1. Introduction

Photoionization dynamics in solution is affected by properties of solvents, such as polarity and viscosity, and is different from that in the gas phase. In order to understand the photoionization in solution, investigation on the dynamics immediately after photoexcitation, both in polar and non-polar environment, is indispensable. Trans-stilbene is a prototype organic molecule that shows many interesting photo-induced dynamics. It is known that photoionization of trans-stilbene occurs in solution to generate the trans-stilbene radical cation. However, the ionization mechanism still remains controversial.

In the present thesis, I have studied photoionization of trans-stilbene in non-polar and polar solvents by using picosecond time-resolved absorption and Raman spectroscopies. The structure and dynamics of a newly found transient species, which is found to exist just after the photoexcitation in acetonitrile and which I identify as a precursor of the trans-stilbene radical cation, are discussed. The same two-step photoionization via a precursor is also found to take place in an ionic liquid. I have found an unknown transient species in non-polar solvent, heptane, that shows similar behavior as the precursor in polar solvents. The lifetime of this unknown species is determined to be a nanosecond by transient Raman spectra. Photoionization mechanisms of trans-stilbene in polar and non-polar solvents are compared and possible implication of the newly found transient species in the non-polar solvents is discussed.

2. Experimental (Chapter 2)

Sub-picosecond time-resolved visible absorption spectra were measured with a laboratory constructed pump-probe time-resolved ultraviolet-visible absorption spectroscopic system. The wavelength and the power of the pump light were 297 nm and ~1.2 mW, respectively. The probe wavelength was 370 to 750 nm. The polarizations of the pump and probe beams were set to make the magic angle (54.7°). The instrumental response time was ~180 fs. Picosecond time-resolved Raman spectra were measured with a laboratory constructed pump-probe Raman spectroscopic system. The wavelength and the power of pump beam were 300 nm and ~1 mW, respectively. The temporal resolution of the instrument was 6 ps.

In order to avoid accumulation of photoproduct and interference from fluorescence from the cell wall, sample solution was circulated to form a flowing thin jet film at the sample position.

3. Photoionization of trans-stilbene in acetonitrile (Chapter 3)

Picosecond time-resolved visible absorption spectra of trans-stilbene in acetonitrile are shown in Figure 1. Strong transient absorption bands are observed at 580 nm and 470 nm. They are known to arise from the S1 state trans-stilbene and the trans-stilbene radical cation, respectively. In addition to these well-known bands, I find a new feature around 440 nm (the rectangle in Figure 1), whose decay time constant coincides with that of the rise of the radical cation.

Picosecond time-resolved Raman spectra of trans-stilbene in acetonitrile measured with 455nm excitation are shown in Figure 2. The 1 ps spectrum shows a small but significant deviation from the radical cation spectrum observed after 98 ps. The peak position of the 1599 cm-1 band is 10 cm-1lower than the corresponding band (1609 cm-1) of the radical cation. This deviation indicates that the 1 ps Raman spectrum is contributed dominantly by a precursor of the radical cation. In order to confirm this assignment, the time dependence of the intensity of the 1599/1609 cm-1 band is examined. Figure 3 shows the time dependence of the integrated intensity of the 1599/1609 cm-1 band. This time dependence can not be fitted with a single exponential function; the intensities before 1 ps are too large to be fitted with a single exponential rise. Rather, fitting with a linear combination of two components, one with an instantaneous rise and a 24 ps exponential decay and the other with a 24 ps exponential rise, reproduce the observed curve very well (full line in Figure 3). This fitting analysis indicates that the 1 ps Raman spectrum is dominated by the bands due to the precursor of the radical cation. Extracted genuine Raman spectrum of the precursor is shown in Figure 4 in comparison with that of the radical cation observed at 1000 ps. Corresponding to the radical cation peaks at 1609, 1349, 1291, 1207, 1162, 994 and 866 cm-1, the precursor spectrum shows similar but appreciably broader peaks at 1599 and 1285 cm-1. The precursor may well be an ion pair that holds an ejected electron still around the cation.

4. Photoionization of trans-stilbene in a room-temperature ionic liquid (Chapter 4)

In order to investigate whether or not the two-step photoionization occurs in other polar-solvents, the picosecond time-resolved absorption and Raman spectra are measured in the solution of a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (emimTFSI).

Picosecond time-resolved visible absorption spectra of trans-stilbene in emimTFSI are measured. The bands at 440 nm, which I assign to the precursor of radical cation in acetonitrile, are also observed immediately after photoexcitation as well as the S1 state trans-stilbene. The rise of trans-stilbene radical cation band is also observed. Picosecond time-resolved Raman spectra of trans-stilbene in emimTFSI measured at -10, 3, 5, 10 and 100 ps with 450 nm excitation are shown in Figure 5. The band at 1595 cm-1 shifts to higher wavenumbers with increasing delay time and eventually reaches 1607 cm-1 at 100 ps, which is consistent with the observation in acetonitrile with 455 nm excitation. The similarity of dynamics in emimTFSI and acetonitrile implies that the photoionization of trans-stilbene in emimTFSI proceeds with the same two-step mechanism via precursor as in acetonitrile.

5. Photoionization of trans-stilbene in heptane (Chapter5)

The photochemical reaction dynamics of trans-stilbene in a non-polar solvent, heptane is also studied by picosecond time-resolved absorption and Raman spectra. As expected, the S1 state trans-stilbene is observed while the band at 470 nm assigned to the trans-stilbene radical cation is not (Figure 6). However, the band at 440 nm, which I assign the precursor of radical cation in polar solvents, is also observed. The assignment of the band is made with reference to the picosecond time-resolved Raman spectra with 455 nm excitation (Figure 7). The transient Raman spectrum at early delay time, 3 ps, resembles the precursor spectrum observed in polar-solvents with a strong broad peak at 1599 cm-1. However, in contrast to the observation in acetonitrile, where spectra gradually transform into the radical spectrum, the precursor-like spectra remain even after 100 ps, with a peak shift smaller than 4 cm-1. This observation may indicate the formation of ion-pair like structure in non-polar solvents as an intermediate for geminate recombination.

Figure 1. Picosecond time-resolved visible absorption spectra of trans-stilbene in acetonitrile measured at delay times (a) -1 ps, (b) 6 ps, (c) 10 ps, (d) 20 ps, (e) 30 ps, (f) 50 ps, (g) 100 ps and (h) 250 ps. Absorption bands surrounded by a rectangle is the new absorption band at 440 nm.

Figure 2. Picosecond time-resolved Raman spectra of trans-stilbene in acetonitrile measured at delay times of (a) -5 ps, (b) -2 ps, (c) 1 ps, (d) 3 ps, (e) 8 ps, (f) 18 ps, (g) 38 ps, (h) 58 ps, (i) 98 ps and (j) 1000 ps.

Figure 3. Time dependence of the integrated Raman intensity in the 1560 and 1656 cm-1 region (filled circles). The full line (a) indicates the least-squares fitted exponential function, A1 exp(-t/τ) + A2{1-exp(-t/τ)} with A2=27000, A2=9500, τ=24 ps. The dotted lines (b) and (c) show the two components A1 exp(-t/τ) and A2{1-exp(-t/τ)}, respectively, with A2=27000, A2=9500, τ=24 ps. The two exponential functions are convoluted with the instrumental function represented by a Gaussian with full-width-half-maximum of 5.5 ps.

Figure 4. Raman spectra of (a) the ion-pair precursor plus the S1 state trans-stilbene and (b) the radical cation.

Figure 5. Picosecond time-resolved Raman spectra of trans-stilbene in emimTFSI measured at delay times of (a) -10 ps, (b) 3 ps, (c) 5 ps, (d) 10 ps and (e) 100 ps.

Figure 6. Picosecond time-resolved visible absorption spectra of trans-stilbene in heptane measured at delay times (a) -10 ps, (b) 10 ps, (c) 30 ps, (d) 50 ps, (e) 80 ps, (f) 130 ps and (g) 280 ps. Absorption bands surrounded by a rectangle is the new absorption band at 440 nm.

Figure 7. Picosecond time-resolved Raman spectra of trans-stilbene in heptane measured at delay times of (a) -3 ps, (b) 0 ps, (c) 3 ps, (d) 5 ps, (e) 10 ps, (f) 20 ps, (g) 40 ps, (h) 60 ps, (i) 80 ps, (j) 100 ps, (k) 300 ps, (l) 500 ps, (m) 700 ps and (n) 1000 ps.

本論文は、ピコ秒時間分解分光法による溶液中におけるtrans-スチルベンの光イオン化初期過程の解明について記述されており、全6章から構成される。

第1章では導入として、本研究の目的が溶液中のtrans-スチルベンの光イオン化機構の理解にあり、そのためにピコ秒時間分解吸収およびラマン分光法を用いることが述べられている。trans-スチルベンは光化学反応の代表的なモデル分子であるにも関わらず、光イオン化機構の初期過程は未だ不明な点が多い。凝縮層における光イオン化反応の総合的な理解のために、光励起直後に生成する化学種についての構造、動的挙動の知見を得ることの重要性が述べられている。

第2章では、本研究に用いた装置と試料系について述べられている。ポンプープローブ法によるピコ秒時間分解吸収およびラマン分光法装置をさまざまな励起波長を用いて測定に使用した。また、高粘度のイオン液体の液膜ジェットの作成に初めて成功し、分光測定に適用した。光照射による副生成物の蓄積を抑制することにより、スペクトルのS/Nの向上したことが示された。

第3章では、アセトニトリル中のtrans-スチルベンの光イオン化について述べられている。ピコ秒時間分解吸収およびラマンスペクトル中に新しいバンドを発見し、バンド強度の時間挙動の解析により、ラジカルカチオンの前駆体と帰属した。前駆体のラマンスペクトルの取得に成功し、ラジカルカチオンと非常に近い構造を持つ化学種であることを示した。前駆体の帰属として光励起により生成した電子がまだ近傍で対として存在している状態のラジカルカチオンである可能性が示唆された。アセトニトリル中のラジカルカチオンの生成の初期過程として前駆体を経る2段階の光イオン化機構が提案された。

第4章では、イオン環境下でのtrans-スチルベンの光イオン化について述べられている。イオン環境は、カチオン、アニオンのみから構成され、常温で液体であるイオン液体を溶媒として用いることにより実現された。イオン液体中での光イオン化を初めて観測し、アセトニトリル中と同様の前駆体のバンドを過渡吸収および過渡ラマンスペクトル中に見出した。イオン環境下でのイオン化機構は、アセトニトリル中と同様に前駆体を経由することが示された。

第5章では、無極性溶媒中でのtrans-スチルベンの光イオン化について述べられている。ヘプタン中では、ラジカルカチオンは観測されなかったが、ラジカルカチオンの前駆体のバンドは吸収およびラマンスペクトル中で観測された。ラマンスペクトルの比較により極性溶媒と無極性溶媒において観測される前駆体は、同じ化学種であることが示された。また、励起後100 psを経るころには、極性溶媒中では従来知られていたラジカルカチオンの生成が確認されることに対し、無極性溶媒中では前駆体のままで存在し続けることが明らかとなった。以上の結果より、溶液中のtrans-スチルベンの光イオン化機構として、極性、無極性溶媒ともに電子とラジカルカチオンが対の状態である前駆体を経る生成機構が提案されている。前駆体のその後の挙動が極性、無極性により異なることが、溶媒和の関与との関連について論じられている。

第6章は以上の研究成果のまとめである。

本研究により、溶液中のtrans-スチルベンの光イオン化について、前駆体の存在が初めて実験的に明らかとなり、前駆体を経由する2段階光イオン化機構が提案された。前駆体の挙動に溶媒の極性依存性を見いだしたことは、前駆体と溶媒との相互作用がその後の光イオン化反応の機構を制御している可能性を示唆しており、これまで詳細が不明であった溶液中における光イオン化反応の溶媒極性依存性の発現する機構を解明する新たな手がかりとなり得る。本研究で明らかになったイオン対前駆体の存在や2段階光イオン化機構は、溶媒とイオンの相互作用についての本質的な理解を深める新たなモデルを提案する。このような新規反応機構の提案とその科学的意義を提示した本論文の内容は高く評価できる。

本論文第3章の主要部分は、Chemical Physics Letters 誌に受理済み、公表予定(吉田匡佑、川手千枝子、島田林太郎、〓屋智久、岩田耕一、〓口宏夫との共著)であるが、論文提出者が主体となって実験および解析を行なっており、その寄与が十分であるので、学位論文の一部とすることに何ら問題はないと判断する。

以上の理由から、論文提出者三木弘子に博士(理学)の学位を授与することが適当であると認める。