Unraveling structural dynamics in isoenergetic excited S1 and multi-excitonic 1(TT) states of 9,10-bis(phenylethynyl)anthracene (BPEA) in solution via ultrafast Raman loss spectroscopy?

Physical Chemistry Chemical Physics Pub Date: 2019-01-22 DOI: 10.1039/C8CP06658B

Abstract

Polyacenes, such as anthracene, tetracene, pentacene etc., have been identified as potential candidates for singlet fission (SF) and triplet–triplet annihilation (TTA) processes in their crystalline and thin film forms as they possess significant singlet and triplet exciton couplings. Interestingly, phenyl-ethynyl substitution to anthracene at the 9,10 positions (9,10-bis(phenylethynyl)anthracene/BPEA) enhances the transverse π-electron conjugation and retains the planar structure even in the excited state. The excited singlet state S1 and the multi-excitonic state 1(TT) in BPEA are separated by ～30 meV (～250 cm?1) making it an ideal system for both SF and TTA applications. BPEA is very effective in photon up-conversion even for low input intensities. Transient absorption measurements of BPEA in n-hexane solution are inadequate for distinguishing the S1 state and the multi-excitonic state 1(TT), since the spectroscopic features are complex (mixed) due to the isoenergetic nature and the existence of an equilibrium between these states. However, ultrafast Raman loss spectroscopy reveals a systematic red shift and a blue shift in the central frequencies of the Raman modes corresponding to C[double bond, length as m-dash]C and C[triple bond, length as m-dash]C vibrational frequencies with time constants of ～2.0 and ～20 ps, respectively. Such a shift in the Raman frequencies is direct evidence of the structural changes that take place while changing from excited singlet state S1 to the multi-excitonic state on the potential surface.

Graphical abstract: Unraveling structural dynamics in isoenergetic excited S1 and multi-excitonic 1(TT) states of 9,10-bis(phenylethynyl)anthracene (BPEA) in solution via ultrafast Raman loss spectroscopy
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