DFT study of the optical properties of triptycene (T)-urea (U) containing compounds and their complexes for targeted analyte sensing

Explosives are energetically very reactive electron deficient substances and have toxic and detrimental effects on all living creatures, especially animals, plants and humans. They are good electron acceptors because of having various electron withdrawing –NO2 groups. The most well-known hazardous explosives including nitro groups are TNT, 2,4-DNT, TNP, 1,3-DNB, PA, HMX, PETN and RDX. Some health problems such as headache, skin diseases and anemia arise from direct contamination of very limited quantity of TNT. In addition to this, exposure to excessive amount of TNT and by-product DNT residues may cause some significant neurological and biological damage for human health such as liver problems, dermatitis, cyanosis and aplastic anemia. Exposure to PA causes both eye problem as yellow-tainted vision and vertigo, nausea, vomiting and diarrhea. The reported minimum level of TNT residues for toxicity musn't be above 2 ng.ml-1 in groundwater. As a result, it is very crucial for our environment and humankind to detect these explosives from soil and water. For this reason, a wide variety of sensors such as fluorescence sensors, electro-chemical sensors and bio-sensors have been designed not only for environmental, clinical and industrial applications but also for homeland security, military and national security purposes. Additionally, these sensors are more applicable for analyses of environmental pollutants such as air and water pollutants, drugs, forensic, food and etc. Especially, fluoresence chemical sensors are mostly preferable with their applications in solution, solid phase or gaseous state as a thin film by means of having very short response time, good selectivity, easy visualization, real-time monitoring, sensitivity and simplicity. Meanwhile, they are inexpensive and reliable sensors not affected by electrical interference, sample flow rate and stir speed. However, fluoresence enhancement is influenced by solubility, flexibility, geometric coordination of molecules, conformation and covalent and noncovalent bonding interactions in molecules. Especially, hydro-thermally stable and moisture resistant and biodegrable polymeric materials are significant for luminescence in fluorescence sensors. For example, organic dyes such as fluorescein, rhodamine, cyanine used as luminescent materials are not found as very sensitive for sensing explosives due to having versatile fluorophores, low absorbance and poor photostability. However, MOFs and COFs have been recently used for their reversible luminescent property due to having permanent porous surface structure. But, they are not moisture resistant enough, especially MOFs posess low hydrothermal stability. To overcome this barrier in sensors, using polycyclic aromatic hydrocarbons having relatively stable covalent C-C, C-H and C-N bonds, which contribute to its exceptional hydrothermal stability are more appropriate. Therefore, fluorescent semiconducting polymers consisting of π- conjugated polymers such as P dots or Q dots are preferable for their perfect brigtness, photostability, fast emission rate and nonblinking features. The applications and preparations of these conjugated polymeric sensors are limited because of their multi-step synthesis and poor molecular recognition. Therefore, to develop highly efficient fluorescence sensors for detecting explosives, S. A Sydlik and T. M Swager's group in MIT have worked experimentally on porous polymers including triptycene polyureas (TPUs) for targeted analyte sensing. Using of triptycene gives rise to numerous advantages in polymers. Triptycene produced from antracene and benzene with Diels-Alder reaction is a paddlewheel shaped, aromatic and bicyclic molecule. Due to its rigid 3-dimensional structure, it has intrinsic free volume in polymeric materials. This property prevents not only aggregation of polymer chains but also gives solubility to polymers with increasing photostability and alignment. Internal free volume facilitates analyte diffusion into polymers. Mesoporosity arised from internal free volume of triptycene interrupts polymer interactions, especially in high frequency environment and gives rise to lower refractive indices and dielectric constants at optical frequencies by modulating the structure of the polymer with air. Triptycene increases toughness, stiffness, tensil strength and ductility. Detection and sensing of some new explosives known as RDX is very challenging owing to having lower vapor pressure of 6 ppt at 250 C compared to TNT with a vapour pressure of 8 ppb. Cyclohexanone that has a high vapor pressure is used as an additive for coupling this kind of explosives and as a solvent for recrystallization. To benefit from the noticable properties of Triptycene in sensing of explosives, TPU was produced from Triptycene and Urea in laboratory conditions since the propety of intrinsic free volume of triptycene in polymers facilitates infiltration of analytes. In order to understand the optical properties of TPU and the role of urea group as hydrogen-bond-donor in the form of complexes with analytes, acetone, DMMP, DMF and cyclohexanone were used as H-bond accepting solvents. It is very crucial that TPU as a polymer has good quantum yields in that case. It is very fluorescent in the presence of DMF and DMSO which are H-bond accepting solvents. However, it is not fluorescent in non-H-bonded solvents. To induce fluorescence, the material is irradiated with a frequency of UV or visible light that it can absorb. This promotes an electron from the valence band into the conduction band, forming a bound electron-hole pair known as exciton. So, electrostatic coulomb force occurs between the positively charged electron hole and the negatively charged bound electron. Some of these excitons relax to the ground state in a radiative fashion by emitting light as fluorescence, whereas the other electrons release their energy non-radiatively as heat in the form of molecular motion. The time interval between the absorption and emission is approximately 1 ns. When encountered with electron deficient explosives, these complexes produce signal transduction by quenching florescence by means of migration of exciton. Although TPU and its complex structures don't have any fluorophores, It is experimentally observed that it becomes emissive in the presence of H-bond between urea group of TPU and the carbonyl group of H-bond acceptor solvents such as cyclohexanone, DMMP and DMF. In order to enhance the fluorescence feature of TPU and its complex structures, two new molecules, MC1 and MC2 were also sytnhesized by Swager et.al. In this thesis, our aim is to study the fluorescence behaviours of H-bonded TU, MC1 and MC2 and analyte complexes with cyclohexanone, acetone and DMMP. For this purpose, the geometries of the TU, MC1 and MC2 molecules naked and in the form of complex were optimized using density functional theory (DFT) method at B3LYP/ 6-31g(d,p) and M06-2X/6-31G(d,p) levels of the theory. The electronic properties such as HOMO, LUMO energies, the (HOMO-LUMO) band gap, the charge densities of hydrogen atoms in urea group and the charge densities of oxygen atoms in carbonyl group before and after the H-bonding interaction and also H-bond length were determined. The optical properties and UV-absorption and emission features were studied using time-dependent DFT (TD-DFT) method at the same level of the theory used for the geometry optimizations. The UV spectra of TU and its complexes originated from the formation of H-bond were compared with each other and also with the experimental values.