The Effect Of Hydrogen Bonding Dynamics On Photophysical Process

The study of hydrogen bond in the excited state is essential for understanding the luminescent intensity or frequency change of luminescent material before and after selective binding to object. In this paper, the complexes formed by luminescent material and object were constructed on the basis of experimental data and phenomena. According to the energies and distances, the reasonable complex was chosen as theoretical model. The density functional theory (DFT) and time-dependent density functional theory (TDDFT) were used for systematically studying the hydrogen bonding interaction between luminescent material and object. To further reveal the effect of gust molecule on the luminescent properties, hydrogen bonding dynamics and photophysical processes were investigated. It can provide the guidance for the design of high sensitive optical sensors.It was found that luminescent metal-organic framework (LMOF) [Zn(3-tzba)(2,2'-bipy)(H2O)]·3H2O is able to interact with formaldehyde through hydrogen bonding to the framework. The luminescent mechanism of the hydrogen-bonded complex is photo-induced electron transfer (PET); while the luminescent mechanism of LMOF [Zn(3-tzba)(2,2'-bipy)(H2O)]·3H2O is ligand-to-ligand charge transfer (LLCT). The intermolecular hydrogen bond was found to be stronger in the excited state than that in the ground state by analyzing the geometry,1HNMR, binding energy and infrared spectrum in different electronic states. Calculated fluorescence radiative rate coefficient (KE) and internal conversion (IC) rate coefficient (kIC) qualitatively indicated a reduced radiative process and an enhanced IC process of the hydrogen-bonded complex. Comparing to the LMOF, the hydrogen-bonded complex exhibits luminescence weakening or even quenching due to the strengthening of the intermolecular hydrogen bond in the excited state. The variable luminescence demonstrated the potential of LMOF [Zn(3-tzba)(2,2'-bipy)(H2O)]·3H2O as luminescent sensor for formaldehyde detection.To study the effect of water molecules on the fluorescence property of 4-(diphenylamino)benzaldehyde dye in micelle and reveal the nature of enhanced fluorescence, hydrogen bonds formed by the aldehyde of the dye and a water molecule were investigated. The study of luminescent mechanism intuitively revealed the percentage change of frontier molecular orbitals compositions of the dye. Combining with the results of electronic excitation energies, the weakened hydrogen bonds in excited state were proven by the analysis of the geometry, hydrogen binding energy and nuclear magnetic resonance, which can lead to fluorescence enhancement of dye.The photophysical processes of diiodo-borondipyrromethene (I2-BODIPY) and the complex formed by I2-BODIPY and an oxygen molecule were studied to explain the oxygen sensing mechanism through steady-state quenching, which enriched the theoretical study of oxygen sensor. Comparison of the energies of different complexes, it can be indicated that the triplet hydrogen-bond complex and halogen-bond complex were formed. The halogen-bond complex was chosen as computational model. The analysis of frontier molecular orbital and electron configuration demonstrated that the electron density of the lowest unoccupied molecular orbital (LUMO) for halogen-bond complex all transits to oxygen molecules. Oxygen molecules can significantly alter the luminescent mechanism of I2-BODIPY. Furthermore, the analysis of rate coefficients for each transition process indicated the mainly dissipative process for I2-BODIPY in the oxygen atmosphere is T1?T0 internal conversion in the excited state.