Molecular Modeling of Photoluminescent Copper(I) Cyanide Materials. Jasprina L Ming Advisor: Craig A Bayse

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1 Molecular Modeling of Photoluminescent Copper(I) Cyanide Materials Jasprina L Advisor: Craig A Bayse Department of Chemistry and Biochemistry, Old Dominion University, Hampton Boulevard, Norfolk, Virginia 23529, and Department of Chemistry. Studies show that solid copper(i) cyanide has interesting photoluminescent properties. Recent work has shown that the UV spectrum of CuCN may be attributed to Laporte-allowed excitations from occupied to unoccupied π-type molecular orbitals (MOs). The emission spectrum is attributed to the relaxation from an excited state triplet with a bent structure due to distortions that remove degeneracies in the partially occupied MOs of the linear triplet. The addition of aliphatic and aromatic amines to CuCN chains leads to substantial changes in the structural and photophysical properties of the material including shifts in λ max for both the excitation and emission spectra and orbital mixing. The study of CuCN materials has been expanded to examine the affect of amines on substituted CuCN (zigzag and helical) chains that were optimized at the DFT (BLYP) level. Time- dependent DFT (TD-DFT) studies on the optimized structures of these model compounds reveal that the excitations for the substituted chains are generally consistent with the unsubstituted chains; however, differences are attributed to type of ligand, structure, and stoichiometry. Introduction Copper (I) cyanide (CuCN) is an interesting substance that forms 1D chains in the solid form and photoluminesces in the UV region of the spectrum. Coordination of an amine to the copper center of the CuCN chain shifts the emission to the visible region of the spectrum 1. The luminescent properties of CuCN can be explained by the excitations of electrons in occupied π type orbitals to unoccupied π type orbitals 10. Intersystem crossing from the lowest excited singlet state to the lowest excited triplet state occurs because it is a lower more stable energy state. The relaxation of the triplet state to the ground state yields the emission spectrum. Moreover, this relaxation has a smaller singlet to triplet gap and the wavelength is in the visible region of the spectrum. The smaller singlet to triplet gap is due to the bent shape of the material from the bonding of the amine to the Cu. Variations can be seen in the singlet to triplet gap depending on the type and amount of amines added to the Cu. The emission wavelength of the materials is highly dependent upon the nature of the amine and the ratio of CuCN to amine. An example of this is the copper cyanide-2-methyl pyridine material. When the ratio between the CuCN and 2-methyl pyridine is 1 to 1, one CuCN and one 2-methyl pyridine, it emits blue light. When the ratio is 3 to 2, three CuCN and two and one 2-methyl pyridine alternating respectively, it emits yellow light. Lastly, with a 2 to 1 ratio, two CuCN and one 2-methyl pyridine, the light emitted is pink. 11 A few of the materials are not photoluminescent; however, some are thermo chromic. The issue that surrounds the design of the photoluminescent materials is that relationship between the wavelength and the color variation between the CuCN amine materials is unknown. This could be used to design display devices also known as light emitting diodes. In contrast to many light sources, display devices are more energy efficient, generate less heat, and have a longer use period. Moreover, display devices have the capability to be created in a variety of colors, which could prove useful for creating customized LED signs and devices. Display devices could also be used for the molecular detection of many gases. The CuCN amine material could also be used to make emergency signs in case of power failure that may occur on shuttles in space. Moreover, it could be used for optical monitoring of surface temperatures for techniques such as landing and used as a precautionary method for hot surfaces. The photoluminescent CuCN amine material could also be useful in photonic computing, which is using either photons or light particles in place of electrons to collect, retrieve, and process data. The photoluminescence of these CuCN amine materials could also be useful in providing more promise for optical communication, in which light is used as the transmission medium for telecommunication. The goal of this project is to enhance the design of CuCN amine materials by using computational methods to model the variable photoluminescent properties of CuCN amine materials. 1

2 Theoretical Methods Gaussian 03 3 was used to optimize the copper(i) cyanide unsubstituted and substituted chains at the DFT level using the BLYP 4,5 exchange correlation (xc) functional. The copper atoms were represented by the Emler- Christiansen relativistic effective core potential (RECP) basis set 6. It was modified to include the 4p contractions of County and Hall 7. Dunning 8 split triple zeta plus polarization functions (TZVP) basis sets were used for the carbon, hydrogen, and nitrogen atoms. Diffuse functions were also added to all carbon and nitrogen atoms in the chain, with the exception of potassium. Potassium atoms were represented by the Hay-Wadt RECP basis set 9. TD-DFT was used to generate the excitation spectrum of the chains. Results and Discussion First, potassium capped three-cucopper(i) cyanide chains were decorated with ammonia ligands to model 1:1 aliphatic amine substituted CuCN materials. Potassium was used as the counterion to sustain a symmetrical environment because it is commonly used as a mineralizing agent for copper cyanide. The ammonia substituted copper(i) cyanide chain was constructed in such a way that each copper ion was bonded to two cyanides and one ammonia and constrained to a planar zigzag symmetry (Figure 1). When this constraint is released, the chain optimizes to the low energy helical conformation. The zigzag chain yields one more excitation band is visible when compared to the linear chain. The lowest energy transition for the zigzag chain is found at a much longer wavelength, nm. Again, the 6πg to 7πu excitation mirrors the excitation for the linear chain. The other major excitations are found at wavelengths nm, nm, and nm. The unsubstituted chain has excitations at wavelengths nm and nm, which closely resemble the excitations for the zigzag chain. The intensities are larger at longer wavelengths for the unsubstituted chain. However, for the zigzag chain, the intensities are a constant 0.11 at all wavelengths. These significant changes need to be explored more experimentally. The variations in the substituted chains cause transformations in the orientation of the molecular orbitals. By making the backbone zigzag, the MOs orientation with respect to each axis is altered slightly. For example, Figure 2 shows that the transition from MO 73 (6πg) to 81 (7πu) is almost exact to that of the transition from MO 59 (6πg) to 66 (7πu). However, the MO is rotated 90 degrees around the x-axis. The helical substituted chain MOs become distorted due to the shape of the backbone. Moreover, there appears to be some mixing between δ and π type orbitals which may also be the cause of the variations in MO orientation. The most significant contribution to these variations is the addition of ammonia to the chain. Adding ammonia to the chains changed the hybridization of the copper from sp 2 to sp 3, which in turn affected the copper-carbon bonding. Table 1 shows a significant increase in the bond distance of both substituted chains for the Cu-C and N-Cu bonds, which are almost identical. The central Cu-C bond distance changes from Ǻ (substituted) to Ǻ and Ǻ, for zigzag and helical respectively. The N-Cu bond distance changes from Ǻ (unsubstituted) to Ǻ and Ǻ, for zigzag and helical respectively. The addition of the ammonia on the chains also affected the λ max. The helical chain yielded two less excitation bands (Table 2). The band that is prominent is at a longer wavelength, nm. In respect to the helical substituted CuCN chain, the π π transitions are in agreement with unsubstituted chain. The lowest, and only, energy transition is at nm and it corresponds to the excitation from the 6πg molecular orbital (MO) to the 7πu molecular orbital (Figure 2). The intensity for the excitation is 0.12 which is extremely close to intensity of the zigzag chain, Figures 3 and Figure 4 show the transition energies that correspond to the zigzag and helical chains. Although there is no direct relationship between intensity and wavelength for the two chains, the figure shows that for the zigzag chain the intensities are constant at around 0.11, they do not change at different wavelengths. As for the helical chain, it is obvious that there is only one major peak. To examine the effect of aromatic amines on CuCN photoluminescence, a pyridine substituted copper cyanide chain was constructed. In each of the chains, the carbon end of the cyanide was facing toward the copper. The pyridine substituted planar CuCN chain has two major excitation bands that are at significantly larger wavelengths (Table 1) than the unsubstituted planar chain. Dissimilar to the unsubstituted chain, the pyridine substituted planar chain showed a significant decrease in intensity, and , for excitations, with 2

3 the λ max at nm and the other major excitation at nm. Figure 5 shows the intensity plotted against the varying angles of the chain for one main excitation. The excitation from 51 to 55 shows an increase in intensity as the angle increase and the excitation from 49 to 55 shows a decrease in intensity as the angle increases. This is due to the effect of the pyridine on the 1 K chain. The dependence of the dominant excitation on the rotation angle of the pyridine may be a factor in the photoluminescence of CuCN decorated with aromatic amines. Table 1: Bond Distances (A) and Bond Angles of the substituted and unsubstituted K-Capped Chains at the DFT/BLYP Level for K-capped linear unsubstituted [Cu n CN (n+1) ] (n=3) [Cu n CN (n+1) (NH 3 ) 3 ] (n=3), CuCN(C 5 H 5 N) (n=3), and CuCN(C 5 H 5 N) (n=1). 3-CuCN Chain Cu-C C-N N-Cu Cu-C C-N N-K Cu-C Average Angle Unsubstituted Zigzag Helical Table 2: BLYP Wavelengths (nm) and Oscillator Strengths (f) for π π Transitions (TD-DFT) for K- capped linear unsubstituted [Cu n CN (n+1) ] (n=3) [Cu n CN (n+1) (NH 3 ) 3 ] (n=3), CuCN(C 5 H 5 N) (n=3), and CuCN(C 5 H 5 N) (n=1). CuCN Chain λ, nm Unsubstituted K-capped f Zigzag K-capped f Helical K-capped f 0.12 Figure 1: Structures for the unsubstituted linear and substituted zigzag and helical CuCN chains (counterclockwise). 3

4 Figure 2: Major excitation transitions for 3 k CuCN linear chain, (CuCN(NH) 3, helical), and (CuCN(NH) 3,zig-zag). 4

5 Figure 3: Transition energies and oscillator strengths for the zigzag K 2 [Cu 3 CN 4 (NH 3 )] + chain. Figure 4: Transition energies and oscillator strengths for the helical K 2 [Cu 3 CN 4 (NH 3 )] + chain. Figure 5: Excitation plot of substituted 1 k at 0 to 90 degree angles. 5

6 Conclusion CuCN unsubstituted linear chains luminesce in the UV and visible regions of the spectrum; however, CuCN amine chains luminesce in the visible region of the spectrum. The alteration of the backbone of these amine bearing CuCN chains from linear to zig-zag and helical affect the shifting of the π π transitions of the chains. It also causes orbital mixing with δ and π orbitals, yielding distorted MOs. The fact that adding ammonia to the chains shift the λ max significantly for the chains and changes the excitation spectrum to yield more or less bands can be attributed to the change in the excitations from occupied to unoccupied π-type molecular orbitals (MOs) between the ammonia and copper. The low energy helical and zigzag conformations are due to the bent triplet formation. The beginning angle is larger for the substituted chains causing for less energy to be used in order to change it to that bent triplet state as compared to the unsubstituted chain. Acknowledgement The authors thank the National Science Foundation for support of this work (CHE ). J.M. thanks the Virginia Space Grant Consortium for an Undergraduate Research Fellowship. Also, J. M. thanks Sonia Antony for training in the basics of the UNIX operating system. References 1 (a) Tronic, T. A.; dekrafft, K. E.; Lim, M. J.; Ley, A. N.; Pike, R. D. Copper Cyanide Networks: Synthesis, Luminescence Behavior and Thermal Analysis. Part 1. Diimine Ligands. Inorg. Chem. 2007,46, (b) Lim, M. J.; Murray, C. A.; Tronic, T. A.; dekrafft, K. E.; Ley, A. N.; debutts, J. C.; Pike, R. D.; Lu, H.; Patterson, H. H. Cyanide Networks: Synthesis, Structure, and Luminescence Behavior. Part 2. Piperazine Ligands and Hexamethylenetetramine. Inorg. Chem. 2008, 47, Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, Becke, A. D. Phys. ReV. A 1988, 38, (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785. (b) Colle, R.; Salvetti, O. Theor. Chim. Acta 1975, 37, Hurley, M. M.; Pacios, L. F.; Christiansen, P. A.; Ross, R. B.; Ermler, W. C. J. Chem. Phys. 1986, 84, Couty, M.; Hall, M. B. J. Comput. Chem. 1996, 17, Dunning, T. H. J. Chem. Phys. 1971, 55, Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, Bayse C. A.; Brewster T. P; Pike P. D., J Inorg Chem. 2009, 48, Unpublished data. R.D. Pike, College of William and Mary. 12 Dunning T H. Jr. J Chem Phys. 1971, 55, Unpublished data. R.D. Pike, College of William and Mary. 3 Frisch, M. J. Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; 6