Ultrafast Dynamics Study Of Singlet Fission Of Rubrene And Its Analogs

In organic photovoltaic materials,singlet fission is a phenomenon of excitation multiplication occurring in specific organic molecules,that is,the absorption of one photon by a molecule can generate two electron-hole pairs.As a multi-exciton effect,singlet fission can effectively avoid energy loss and improve photoelectric conversion efficiency.With the emergence and development of ultrashort laser pulses and time-resolved technology,researchers have continued to study the singlet fission,but its physical essence is still unclear.As a common singlet fission material,rubrene has been studied continuously for single crystal,polycrystal,and thin film.However,there are few reports about rubrene and its analog C₃₈H₂₄S₂in the liquid phase.In rubrene solution,there is still much controversy in the process of the singlet state transition to the intermediate state and the intermediate state evolution into two independent triplet excitons.Most literatures have reported that the intermediate state splits directly into two independent triplet excitons,but current reports on the splitting process state that there are not only intermediate states but also triplet exciton pairs in the transition state.The rubrene analog C₃₈H₂₄S₂is a tetracene derivative in which a pair of symmetric phenyl groups in a pendant molecular group of rubrene are replaced with a thienyl group,and has excellent photooxygen stability compared to rubrene.In the present study,femtosecond time-resolved transient absorption spectroscopy is used to systematically analyze the singlet fission process of singlet excitons to triplet excitons in rubrene and its analog C₃₈H₂₄S₂.The research results and details are as follows:(1)The kinetics of singlet fission of rubrene in toluene solution:under the excitation of pump wavelength 470 nm,the rubrene solution exhibited a ground state bleaching signal at a wavelength of?526nm and a triplet-triplet-induced absorption signal at a wavelength of?510 nm and?725 nm and the stimulated emission signals appeared at wavelengths of?562 nm and?590 nm.According to the analysis,under the initial photoexcitation of rubrene,the molecule is excited from the ground state to an excited state with a high vibrational energy level,and is resonantly coupled with the triplet state;the excited singlet state and the associated triplet exciton pair vibration relaxation occurred,and the time was about 2 ps.Subsequently,the associated triplet exciton pair spatially separated and generate a spatially separated triplet exciton pair,which took about 95 ps.In the rubrene solution,the spectral characteristics of the associated triplet exciton pair and the spatially separated triplet exciton pair are clearly identified by the transient absorption spectrum image,which proves the electronic resonance and vibration relaxation play a very important role in the process of light conversion from singlet to triplet exciton.(2)The excited state kinetics of rubrene analog C₃₈H₂₄S₂dissolved in toluene solution under different excitation conditions:the rubrene analog C₃₈H₂₄S₂is excited by pumping wavelengths of 550 and 365 nm,and its transient absorption spectrum is near a wavelength of 444 nm.The excited singlet and ground state molecules collide at 8.2 ps and 3 ps at 510 nm.An associated triplet exciton pair was generated,but no obvious independent triplet exciton signal was observed under 550 nm light excitation;the associated triplet exciton shifted the induced absorption signal under 365 nm light excitation.The independent triplet exciton absorption was observed near the wavelength of 550 nm.Under the excitation of light at 460 nm,the spectral signals of the components in rubrene analog C₃₈H₂₄S₂overlap with each other.By using the method of singular value decomposition,the overlapping spectral signals are separated.And the spectral characteristics of singlet and independent triplet excitons are identified from the transient absorption spectrum image.