Evidence of ultrafast intersystem crossing in Pt(TPBP) confirmed by near infrared transient spectroscopy

Abstract

Platinum complexes are well known as oxygen sensors, guest materials in OLEDs, and sensitizers in up-conversion systems1,2,3. Among them, Platinum (II) 5, 10, 15, 20-tetraphenyltetrabenzoporphyrin (Pt (TPBP): Fig.1(a)) is a phosphorescent material that emits red light with a quantum yield of more than 50%4. However, the reason for the high quantum yield is not clear. To obtain Pt (TPBP) phosphorescence by the light excitation, singlet excitation states have to be converted to triplet excited states, i.e., intersystem crossings. We report on the analysis of the intersystem crossing by transient absorption measurements in the near-infrared wavelength region using a femtosecond laser source to explore the factors responsible for the extremely high quantum yield of Pt (TPBP).A chloroform solution of Pt (TPBP) was used as the sample. The absorption spectrum is shown in Fig. 1 (b), where the characteristic structures called the Soret band and Q band appear at 430 nm and 610 nm, respectively4. Fig. 1 (c) shows the near-infrared transient absorption signal at the excitation wavelength of 610 nm. A fast decay component is observed at 1,040 nm with a lifetime of 520 fs. Followingly, a rise of the long-lived (> 1 ns) component was observed in the wavelength region shorter than 900 nm, which is attributable to phosphorescence.Phosphorescence lifetime was determined as 43 µs by time-resolved emission spectroscopymeasurements using a streak camera. Strangely, when the sample was evaluated with the nearinfrared transient absorption system with 400 nm excitation light, no signal was observed around 1,040 nm (Fig. 1 (d)). Since this signal is specific to 610 nm excitation, we can conclude that the signal indicates S1-Sn transition (n>3), and energy difference between S0 and Sn is calculated as 384 nm. To the best of our knowledge, this is the first report on the experimental confirmation of S1-Sn transition enabled only by near-infrared transient absorption spectroscopy. Furthermore, there is no contradiction with Roberts’s work5, where intersystem crossing of Pt (TPBP) occurs with a lifetime of 400 fs which is 105 times faster than ordinary systems.