Transient absorption spectroscopy spectra

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

The UV-visible absorption and fluorescence spectra of NKX-2311 in solution are measured. The absorption and fluorescence maxima are located at 490 and 555 nm, respectively. The fluorescence spectrum is shown in Fig. 1. This figure also shows the transient absorption spectrum of NKX-2311 in deuterated methanol (CD3OD) solution measured by the nanosecond laser system. The spectrum is ascribed to the excited singlet state (dye ). The absorption spectrum of the oxidized form of NKX-2311 (dye+) was measured by nanosecond transient absorption spectroscopy after adding electron acceptor, 1,4-benzoquinone, in NKX-2311 solution of CD3OD. As shown in Fig. 1, there are two characteristic peaks at around 875 and 1010 nm. 

In the case of compound 32, no change in the absorption spectrum compared with that of 33 could be observed, neither in cyclohexane nor in benzonitrile. This indicates that intramolecular CT interaction is negligible in the ground-state. Nevertheless, fluorescence and transient absorption spectroscopy show that processes after excitation strongly depends on solvent polarity. In cyclohexane the Si state... 

Transient absorption spectroscopy, wherein one measures the electronic absorption spectrum of a molecule in an excited state, is still in its infancy, but the growing availability of ultra-high-speed, rapid-scan spectrometers augurs well for this area of spectroscopy. Thus one may, in the future, routinely probe excited state absorption spectra as well as ground state absorption spectra. The former can be expected to be as valuable in obtaining information about the excited state as is the latter for the ground state. 

These conclusions were supported by transient absorption spectroscopy, which revealed signals corresponding to the formation of the diimine radical anion, with lifetimes in close agreement with the luminescence lifetimes. Time-resolved infrared spectroscopy of the acetylide C = C bonds provides further conclusive evidence for the MLCT assignment. Thus, in the ground state IR spectrum of 4, there are two v(C=C) bands at 2115 and 2124 cm-1, whilst the step-scan FTIR difference spectrum obtained 50 ns after irradiation at 355 nm reveals bleaching of the parent bands, and the formation... 

Therefore, a molecule in the TICT state can be regarded as a rigidly linked radical anion-radical cation pair. Experimental proof for this expectation can be gained from transient absorption spectroscopy. In the simplest case, the absorption spectrum of the TICT excited state is expected to be the sum of the individual ion spectra. This was indeed found in a few cases, with some perturbations which can be explained by the interaction of the closely-spaced radical ions. Thus, the transient absorption spectra of DMABN [135], of DMABK (or DMABA) [137] and of BA [58] resemble the spectrum of benzonitrile and acetophenone radical anion (the absorption of the dimethyl-amino radical cation is expected to be situated in the UV region and could not be observed) and to the sum of anthracene anion and cation absorption spectra, respectively. 

In this paper we investigate the time dependent ground state hole spectrum of cresyl violet in polar solvents by means of subpicosecond transient absorption spectroscopy. The time correlation function expressed by eq (7) showed large difference in time profiles compared with the reported one expressed by eq (6). Possible mechanisms will be discussed. 

The P +-Q state can be observed by transient absorption spectroscopy because the absorption spectrum of the charge-separated state differs from that of the porphyrin first excited singlet state. Measurement of the transient absorption spectrum of a benzonitrile solution of 8 as a function of time following excitation with a 150 to 200-fs laser pulse at 590 nm allowed determination of the rate constant for decay of P" -Q back to the ground state (step 3 in Figure 3). The value obtained for kj, is 5.3 X 10" s- [61]. 

The absorption spectrum of 22 is nearly identical to the sum of the spectra of unlinked model compounds. The long wavelength band of the free base porphyrin is observed at 650nm in chloroform, whereas that of the zinc porphyrin is at 590nm. Excitation of a chloroform solution of 22 with a 15 ns pulse of 650 nm laser light leads to the formation of a carotenoid radical cation which can be detected by transient absorption spectroscopy (Figure 14). This ion arises from the charge separated state C -Pzh-P-Qa-Qb, and is formed with a quantum yield of 0.83. The lifetime of the species is 55/iS. 

Fig. 7.16. Characterization of the photoinduced charge-separated state of a porphyrin-fullerene diads with flexible polyether linkers by transient absorption spectroscopy. The differential absorption spectrum is given for a 50 ns delay after excitation at 532 nm with a 10 ns laser. (Reprinted with permission from ref. [41]).

Fig. 11. (A) Decay-associated difference spectra (DADS) of the 3-, 28-ps and the non-decaying components of Synechocystis PS-I core complex in the 380-500 nm region under reducing conditions and at room temperature. (B) The absorbance-difference spectrum AA [Ao"-Ao] (solid line) the same spectrum [see Fig. 9 (A), left panel] obtained from spinach is included for comparison. Figure source Mi, Lin and Blankenship (1999) Picosecond transient absorption spectroscopy in the blue spectral region of photosystem I. Biochemistry 38 15234. 15235.

In the case of 5-membered rhodopsin, only a long-lived excited state (r = 85 ps) was formed without any ground-state photoproduct (Fig. 4.5D), giving direct evidence that the CTI is the primary event in vision [39]. Excitation of 7-membered rhodopsin, on the other hand, yielded a ground-state photoproduct with a spectrum similar to photorhodopsin (Fig. 4.5C). These different results were interpreted in terms of the rotational flexibility along the C11-C12 double bond [39]. This hypothesis was further supported by the results with an 8-membered rhodopsin that possesses a more flexible ring. Upon excitation of 8-membered rhodopsin with a 21 ps pulse, two photoproducts - photorhodopsin-like and bathorhodopsin-like products - were observed (Fig. 4.5B) [40], Photorhodopsin is a precursor of bathorhodopsin found by picosecond transient absorption spectroscopy [41]. Thus, the picosecond absorption studies directly elucidated the correlation between the primary processes of rhodopsin and the flexibility of the Cl 1-02 double bond of the chromophore, and we eventually concluded that the respective potential surfaces were as shown in Fig. 4.5 [10,40]. 

The photoinduced electron transfer reactions have also been studied with nanosecond transient absorption spectroscopy [67]. The transient absorption difference spectrum for the reaction of 12a and 4-(methoxycarbonyl)-A/-methylpyr-idinium is shown in Fig. 4. The difference spectrum is characterized by a sharp intense absorption at approximately 390 nm, a Iowa- intensity band at 484 nm, and an intense broad absorption band at approximately 693 nm. The sharp band at around 390 nm is characteristic of pyridinyl radical absorption. The reaction mechanism is depicted in Scheme 1. 

The photoinduced electron transfer behavior has also been confirmed by nanosecond transient absorption spectroscopy [102,103,108,111]. A representative transient absorption difference spectrum recorded 10 ps after laser flash excitation of 24a (0.05 mM) and 4-(methoxycarbonyl)-A[-methylpyridinium hexaflu-orophosphate (13 mM) in degassed acetonitrile (0.1 M "Bu4NPF6) is shown in... 

The spectrum (Amax = 410nm) and kinetics of singlet ortfto-biphenylnitrene 67a were recorded by LFP of 65a in glassy 3-methylpentane at 77 K. The lifetimes of 67a and 67-d9 at 77 K are equal to 59 3ns and 80 + 2ns, respectively. A similar spectrum was detected recently at room temperature using femtosecond transient absorption spectroscopy (Figures 11.16 and 11.17). " ... 

As in the case of 65a, the transient absorption spectrum of singlet nitrene 67b was detected at ambient temperature using a fs transient absorption spectroscopy and at 77 K... 

Ns laser photolysis measurements on (1-pyrenyl) - (CH2 ) (p-N,N-dimethylaminophenyl) (abbreviated as P-(CH2)n"D) were performed in some solvents (18). Their absorption spectra are reproduced by the superposition of bands of the donor cation and the acceptor anion. Only the bandwidth is dependent upon solvent polarity and the number of CH2 groups, which is reduced to the relative geometrical structure of the donor and the acceptor. Exciplex formation dynamics of these compounds has been established by ps transient absorption spectroscopy (19). The absorption spectrum of P-CH2-D in 2-propanol is almost independent of the delay time. On the other hand, P-(0112)3-0 gives a similar spectrum at about 1 ns after excitation, and its spectral shape broadens with a time-constant of 1.2 ns. 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

---