As mentioned above, ESIPT reaction involves transient chemical changes and thus the electronic properties, especially the dipole moment and polarizability. Introducing an electron-donating or -withdrawing group to ESIPT molecules, therefore, can be a good molecular design strategy to intensify their transient alternations, bringing about excited-state intramolecular charge transfer (ESICT) coupled reactions. In that case, not only the proton transfer reaction which transforms E* form into K* form but also the charge transfer reaction which results in forming a charge transfer species CT*, subsequently giving rise to solvent relaxation to reach the equilibrium polarization, take place upon photoexcitation. According to the rate of each reaction, the following two different types of ESICT coupled ESIPT reaction are possible: one is that ESICT occurs prior to ESIPT and another is vice versa.21, 30 Because these two different reaction routes involve a lot of interesting and unique photophysical features, numerous studies to rationalize their dynamics and also to reveal the mechanistic origins have been reported, so far. Recently, detailed reviews on these issues were published by Chou and colleagues.21, 22 In this section, we will describe, therefore, only a brief summary of the different solvatochromic shift in the emission spectrum of ESICT coupled ESIPT molecules in the steady-state spectroscopic viewpoints.
As mentioned above, ESIPT reaction involves transient chemical changes and thus the electronic properties, especially the dipole moment and polarizability. Introducing an electron-donating or -withdrawing group to ESIPT molecules, therefore, can be a good molecular design strategy to intensify their transient alternations, bringing about excited-state intramolecular charge transfer (ESICT) coupled reactions. In that case, not only the proton transfer reaction which transforms E* form into K* form but also the charge transfer reaction which results in forming a charge transfer species CT*, subsequently giving rise to solvent relaxation to reach the equilibrium polarization, take place upon photoexcitation. According to the rate of each reaction, the following two different types of ESICT coupled ESIPT reaction are possible: one is that ESICT occurs prior to ESIPT and another is vice versa.21, 30 Because these two different reaction routes involve a lot of interesting and unique photophysical features, numerous studies to rationalize their dynamics and also to reveal the mechanistic origins have been reported, so far. Recently, detailed reviews on these issues were published by Chou and colleagues.21, 22 In this section, we will describe, therefore, only a brief summary of the different solvatochromic shift in the emission spectrum of ESICT coupled ESIPT molecules in the steady-state spectroscopic viewpoints.
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