| The intramolecular proton transfer reactions have attracted great experimental and theoretical interests due to its importance to chemistry and biochemistry. The experimental results show that the excited state intramolecular proton transfer (ESIPT) of 2-(2′-hydroxyphenyl)-5-phenyl-1, 3, 4-oxadiazole (HOXD) can take place in the first singlet excited state (S1) and dual fluorescence phenomenon can be observed. The reverse ESIPT can occur in the first triplet excited state (T1) to form the enol form which emits phosphorescence. So the electroluminescent efficiency is greatly raised that is the demanding for electroluminescence materials of organic light-emitting diodes (OLEDs) in flat panel display technologies. The intramolecular proton transfer, rotational process and optical properties of 2-(2′-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole (HOXD) and its O/"NH"and O/"S"substituted derivatives: 2-(2′-hydroxyphenyl)-5-phenyl-1,3,4-triazole (HOT) and 2-(2′-hydroxyphenyl)-5-phenyl-1,3,4-thiadiazole (HOTD) have been respectively studied. The main results are outlined as follows:We performed theoretical studies on the intramolecular proton transfer (cis-enol→keto), rotational process (cis-enol→trans-enol) and optical properties of HOXD in the ground (S0) and excited states (S1 and T1). The geometries were optimized at HF and DFT levels for S0, while CIS method was employed to optimize the geometries for the excited states. The 6-31G* and 6-31G** basis sets were used to perform geometry optimizations. To introduce the dynamic electron correlation, single-point energy calculations for the ground and excited states have been done at the DFT and TD-DFT level, respectively, with B3LYP functional, using the 6-31+G** basis set. The theoretical results showed that the high forward energy barrier inhibits the proton transfer in S0, while the ESIPT can take place through a low energy barrier in S1. In T1, the reverse ESIPT (keto→cis-enol) can occur. In S0, cis-enol and trans-enol can coexist, but the rotation between the two forms should be impossible because of the high energy barriers in S1. All theoretical results are in agreement with the experimental results. Furthermore, we also studied its optical properties using the TDDFT method and found that TD-B3LYP/6-31+G** method provides reasonable results for the absorption (λabs) and fluorescence (λfl) wavelengths, while underestimates the phosphorescence wavelength (λph). Theλph obtained at the B3LYP/6-31+G** level is in good accordance with the experimental result.On the basis of the suitable method for HOXD, we performed theoretical studies on HOT and HOTD at the same level as HOXD. It was revealed that the proton transfer reaction is very unlikely in S0 but energetically favored in S1 and the forward energy barriers in S1 are in the following order: HOXD > HOT > HOTD. The reverse proton transfer can occur in T1 for HOXD or HOTD, and O/"S"substitution decreases the reverse energy barrier. The rotation process is feasible in S0, while impossible in S1 because of the high energy barriers for each of the three investigated molecules. The theoretical studies on HOT and HOTD revealed thatλabs andλfl of HOT show hypsochromic shift, whileλabs,λfl andλph of HOTD show bathochromic shift relative to HOXD. Furthermore, the O/"S"substitution shows a more pronounced influence onλabs of 1Ec andλfl of 1Et* and 1K*, while the O/"NH"substitution has a more significant influence onλabs of 1Et. The O/"S"substitution has a more remarkable influence on theλph thanλabs andλfl, 101 nm red-shift for phosphorescence were predicted for HOTD compared with HOXD, making HOTD a good candidate as the organic light-emitting materials for the phosphorescence emission. For HOT, the keto form is more stable than the cis-enol form in T1, so the chance to observe the phosphorescence is rare. |
