular excited states

“…a linear ground state potential used by Williams and Hebbs, 56 eqn ( 14) is the analytical expression for the FWHM of the emission spectra. If the deviation of the line shape of the emission spectra from an overall Gaussian line shape is reasonably small, fitting eqn (14) to the temperature dependent FWHM is still acceptable to approximate the excited state potential. We determine E 0,X to be (14.7 AE 0.4) meV (Note 4 in the ESI †), which translates to a vibrational energy quantum of E X,vib = (29.4 AE 0.8) meV that will be used as a starting point for the fit algorithm.…”

“…11 Emission from excited states which emerge from intermolecular geometry relaxation, e.g. excimers or exciplexes (vide infra), is common in molecular single crystals as well as polycrystalline thin films 2,[12][13][14] and is often found in organic light emitting diodes (OLEDs) [15][16][17][18] and solar cells. 19,20 Usually, the emission from such states is characterized by a broad and unstructured spectrum which is considerably red shifted compared to the single molecule emission.…”