Performance of DFT Methods in the Calculation of Optical Spectra of Chromophores
Organic chromophores possessing a high degree of π -conjugation are ideal materials for advanced electronic and photonic applications including optical information processing, photovoltaic cells, photodynamic therapy agents, and many other applications. These properties are due to the stable macrocyclic conjugated network of π -electrons leading to high electrical polarizability and rapid nonlinear optical response (NLO) of the charge density to the applied intense electromagnetic fields. Changing the substitutional groups and substitution pattern, conjugation and molecular electronic structure can conveniently modify the optical spectra and NLO properties of chromophores. Due to almost limitless optimization space of chromophore structures computational tools are increasingly being used to assist experimental efforts in designing the optimal chromophores. By far the most often used computational approaches are semi-empirical and DFT methods. The semiempirical computational methods, such as ZINDO/CIS are fast and accurate for chromophores similar to systems for which the method was parameterized. DFT, on the other hand, offers the best compromise between accuracy and computational performance for typical chromophores of about 100 atoms. The accuracy of the methods depends strongly on the type of chromophores. For example, the ZINDO/CIS method is excellent for polyphenylacetylene dyes, but fails for many cyano-based acceptors of chromophores; DFT may be better suited for tricyanofuranyl complexes but leads to large errors in spectra of the polyphenylacetylene dyes. The failure of DFT calculations has been associated with the incorrect asymptotic behavior of the typical exchange-correlation (XC) potential. The approximate XC potential does not posses the correct 1/r asymptotic behavior, which leads to the self-interaction error (SIE). As a result, the excited states of dyes, in particular the charge-transfer (CT) chromophores are poorly described. A variety of approaches have been developed to treat this problem. The exact (i.e., Hartree Fock (HF)) exchange is frequently added to partially remove the SIE error resulting in several hybrid methods with a constant fraction of HF contribution. The well-known models include B3LYP, PBE1PBE and BH-LYP incorporating 20, 25 and 50% of HF exchange, respectively. Recently, a new class of functionals has been proposed which includes a growing fraction of exact exchange as the distance increases. These long-range (LC) corrected functionals have been found to provide improved correlation between the calculated and experimental optical properties of several dyes. The recently proposed BNL functional, which falls into this LC-class, is of particular interest for studying the push-pull dyes because it has been shown to exactly reproduce the charge-transfer excitation for model systems. In this paper we have implemented the BNL method in the parallel NWCHEM program package and applied it to calculate the optical spectra of highly active NLO chromophores such as 2-dicyanomethylen-3-cuano-4-{2-[E-(4-N,N-di(2-acetoxyethyl)-amino)-phenylene-(3,4-dibutyl)thien-5]-E-vinyl}-5,5-dimethyl-2,5-dihydrofuran (denoted FTC) and YLD_124. Extensive validation of the BNL functional and comparison to other functionals was performed. In addition, a TCV chromophore with a strong acceptor ( tricyanovinyl) and donor (dimethylamine) was also studied. These chromophores allow for accurate validation of the method because the UV-Vis spectra in the entire visual region were recently obtained.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 947476
- Report Number(s):
- PNNL-SA-61071; KP1504020; TRN: US200909%%103
- Resource Relation:
- Conference: Proceedings of the DoD HPCMP User Group, 235-240
- Country of Publication:
- United States
- Language:
- English
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