Ultrafast Dynamics of the 9,10-Bis[(triisopropylsilyl)ethynyl]anthracene Nanoaggregate and Thin Film: Influence of the Molecular Packing and Sample Morphology on Singlet Fission and Excimer Formation

Abstract

Photophysics and ultrafast exciton dynamics of 9,10-bis­(triisopropylsilylethynyl)­anthracene (TIPSAn) nanoaggregates and thin films are studied. The effect of molecular packing on singlet exciton relaxation is evaluated by comparing ultrafast dynamics in nanoparticles and vapor-deposited thin films. As compared to the strong emission of TIPSAn in solution, significant quenching of fluorescence in nanoaggregates and thin films suggests fast nonradiative relaxation of singlet excitons. In nanoaggregates, disordered molecular packing leads to long-lived excimer-state outcompeting singlet fission (SF)-mediated triplet formation. In microcrystalline thin films, exciton relaxation to the excimer state is inhibited due to ordered molecular packing and a significant fraction of singlet excitons undergo singlet fission, resulting in long-lived triplet excitons. However, the singlet fission yield in TIPSAn is much smaller than that of the structurally similar but strongly coupled anthracene derivative, namely 9,10-bis­(phenylethynyl) anthracene (BPEA). The low yield of SF-mediated triplets in TIPSAn is attributed to weak intermolecular interactions due to larger intermolecular separation introduced by bulky triisopropylsilyl groups in crystalline thin films and additional competing exciton trapping to the excimer state in disordered nanoparticles.

Singlet fission in TIPS-anthracene thin films

“…20,21 Recently, thermally evaporated thin films of TIPS-Ac were reported to undergo SF with free triplet yields of less than 40% (out of a possible 200%). 22 These low yields contradict previous computational studies that estimated the SF yield to be 150%. 23 This implies that there is a loss mechanism that competes with SF in TIPS-Ac.…”

“…However, the presence of Davydov splitting in our thin film indicates that its structure is most similar to the TIPS-Ac polymorph that has been observed to undergo singlet fission. 22 The arrangement of non-parallel chromophores undergoing singlet fission is observed in thin films of pentacene and tetracene, two of the most well-studied singlet fission chromophores. 11,33–35 The photophysics of TIPS-Ac was further investigated with TA.…”

“…This low yield agrees with the previously estimated 40%. 22 Indeed, it implies that SF is subject to loss mechanisms. To investigate further, magnetic field effect and transient photoluminescent measurements were undertaken.…”

Substitution effects on the photoinduced excited state dynamics of perylenemonoimides in solution and thin films

“…The presence of intermolecular coupling in the thin films leads to a difference in TA spectral shape, to the spectra obtained in solution, where the molecules exist in monomer form. 50,51 The TA spectra of BrPMITB , PMITB and APITB exhibit GSB at 480 nm and 529 nm, which corresponds to the vibronic transition obtained from steady state absorption spectroscopy for each derivative. ESA was observed within the ∼550–750 nm range, attributed to the photoexcited S 1 state.…”

Triplet Formation in a 9,10-Bis(phenylethynyl)anthracene Dimer and Trimer Occurs by Charge Recombination Rather than Singlet Fission

“…The mechanism of charge recombination to form the triplet excited state has been proposed in other thin film systems, such as in the indigo derivative cibalackrot, in which case transient intermediates with charge-transfer character are also observed prior to triplet formation . It is also worth noting that the interpretation of the spectral feature as a shifted and broadened S 1 absorption in BPEA thin films could be entirely accurate, given that the excited state absorption of various compounds, including polyacenes, are known to differ significantly between solution and film states. − …”