Spectroscopy and Femtosecond Dynamics toward the Excited State Proton/Electron Transfer Coupled Reactions

Part 1:
Detailed insights into the excitation behaviors for charge versus proton transfer in para-N,N-ditolylamino-salicylaldehyde (Ia) have been gained via luminescence spectroscopy and femtosecond dynamics. In cyclohexane, following an ultrafast rate (~2.0x1012 s-1) of excited-state intramolecular proton transfer (ESIPT), fast equilibrium takes place between normal (N*) and tautomer excited states (T*), resulting in dual fluorescence maximized at 450 and 540 nm, respectively, with a common population decay rate of 360 ps-1. The normal emission exhibits drastic solvent-polarity dependence and has been concluded to originate from a charge-transfer species incorporating excited-state intramolecular charge transfer from ditolylamine to carbonyl oxygen. In dipolar solvents, competitive rates between ESIPT and solvent relaxation were observed, and the solvated charge-transfer state is thermodynamically more favorable, so that the T*-N* reverse proton transfer takes places. Supplementary supports were provided by the corresponding experiments for the methoxy derivative of Ia as well as other relevant analogues. The results shed light on detailed proton/charge transfer coupled dynamics as well as the associated solvent-relaxation dynamics at an early time domain.
Part 2:
The excitation behaviors for 4’-N,N-diethylamino-3-hydroxyflavone (Ia) and 4’-N,N-diethylamino-3-methoxyflavone (Ib) have been investigated via femtosecond fluorescence upconversion approaches to gain detailed insights into the mechanism of proton/charge transfer coupling reaction. In polar solvents such as CH2Cl2 and CH3CN, Ib undergoes an excited state intramolecular charge transfer (ESICT) reaction, and its early fluorescence decay correlates well with the solvent relaxation dynamics, resulting in a continuously time-dependent Stokes shifted emission. For the case of Ia, in addition to a slow, solvent-polarity dependent rate (a few tens of picoseconds-1) of excited state intramolecular proton transfer (ESIPT) reported previously, early femtosecond relaxation dynamics clearly reveal that the proton-transfer tautomer emission consists of a rise component of a few hundred femtoseconds. The temporal spectral evolution at the time domain of zero to a few hundred femtoseconds further resolves two distinct emission bands consisting of a proton transfer tautomer emission and a time-dependent Stokes shifted charge transfer emission similar to that observed in Ib. The results, in combination with ab initio calculations on the dipolar vectors for normal and tautomer species in both ground and excited states, lead us to unveil the importance of the relationship of the dipolar vectors among various states, and hence the corresponding solvation energetics, in order to shed light on the overall ESICT/ESIPT coupled reaction in Ia. We conclude a similar dipolar character between ground-state normal (N) and excited proton-transfer tautomer (T*) species, whereas due to ESICT, the normal excited state (N*) possesses a large dipolar change with respect to N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvation relaxation process. After reaching the solvent equilibration, there exists an equilibrium between N* and T* states in e.g. CH3CN. Due to the greatly different equilibrium polarization between N* and T*, both forward and reversed ESIPT dynamics are associated with a solvent induced barrier. The latter viewpoint of the equilibrium type of ESIPT in Ia is in agreement with the previous reports based on steady state,10 picosecond,11, and femtosecond , dynamic approaches. The generalization of mechanism of the dipolar tuning ESIPT/ESICT reaction is also discussed.
Part 3:
Comprehensive excitation behaviors of 7-N,N-diethylamino-3-hydroxyflavone (I) have been investigated via steady state, temperature dependent emission, and fluorescence upconversion to probe the excited state intramolecular charge/proton transfer (ESICT/ESIPT) coupled reaction. For I in polar solvents such as CH2Cl2 and CH3CN, in addition to a relatively slow, solvent-polarity dependent rate (a few tens of picoseconds-1) of ESIPT, femtosecond dynamics clearly reveals the competitive rates between solvation and proton-transfer dynamics. Firm supports are rendered by the spectral temporal evolution, which resolves two distinct bands, i.e., ESICT and ESIPT emission at < few ps. The results, in combination with ab initio calculations on the dipolar vectors for various corresponding states, lead us to conclude that excited-state normal (N*) and excited proton-transfer tautomer (T*) possesses very different dipole orientation, whereas the dipole orientation of the normal ground state (N) is in between that of N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to solvent relaxation dynamics induced by ESICT. Unlike the well-known ESICT/ESIPT system, 4’-N,N-diethylamino-3-hydroxyflavone, in which equilibrium exists between N* and T*, N*-T* ESIPT for I is a highly exergonic, irreversible process in all solvents studied. Further temperature dependent studies deduce a solvent-polarity perturbed energy barrier of 3.6 kcal/mol in CH3CN. To extend the dipolar tuning ESICT/ESIPT mechanism, 7-N,N-diethylamino-4’-N,N-dimethyl-3-hydroxyflavone (II) was synthesized via addition of the 4’-N,N-dimethyl substituent to I. As a result, the changes of dipolar vector are largely cancelled out, and ESIPT behavior is similar to that of the parent 3-hydroxyflavne in that the proton transfer dynamics is only slightly dependent of solvent polarity. The proposed dipolar tuning mechanism thus generalizes the ESICT/ESIPT coupled reaction dynamics in polar, aprotic solvents.