Ground state recovery

The mechanism in Scheme 15 suggests the rapid formation of an intermediate that is cis about the central (3-methine bond, which leads eventually to the ring-closed form but which can also re-form the state as the equilibrium concentration depletes. Scheme 16 is based on the fact that two possible conformers for the merocyanine have been identified (TTC and TTT) [36] and these may have different ground-state recovery times, but TTC and TTT generate the closed form with equal efficiency. Both of these schemes have merit and both have possible flaws however, what is clear is that there is no triplet-state involvement and that the bleaching reaction generates the spiropyran closed form essentially within 1.5 nsec. 

Therefore, we have developed a pump/pump-probe experiment to obtain more informations on the structures of these geminate ion pairs. It allows the investigation of the excited states dynamics of the transient species at different time delays after photo-triggering the charge transfer, by monitoring the ground state recovery (GSR) of those transient species (Fig. lb). In the present study, we have used perylene (Pe) as fluorescer (electron donor) and either trans-l,2-dicyanoethylene (DCE) or 1,4-dicyanobenzene (DCB) as quencher (electron acceptor) in acetonitrile (ACN). 

Fig. 1. a) Formation of the geminate ion pair upon electron transfer b) Energy diagram for ground state recovery measurements on the electron transfer product. 

In order to understand the dynamics of the solvent fluctuation, many experimental as well as theoretical efforts have been made intensively in the last decade. One of the most convenient methods to observe solvent reorganization relaxation processes within the excited state molecule is time resolved fluorescence spectroscopy. By using time resolved techniques a time dependent fluorescence peak shift, so ( ed dynamic Stokes shift, has been detected in nanosecond picosecond >, and femtosecond time regions. Another method to observe solvent relaxation processes is time resolved absorption spectroscopy. This method is suitable for the observation of the ground state recovery of the solvent orientational distribution surrounding a solute molecule. 

Ground State Recovery of Phenol Blue in Fluids near the Gas-Liquid Critical Density Yoshifumi Kimura and Noboru Hirota... 

For these purposes we have studied the solvatochromic dye of Phenol Blue (PB, Scheme 1) in trifluoromethane (CF3H) near the critical density. Although betaine-30 is more preferable for the comparison with liquid solvent, it was impossible due to the extremely low solubility of betaine-30 to CF3H. PB is more soluble in CF3H, and has absorption around 560 nm which is accessible by our laser system. Unfortunately the photophysics of PB has not been studied until now even in liquid solution. Therefore we have also studied the ground state recovery in several liquid solvents. According to the theoretical calculation, the charge... 

King SM, Rothe C, Dai D, Monkman AP (2006) Femtosecond ground state recovery Measuring the intersystem crossing yield of polyspirobifluorene. J Chem Phys 124(23) 234903... 

Transient absorption changes of spheroidene in cyclohexane at 540-nm induced by 480-nm pulses showed the ground-state recovery, Sj Sq, atroomtemperature tobe9.1 ps. However, similar scans made with a higher time resolution showed an initial bleaching (attributable to the Sq->S2 transition) which rapidly decayed in 0.34ps to a longer-lived state. The 0.34-ps lifetime of this S2 state is consistent with the lifetime indirectly estimated from the quantum yield of fluorescence, 0. ... 

The same authors also investigated the bleaching and recovery of the ground state of the dye after a 1-ps laser excitation [48]. Since these experiments were performed under open-circuit conditions, all the electrons transferred from the excited dye into the conduction band of SnS2, had to return to the oxidized dye. The corresponding ground state recovery was found to occur within about 10 ps. Other investigations on transfer rates will be discussed in Section 10.2.7. 

It should be mentioned that direct recombination from the excited molecule to the ground state, and also the ground state recovery due to recombination and transfer of electrons via surface states (compare with Fig. 10.21), have been neglected in this derivation. According to Eq. (10.17), the exponential decay is determined by k. Since in heavily doped electrodes, ki includes the tunneling rate constant kj, the decay should... 

In the case of vibrational responses the population relaxation times may be dominating the coherence decays. In addition, it can be essential to incorporate the multilevel nature of molecular vibrators into the response. The rate of repopulation of the ground state is seldom equal to the decay of the fundamental V = 1 state, so there can be bottlenecks in the ground state recovery. Following... 

Abouaf-Marguin et al. have excited the lowest frequency C-F stretching modes of CHjF and CDjF using a pulsed CO2 laser and monitored the ground-state recovery by means of a weak CW probe laser. They again find an order of magnitude slower relaxation in the deuteride. In both of these studies the results were rationalized by accepting the vibrations rotation relaxation mechanism. 

The films and LEDs also exhibited PL-, EL-, and a-detected triplet exciton resonances. The primary PL-detected patterns were attributed to triplet excitons localized on the thiophene and phenylene rings, and only slightly larger. However, patterns due to other distinct triplets were observed in some films and LEDs. The nature of the triplet exciton ODMR was discussed in relation to the role of triplets as Si quenching sites, ground state recovery , and triplet-triplet fusion to S,. ... 

The estimated lifetime of IB is 100 fs(i ). Absorption experiments with picosecond time-resolution have shown, however, that typical ground state recovery times (i ) are about 10 ps (2,6). This is again a strong indirect evidence for a state between IB and the ground state. We can thus give the scheme in Fig. 5 for the lowest states. 

In Fig. 6 the kinetic curve at 548 nm showing the energy transfer in the B880 complex of Rsp. rubrum is displayed. The transfer time is estimated to be 1-3 ps. i.e. it competes with the 10 ps ground state recovery lifetime observed in vitro (2,5,6). 

---