(Invited) Ultrafast Formation of Long-Lived Charge-Separated State at High Energy Level By Two-Photon Ionization
Authors: Sotome Hikaru , Kawakami Tomomi , Koga Masafumi , Miyasaka Hiroshi 
Published: July 2022

Photoinduced electron transfer reactions play an important role in various photo-energy conversion processes. A key transient species enabling these processes is the charge-separated (CS) state produced by electron transfer, and its large formation rate constant, high energy level and long lifetime are required for its efficient utilization to the subsequent energy conversion and photoredox reactions. However, the formation rate constant is mutually correlated with the energy gap of electron transfer reactions, and it is difficult to optimize these parameters simultaneously, as shown by the Marcus theory and experimental investigations accumulated to date. In the present study, to create the CS state with the large formation rate, high energy level and long lifetime, we experimentally demonstrated an electron transfer reaction driven by two-photon ionization using a model system of pyrene (Py) as an electron donor and biphenyl (BP) as an acceptor. Photoionization enables ultrafast electron ejection from the donor to the acceptor which is 3-4 nm away from the donor. This long interionic distance effectively inhibits the geminate recombination, and the high energy level of the produced CS state is realized thanks to total energy of two photons

We first investigated two-photon ionization of pyrene in the absence of the electron acceptor. Photoinduced dynamics of pyrene in acetonitrile solution was investigated under various excitation intensities using transient absorption spectroscopy. Upon photoexcitation at 355 nm, the absorption band at 445 nm, which is ascribable to the radical cation of pyrene, was observed with the S1 state absorption under higher excitation power, while only the S1 absorption appears at low intensities. This result shows that pyrene is ionized by the multiphoton process. Furthermore, the excitation power dependence of this ionization was simulated by a numerical simulation to quantitatively analyze the ionization quantum yield in higher excited state. The absorption signal intensities of the S1 state and radical cation were reproduced by the simulation, and the ionization yield was estimated to be 0.20.

The excitation wavelength effect on the ionization was also investigated for optimization of the condition to produce the radical cation. The ionization dynamics was systematically measured by photoexcitation at 340, 355 and 370 nm. The absorption peak due to the radical cation is scarcely seen under 340 nm excitation condition. Under the 370 nm excitation, on the other hand, the quantity of the produced radical cation was maximized among the three conditions. This difference can be interpreted by distinct resonance of the excitation wavelength with the S1-S0 absorption. That is, under 370 and 355 nm excitation where the molar absorption coefficients were respectively 180 and 340 M-1cm-1, the S1 state produced by the first photon absorption can further absorbs the second photon in the excitation pulse. However, under 340 nm excitation at which the absorption coefficient is 4600 M-1cm-1, the second photon absorption by the S1 state is inhibited by the large absorption of the major remaining ground state molecules, which behaves as an intrinsic filter to hide the Sn-S1 absorption. Thus, the stepwise two-photon excitation does not effectively proceed under this condition. Taken together, the photoexcitation at the wavelength region, where the ground state absorption is not prominent, is crucial for efficient ionization driven by stepwise two-photon excitation.

We finally performed the proof-of-concept experiments on two-photon ionization-driven electron transfer of the pyrene-biphenyl system. Under the intense excitation condition optimized above, pyrene was excited in a two-photon manner, which can be confirmed by the instantaneous appearance of the radical cation. Other absorption bands show up around 400 and 640 nm in a few tens of picoseconds. These bands are attributable to the radical anion of biphenyl. A detailed global analysis revealed that the anion radical is produced with a time constant of 200 fs, which is followed by vibrational cooling of the resultant ion radicals and the S1 state with a time constant of 16 ps. This observation demonstrates that an electron ejected from pyrene is directly captured by biphenyl on an ultrafast timescale. The energy level of the CS state thus formed was estimated to be higher than that of the S1 state of pyrene by 0.53 eV, which is also confirmed by non-quenching behavior of pyrene fluorescence by biphenyl. In addition, the subsequent ionic dissociation without a remarkable geminate recombination in the sub-nanosecond to nanosecond time region effectively avoids the quantity loss of the CS state. The results in the present work show that two-photon excitation method enables ultrafast formation of the long-lived CS state at a high energy beyond the traditional framework of electron transfer reactions.

Figure 1