TRADITIONAL use of colorants has the aesthetic purpose while functional uses emphasize on the behavior of colored molecules in the presence of electromagnetic radiation. The interaction of light with matter is today fairly well understood and we are in a position to design a molecular material with specific phtophysical and physicochemical properties. All this success is attributed to the confluence of individuals striving to understand color and constitution aspects. The constitutional aspects have been mainly the forte of chemists, and color aspects that of physicists – both theoretical and experimental.
Prediction of color of a substance at all levels has been widely researched with a view to develop an a priori approach for making and understanding functional colored materials. Even before the theory of chemical bonding, hybridization carbon atom in the formation of organic molecules and similar aspects the relationships between color and chemical constitution were advanced out of sheer inquisitiveness. Color chemists define a colored materials as a molecular framework consisting of at least the three unique chemical groups, chromophore, auxochrome, and solubilizing group. Each of these three groups has a specific function in deciding the application of colorants in both textile and functional domain. This kind of approach together with the art of manipulating the chemical groups in molecules is really a great deal if one wants to artificially prepare a colorant with desired functional properties.
Alongside this vast challenging synthetic chemistry one also wants to know how these functional groups alter the color in the desired manner. So the theory of color and chemical constitution has been occupying a central and pivotal role in the progress of color chemistry. In the earlier days several qualitative concepts based on observations, empirical, and semi empirical approaches were developed and the subject has been periodically reviewed in several text books as well as reference materials.
The real understanding of color in relation to chemical constitution is attributed to physics in general and physics of electrons and electromagnetic radiation in a non-classical paradigm. Experimental quantum physics, atomic spectroscopy, and associated concepts in this realm triggered interest at all corners of the world. This became the need of the hour as a priori approach for the design of colorants when we want to have functional colorants.
Looking at the aspect of color from ground realities is of prime importance here. For a chemist it would serve as a healthy break to make a brief excursion to physical aspects and then toss around only with a few structures to make and study in his chemistry laboratory.
We say a molecule interacts with electromagnetic radiation causing electronic transitions. When this electronic transition occurs with the energy provided by the visible light, then we say the material absorbs visible light and hence colored. When it absorbs the entire visible radiation it looks black, when it does not absorb anything it appears white, and when it selectively absorbs a specific portion of visible light then it appears colored and the color is decided by what portion it absorbs. This treatment is applicable to a single molecule, a surface deposited with these molecules, as well as solutions containing these molecular materials.
It is not only one molecule which determines the color but the microenvironment which surrounds. Whether the same molecule is surrounded as in clusters, aggregates, agglomerates, or in a crystalline lattice, whether the interstices or voids are filled by solvent, or surfactants are all very important. On absorption of visible radiation the molecule is said to have been elevated to an energetically higher state called first excited state. The lifetime and the other characteristics of this excited state and its closeness to the ground state (before absorbing light) determines the color, its strength and other properties like fluorescence or phosphorescence. If it is a particulate matter then the wave characteristics of interacting electromagnetic radiation (like scattering, diffraction) come into play. Many of us hail this as an art, but crystal engineering and optics need to be understood here.
Thus understanding electronic transitions in a colored molecule happens to be the starting point. Now understanding the behavior of electrons in a molecule or in an atom needs an enormous knowledge of what we call quantum mechanics both in the non-relativistic and relativistic domain. With the help of quantum mechanical description of a molecule we can understand the color of different kind of molecules and start bench marking the whole aspect.
Developing methodological as well as approaches able to accurately deliver the transition energies corresponding to electronically excited-state of organic molecules particularly dyes remains a major challenge for theoretical chemists. It is a fact that we are today in need of a very assertive and robust methodology capable of bridging the different spatial and time scales that are quite appropriate to the description of photo-initiated processes in single molecules, nano-structures, bio-molecules, and extended systems. Absorption spectra of polyenes, cyanines were all obtained with reasonable accuracy using the traditional quantum mechanical treatment of a particle in one dimensional box formalism. But this could not be safely extended to other molecules.
The first computational schemes developed for understanding molecular absorption characteristics relied on semi-empirical formalisms. A quick estimate was successfully obtained using the so called ZINDO model and it still remains popular today. However, the quantitative aspect of the obtained results (absorption wavelengths and transition probabilities) was found to be highly system dependent and not consistent. Several other claims remain to be tested for larger molecules like dyes.
However, there is a general acceptance that computational studies can be really helpful in providing some hints about the important aspects of the studied molecules, like the prediction of transition energies, oscillator strengths, or the electronic nature of the ground and excited states. At the outset these methods must be validated in the computation of the target molecules, and the comparison to experimental measurements is mandatory. These results should provide a standardized procedure to be applied in the study of similar sets of organic dyes. Therefore the days have come that we can say a priori approach is definitely possible. Of course a level of approximation, such as Hartree-Fock, and technical details, such as a basis set, had to be chosen first, with each choice yielding what later came to be known as a “Model Chemistry”. Some very robust approaches have also come into vogue. They are the different truncations of configuration interaction, various orders of many-body (Møller-Plesset) perturbation theory, the coupled-cluster approach that involves summing certain perturbation theory terms to infinite order, and the complete active space (CAS) method. It also has to be admitted that today the electron correlation problem for strongly multi-configurational systems still cannot be regarded as solved in a satisfactory manner.
The time tested Schrodinger equation and the realistic Born-Oppenheimer approximation happen to be the two pillars on which the quantum chemical computations rest. For almost fifty years, it has been appreciated that relativistic effects (included via the Dirac equation or its various approximations) are important for heavier atoms and molecules. For a better accuracy even the other molecules may need the same. In recent years, a new scientific frontline in quantum chemistry has considered the effects of quantum electrodynamics (QED). Not only the relativistic effects but also the approximations also need to be readdressed for better accuracy.
As it is often seen than said, change is always permanent. The so-called ab initio calculations emerged with a definite goal soon after the Second World War, and the chemical conclusions are obtained with useful numbers derived computationally from the general principles of quantum mechanics. Color chemists are usually keen in understanding the so–called theoretical but actually real description of the interaction of organic molecules with time-dependent electromagnetic radiation (one should not forget that the Maxwell's theory of electromagnetic radiation considered to be theory has practical inroads into engineering discipline). In fact even today, we are still lacking a definitive and systematic methodology which can bridge the different spatial and time scales that are relevant for the description of light-induced processes in nanostructures, bio-molecules and extended systems with predictive power. At the same time we know that intricate photo-induced process are very important in the evolution.
A conceptually realistic understanding both the ground and excited states of organic molecules comes first in such approaches (these aspects are very essential in visualizing performance of devices based on such functional dyes). Particularly in the computation of electronic excited states, a large variety of molecular quantum chemistry methods is available. Many a times the hierarchy of the approximations defines the properties of the models, classifying them at different levels of complexity. At a particular stage one stops and starts interpreting the computational results reconciling with the experimental observations with a deeper understanding. Of course, the chosen computation level of theory will directly affect on the accuracy of the computed energy and other derived values, but it also determines its computational cost. Although quite high accuracy is desired in the calculation of transition energies of organic dyes (ideally, one would like to achieve an accuracy better than ~0.05eV), computational demands are rather worrisome, especially due to the exorbitantly large size of the molecules with the targeted properties. Therefore, accurate models in the determination of single electron character excitation energies like the equation-of-motion coupled cluster (EOM-CC) family of methods, the closely related linear response coupled-cluster (LR-CC), or the symmetry-adapted cluster configuration interaction (SAC-CI) methods cannot be considered in routine screenings of transition energies of such large molecules. So the wave function-based for larger molecules become more demanding. One need to understand the charge transfer character as well as dispersion character of electrons operating simultaneously in a molecule. These limitations become even stronger in multi-configurational-based wave function methods, like multi-reference configuration interaction (MRCI), multi-reference coupled-cluster (MRCC), or second order perturbation correction to the complete active space self-consistent field (CASPT2 or MCQDPT2 for example), which seem, in general, computationally prohibitive. Methods allowing for larger active spaces for the multi-configuration SCF wave function (MCSCF) reference are available, in particular, second-order perturbation theory (PT2) based on restricted active space SCF (RASPT2)24 or PT2 based on general MCSCF (GMCPT2).
Density functional theory and the associated time-dependent density-functional theory has repeatedly demonstrated in the last decade its usefulness when investigating this challenge associated with the wave function based approaches. The singular reason is the unparalleled balance between the computational load that it requires and the accuracy that it provides. The growing interest that these methods have been creating in the scientific community can be clearly measured by the exponential growth on the number of articles published in this field (similarly to what happened to standard density functional theory twenty years ago), and by the number of high-level scientific meetings focusing on time dependant density functional theory.
The fact however remains that time-dependent density functional theory represents a very attractive alternative even though the standard density functionals exhibit a sizable delocalization and static correlation error as well as the self-interaction error. The latter is crucially responsible for large errors in long-range charge transfer transition energies and becomes rather relevant in the kind of excitations occurring in molecular dyes. In spite of these serious limitations they are popular tools when comparative benchmarking as against absolute benchmarking happens to important, which is always the case. Therefore, one has to be very careful in drawing some conclusions derived from the application of such a methodology. Nevertheless today many research findings are supported by computational approaches using density functional theory based approaches.
No wonder, any research program on functional colorants has become heavily dependent on computational capability of the team.
Prof. N. Sekar
Dyestuff Technology Department,
Institute of Chemical Technology, Matunga, Mumbai - 400 019.