(Invited) Transient Absorption Study of Singlet Excitons in Crystalline Peropyrene Derivatives: Examination of Singlet Fission Process

Abstract

Transient absorption study of singlet excitons in crystalline peropyrene derivatives: examination of singlet fission process Akihiro Furube*1, Daiki Yamanaka1, Shinichiro Yanagiya1, Hiroyuki Matsuzaki2, Kazuyuki Uchida3, Takashi Kubo3 1Tokushima Univ,. 2-1, Minamijosanjima-cho, Tokushima, 770-8506, Japan 2AIST, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan 3Osaka Univ., 1-1, Machikaneyama, Toyonaka, Osaka 560-0043, Japan Furube.akihiro@tokushima-u.ac.jp Singlet fission process in organic crystal is attracting considerable interest due to its potential application of organic photovoltaic (OPV) with the internal quantum efficiency of 200%, since one absorbed photon can produce two triplet excitons that can generate two electron-hole pairs at the interface between the organic crystal and a suitable electron donor material such as fullerene derivatives and carbon nanotubes. Peropyrene, which is one of polycyclic aromatic hydrocarbon compounds, is expected to exhibit singlet fission in the crystal form. The SF is a reaction in which the excited singlet state (S1) spilt into two triplet excited states (T1 + T1). Energy matching (S1 ~ T1 + T1) reported [1] for peropyrene molecule interests researchers for development of derivatives of this molecule to control the energy levels and crystal structure for efficient singlet fission. In the present study, two kinds of 2,9-disubstituted peropyrene derivatives, one with two phenyl groups and the other with two n-butyl groups, were newly synthesized and tested. Time-resolved transient absorption spectroscopy in the femtosecond and nanosecond timescales, time-resolved fluorescence spectroscopy, and steady-state absorption/fluorescence measurements were carried out in order to reveal the relaxation processes of the singlet excitons of these peropyrene derivatives in the crystalline and solution phases. Transient absorption spectra and kinetics of phenyl peropyrene in the crystalline state are shown in Figure 1a. Here, diffuse reflectance spectroscopy was applied [2, 3]. At 10 ps after excitation, a broad absorption band appeared around 700 nm, which showed gradual decay up to 500 ps. It should be noted that the excitation laser intensity (at 400 nm wavelength) was adjusted to be weak enough so as not to undergo any non-linear decay process such as exciton–exciton annihilation. Since the absorption peak wavelength (700 nm) is close to the one observed in solution (775 nm), the absorption band can be ascribed to the singlet exciton in crystalline state. The decay component in the picosecond timescale showed a time constant of 150 ps. The involvement of singlet fission in the decay of a singlet exciton would result in the appearance of the T1absorption band being similar to the band observed in solution around 525 nm. However, we did not observe such an absorption band in the picosecond time scale. This experimental finding implies that the energy level of the S1 state is lower than twice the energy of the T1 state in the crystalline state. In this case, singlet fission is expected to occur from highly excited singlet states (Sn). Under high-density excitation conditions, two singlet excitons collide each other to produce Sn and S0 through singlet-singlet exciton annihilation (S1 + S1 → Sn + S0). Thereafter the resulting Sn state can undergo singlet fission. Figure 1b shows the transient absorption. Indeed, a broad absorption band centered at 570 nm was observed in the transition absorption spectrum measured at 1 ns, after the disappearance of the absorption band of the S1 state around 700 nm. This spectral feature is similar to that of the T1 state observed in solution, suggesting that the T1 state is generated from the highly excited singlet state Snthrough singlet fission. Interestingly, n-butyl peropyrene crystal did not show singlet fission either in weak or strong excitation condition. Based on these experiments, it was revealed that S1exciton in phenyl peropyrene is energetically low for undergoing singlet fission, therefore requiring additional energy. Further detailed discussion is given with other experimental results. Figure.1Transient absorption spectra of phenyl peropyrene crystal power samples excited by 400 nm laser with weak (a) and strong (b) intensity, respectively. Acknowledgement: A part of this work is supported by KAKENHI (16K13669) Japan.