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Aggregation-caused quenching (ACQ) has long been a problem that inhibits the application of organic light-emitting materials in organic light-emitting diodes, especially near-infrared (NIR) materials. Figuring out the reasons that induce ACQ is important for the quantum efficiency enhancement of NIR materials. In this paper, an NIR molecule (TPA-QCN) with thermally activated delayed fluorescence (TADF) is studied based on first-principles calculations and excited-state dynamics investigation in both toluene and in the aggregation state. Our calculation indicates that aggregation can induce a smaller energy gap between the first singlet excited state and the first triplet excited state, which is favorable for TADF. Both the decreased fluorescent rate and the increased nonradiative rate will induce emission quenching in the aggregation state. Based on detailed analyses of the reorganization energy and intermolecular interaction, we find that the hydrogen bond will induce enhanced contribution to the reorganization energy from C–H stretching vibration modes and thus a larger nonradiative rate in the aggregation state than in toluene. A new mechanism of ACQ is proposed, and it could help in the design of new types of NIR-TADF molecules with enhanced fluorescence efficiency.

The paper presents the formalism, implementation, and performance of the analytical approach for the excited-state Hessian in the time-dependent density functional theory (TDDFT) that extends our previous work [J. Liu and W. Z. Liang, J. Chem. Phys. 135, 014113 (2011)] on the analytical Hessian in TDDFT within Tamm-Dancoff approximation (TDA) to full TDDFT. In contrast to TDA-TDDFT, an appreciable advantage of full TDDFT is that it maintains the oscillator strength sum rule, and therefore yields more precise results for the oscillator strength and other related physical quantities. For the excited-state harmonic vibrational frequency calculation, however, full TDDFT does not seem to be advantageous since the numerical tests demonstrate that the accuracy of TDDFT with and without TDA are comparable to each other. As a common practice, the computed harmonic vibrational frequencies are scaled by a suitable scale factor to yield good agreement with the experimental fundamental frequencies. Here we apply both the optimized ground-state and excited-state scale factors to scale the calculated excited-state harmonic frequencies and find that the scaling decreases the root-mean-square errors. The optimized scale factors derived from the excited-state calculations are slightly smaller than those from the ground-state calculations.

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The dodecamer d(CGCGAATTCGCG) was the first oligonucleotide to be crystallized as a B-DNA duplex. Its structure was analyzed in detail in the early 1980s. Here we show that, in the presence of Ca 2؉ , it crystallizes in a different way (R3 space group). The dodecamers form parallel columns of straight duplexes with ten base pairs in the B form. The terminal cytosines in each molecule are disordered, whereas the terminal guanines are placed in the minor groove of neighbor duplexes. The central GAATTC region is practically identical to that found in the classic structure of the same dodecamer crystallized in the P2 1 2 1 2 1 space group in the presence of Mg 2؉ and spermine. Its structure is thus independent of the crystallization conditions which have been used.The first detailed structure of a DNA oligonucleotide in the B-form determined by single crystal x-ray diffraction methods was published in this journal in 1982 (1). The structure of a bromine derivative was compared with that previously determined (2, 3) for the native dodecamer at different temperatures. Its features were analyzed in detail in several other publications (4 -7). Many other derivatives from the same oligonucleotide and related sequences, alone and in association with drugs, have also been studied. Coordinates and references may be found in the Nucleic Acid Database (8). Most of them are in practically identical unit cells in the P2 1 2 1 2 1 space group. The original work (1-3) was a landmark in the study of DNA because it confirmed unequivocally the double helical structure of B-form DNA and, at the same time, showed many features of conformation as a function of sequence. More recently, the same dodecamer has been crystallized under various ionic conditions (9 -13). A high resolution (1.4 Å) was obtained (9) in the presence of Na ϩ /Mg 2ϩ /spermine, giving a structure essentially similar to those first reported by Dickerson and co-workers (1-3). Given the higher resolution of that structure (9), we used it for comparison with our results.Most of the work described in the previous paragraph was carried out with crystals obtained in the presence of Mg 2ϩ and spermine. Recently we discovered (14) that, in the presence of a high concentration of Ca 2ϩ ion, this oligonucleotide could crystallize in the R3 space group. Here we report the results we have obtained under the latter crystallization conditions at a higher resolution than that previously reported (14). Our results show that the central sequence GAATTC is practically identical in both cases, whereas the conformation of the terminal CGC/GCG sequences is much more variable because of the interaction with neighbor molecules in the crystal. EXPERIMENTAL PROCEDURESCrystallization-The crystal was grown by the vapor diffusion hanging drop method. The crystallization solution contained 0.4 mM dodecamer (NH 4 ϩ salt), 300 mM CaCl 2 , 10% MPD (2-methyl-2,4-pentanediol), and 20 mM cacodylic acid (pH 7.0). The concentration of monovalent cations (Na ϩ from the buffer and NH 4 ϩ from the dodecame...