DENSITY FUNCTIONAL THEORY STUDIES OF PHOTOINDUCED ELECTRON EXCITATION AND TRANSFER OF ORGANIC DYES FOR PHOTODYNAMIC THERAPY, SOLAR CELLS, AND FLUORESCENCE SENSOR APPLICATIONS

The main aim of work presented here is to understand photophysical processes of organic dyes and to design better organic molecules/systems which can be applied in many applications such as solar cells, photodynamic therapy, and fluorescence sensors. Developments of novel multichromophore organic materials for the above mentioned applications were made using computational tools. A brief description of the history of computational chemistry was given based on the photochemistry of organic dyes in the introductory chapters and also the importance of basis sets and functionals was discussed in order to produce accurate computational results. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed to understand the photophysical processes in the porphyrin-perylene bisimide (HTPP-PDI) dyad that exhibited long-lived triplet states. The DFT results show that breaking the rigidity of PDI in HTPP-PDI was responsible for the generation of long-lived triplet states. Furthermore, six porphyrin derivatives were designed by introducing a 4,4’-dicarboxybutadienyl functional group to the porphyrin moiety and studied to investigate the substituent effects on the non-coplanarity, molecular orbitals, and excitation wavelength of the porphyrin donor. Five of the six proposed porphyrin derivatives are promising donors in the HTPP-PDI dyad to replace HTPP for its potential use in photodynamic therapy. Six donor- accepter(s) systems were designed for their potential application in solar cells. Four D-A1-A2 architectural triads, MTPA-TRC-AEAQ, MTPA-TRC-HTPP, MTPA-TRC-PDI, and MTPA-TRC-PBI were designed. The cascade electronic energy levels were obtained and experimentally observed, which lead to sequential electron transfers from 1MTPA* to TRC and then to AEAQ (HTPP/PDI/PBI) module as well as a hole transfer from 1AEAQ*(1HTPP*/1PDI*/1PBI*) to MTPA module. Therefore, all the D-A1-A2 systems we have designed are ambipolar. Interestingly, the lifetime of charge separated states of the newly designed MTPA*+-TRC-AEAQ*- was elongated to 650 ns, an eightfold of that of the donor-acceptor MTPA-TRC parent molecule (80 ns). However, different charge separated state lifetimes were obtained for MTPA*+-TRC-PDI*-(22ns) and MTPA*+-TRC-PBI*-(75ns). The photophysical results suggested that the charge separated state may decay to the triplet state when the charge separated state exhibits a higher energy level than the triplet state. Further, the photovoltaic tests indicated potential applications of MTPA-TRC-AEAQ in solar cells. DFT and TDDFT calculations were performed together with experimental studies to explore the nature of fluorescence enhancement in the anthracene-based sensor after the addition of Zn2+. A 23-fold fluorescence emission was quenched via non-radiative decay pathway in the absence of Zn2+. However, when the Zn2+ chelated to the sensor fluorescence intensity was increased remarkably. A 32-fold fluorescence increase was overserved and calculation results suggested this could be due to the inhibition of the electron-transfer pathway and enhanced rigidity of sensor-Zn2+ complex. The response selectivity of Zn2+ over Ca2+, Mg2+, Cu2+, and Hg2+ ions was also studied using DFT calculations and it was found that Zn2+ has a strong binding affinity to the sensor, which could be a potential application in the detection of Zn2+

DFT and TDDFT Studies of Silicon Analogs of Fluorescein Derivatives

Fluorescein derivatives play an important role in the field of biological and fluorescent sensors. To tune the spectroscopic properties many attempts have been made including extending conjugation, substituting the central carbon by nitrogen or introducing electron withdrawing groups and replace the oxygen bridge atom by other elements such as N, C, S, Se, and Te. In this paper we report density functional theory (DFT) and time-dependent DFT (TDDFT) studies of silicon analog of fluorescein derivatives with oxygen replaced by Si, C, and Ge. Among the different silicon analogs, the most conjugated molecule 4 showed red shift in absorption wavelength (495 nm). The OH position of molecule 2 has a significant effect on the spectral properties of the silicon analogs of the fluorescein. Since aggregation is very common in most of the fluorescein and it is interesting to study the effect of aggregation, we also studied dimerization of molecule 1 in silicon analog of fluorescein derivative and the results show that two absorption bands are formed with red shift compared to monomer

DFT studies of fluorescence probe for selective detection of Zn2+ in the presence of Ca2+, Mg2+, Cu2+, and Hg2+ ions

Fluorescence sensors are an important analytical tool for monitoring biologically relevant analytes. For instance, an anthracene based sensor was designed and characterized for the detection of Zn2+ in our previous studies. However, the selective detection of such a sensor in the presence of other metal cations is a critically important factor for its practical application. In this work, we employed density functional theory calculations to study the selectivity of this anthracene sensor in the detection of Zn2+ in the presence of Ca2+, Mg2+, Cu2+, and Hg2+. DFT results indicate that the selectivity of the sensor on Zn2+ detection over the cations Ca2+, Mg2+, and Hg2+ due to the binding selectivity as Zn2+ binds favorably to the sensor while Ca2+, Mg2+, and Hg2+ are no binding. Although Cu2+ binds to the sensor stronger than Zn2+, the chelated sensor by Cu2+ reduces the UV-Vis absorption at the free sensor wavelength by 10 times and the fluorescence pathway is also enhanced by the chelation, thus resulting response selectivity of Zn2+ over Cu2+ . Therefore, the present DFT study shows that the sensor selectivity on Zn 2+ detection in the presence of Ca2+, Mg2+, Cu2+, and Hg2+ is due to a combination of binding selectivity and response selectivity

DFT studies of binding of fluoride anions with silyl-fluorescein derivatives

Fluoride is abundantly used in dental healthcare and pharmaceutical industry, it is harmful if it is found over certain concentration in drinking water and in soil. It is therefore essential to develop portable tools to detect fluoride. Fluorescence sensing can be used as a simple and sensitive tool to detect fluoride anions. Fluorescent detection of anions is less widely studied compared to metal cation and neutral organic small molecules. As such, the interactions between F- anion and two silyl-fluorescein based sensors were studied here using density functional theory (DFT). Chemical shifts and electronic properties of the free sensor and the sensors in the presence of different number of fluoride anions were obtained and compared. The DFT results show that strong binding responses can be found as a function of fluoride concentration. The change in UV-Vis spectra due to the presence of fluoride indicates they are promising sensor candidates for fluoride detection and further studies on fluorescence response of these sensors are worthwhile

Surface Chemical Properties of Mo2C, W2C, Mo2N and W2N Probed with CO, CO2and O2 Adsorption: A DFT Analysis

作为具有吸引力的电极材料，过渡金属碳化物与氮化物被应用在许多电化学储能及能量转换领域. 本工作中，通过密度泛函理论计算，以及一氧化碳 （CO）, 二氧化碳（CO2）和 氧气（O2）分子的吸附来表征钼和钨的碳化物及氮化物，如碳化钼（Mo2C）、碳化钨（W2C）、氮化钼（Mo2N）和氮化钨（Mo2C）的表面化学性质. 这些探针分子可为研究钼和钨的碳化物及氮化物表面在酸性/碱性的氧化还原性质提供衡量方法. 计算结果表明，CO2分子的吸附发生在路易斯碱位，其碱性降低顺序为α-W2C(001) > α-W2N(001) > β-Mo2C(001) > γ-Mo2N(100). 此外，CO和O2分子吸附可用于评估上述碳化物及氮化物的还原能力，其还原性减小顺序为β-W2C(100) > α-Mo2C(100) > α-W2N(001) > α-W2C(001) > β-Mo2C(001) > γ-Mo2N(100). 由于还原本性，使得上述这些碳化物和氮化物成为在各种催化反应中有可能取代贵金属的良好候选材料.Transition metal carbides and nitrides are attractive materials for electrodes in many electrochemical energy storage and conversion applications. In the present study, we use density functional theory slab calculations to characterize the surface chemical properties of molybdenum (Mo) and tungsten (W) carbides and nitrides, namely, Mo2C, W2C, Mo2N and W2N with the adsorption of CO, CO2 and O2. These probing molecules provide measures of in both acidity/basicity and redox property of for the surfaces of these carbides and nitrides. Our results show that Lewis basic sites were responsible for CO2 adsorption and the basicity follows followed an order of α-W2C(001) > α-W2N(001) > β-Mo2C(001) > γ-Mo2N(100). Both CO and O2 adsorption provide measures of in the reducing ability of these carbides and nitrides. The results showed a reducing ability in the order of β-W2C(100) > α-Mo2C(100) > α-W2N(001) > α-W2C(001) > β-Mo2C(001) > γ-Mo2N(100). The reducing nature of these carbides and nitrides make them good candidates to substitute noble metals in various catalytic reactions.We acknowledge the support of NSF-CBET program (Award no. CBET-1438440).We acknowledge the support of NSF-CBET program (Award no. CBET-1438440).作者联系地址：美国南伊利诺伊大学化学与生物化学系, 卡本代尔, 伊利诺伊 62901Author's Address: Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, IL 62901, US

Effect of Triplet State on the Lifetime of Charge Separation in Ambipolar D‑A₁‑A₂ Organic Semiconductors

An ambipolar organic semiconductor with styrene based triphenylamine derivative (MTPA) as an electron donor (D), triazine group (TRC) as an electron acceptor (A₁), and 9,10-anthraquinone (AEAQ) as a second electron acceptor (A₂) has shown an 8-fold increase in the lifetime of charge separation with a high performance as solar cell materials with respect to the D-A₁ architecture and demonstrated a general D-A₁-A₂ architecture as a promising materials design strategy for photovoltaics. Here we synthesized and characterized two new D-A₁-A₂ compounds with perylene bisimide derivatives (PDI and PBI) as A₂ using an integrated experimental and computational method to study and compare the kinetics of three MTPA-TRC-A₂ systems. A two-step sequential decay pathway was observed in both MTPA-TRC-PDI and MTPA-TRC-PBI but a direct decay pathway in MTPA-TRC-AEAQ. The charge separated state with a lifetime of 22 ns in the PDI system and 75 ns in the PBI system relaxes to the corresponding triplet state followed by the decay to ground state in 827 ns and 29.2 μs, respectively. Thus, a triplet state with a lower energy than the charge separated state shortens the lifetime of the charge separated state but increases the overall lifetime of excited states

Enhancing Photoinduced Charge Separation through Donor Moiety in Donor–Acceptor Organic Semiconductors

Three systems were designed, synthesized, and characterized to understand decay processes of photoinduced charge separation in organic semiconductors that are imperative for efficient solar energy conversion. A styrene-based indoline derivative (YD) was used as donor moiety (D), a triazine derivative (TRC) as the first acceptor (A₁), and 9,10-anthraquinone (AEAQ) as a second acceptor (A₂) in constructing two systems, YD-TRC and YD-TRC-AEAQ. The lifetime of the photoinduced charge-separated states in YD-TRC, a D–A₁ system, was found to be 215 ns and that in YD-TRC-AEAQ, a D–A₁–A₂ system, to be 1.14 μs, a 5-fold increase with respect to that of the YD-TRC. These results show that YD is a more effective donor in YD-TRC and YD-TRC-AEAQ systems at forming long-lived charge-separated states compared to a previously reported atriphenylamine derivative (MTPA) that generated charge-separated states with a lifetime of 80 ns in MTPA-TRC and 650 ns in MTPA-TRC-AEAQ. The third system was constructed using a metal-free porphyrin derivative (MHTPP) to form a MHTPP-TRC-AEAQ structure, a D–L (linker)–A system with a charge separation lifetime less than 10 ns. Therefore, the D–A₁–A₂ architecture is the best at generating long-lived charge-separated states and thus is a promising design strategy for organic photovoltaics materials

Enhancing Photoinduced Charge Separation through Donor Moiety in Donor–Acceptor Organic Semiconductors

Three systems were designed, synthesized, and characterized to understand decay processes of photoinduced charge separation in organic semiconductors that are imperative for efficient solar energy conversion. A styrene-based indoline derivative (YD) was used as donor moiety (D), a triazine derivative (TRC) as the first acceptor (A₁), and 9,10-anthraquinone (AEAQ) as a second acceptor (A₂) in constructing two systems, YD-TRC and YD-TRC-AEAQ. The lifetime of the photoinduced charge-separated states in YD-TRC, a D–A₁ system, was found to be 215 ns and that in YD-TRC-AEAQ, a D–A₁–A₂ system, to be 1.14 μs, a 5-fold increase with respect to that of the YD-TRC. These results show that YD is a more effective donor in YD-TRC and YD-TRC-AEAQ systems at forming long-lived charge-separated states compared to a previously reported atriphenylamine derivative (MTPA) that generated charge-separated states with a lifetime of 80 ns in MTPA-TRC and 650 ns in MTPA-TRC-AEAQ. The third system was constructed using a metal-free porphyrin derivative (MHTPP) to form a MHTPP-TRC-AEAQ structure, a D–L (linker)–A system with a charge separation lifetime less than 10 ns. Therefore, the D–A₁–A₂ architecture is the best at generating long-lived charge-separated states and thus is a promising design strategy for organic photovoltaics materials