In this paper we present a computational study of linear optical absorption in phenacene class of... more In this paper we present a computational study of linear optical absorption in phenacene class of polyaromatic hydrocarbons. For the purpose, we have employed a correlated-electron methodology based upon configuration-interaction (CI) approach, and the Pariser-Parr-Pople (PPP) π-electron model Hamiltonian. The molecules studied range from the smallest one with three phenyl rings (phenanthrene), to the largest one with nine phenyl rings. These structures can also be seen as finite-sized hydrogen-passivated armchair graphene nanoribbons of increasing lengths. Our CI calculations reveal that the electron-correlation effects have a profound influence not just on the peak locations, but also on the relative intensity profile of the computed spectra. We also compare our phenacene results with isomeric oligo-acenes, and find that in all the cases former have a wider optical gap than the latter. Available experiments based upon optical absorption and electron-energy-loss-spectroscopy (EELS) are in very good agreement with our results.
Several years back Angliker et al. [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the... more Several years back Angliker et al. [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the first linear acene with the triplet state 1 3 B 2u as the ground state, instead of the singlet 1 1 A g state. However, contrary to that prediction, in a recent experimental work Tönshoff and Bettinger [Angew. Chem. Int. Ed. 2010, 49, 4125] demonstrated that nonacene has a singlet ground state. Motivated by this experimental finding, we decided to perform a systematic theoretical investigation of the nature of the ground, and the low-lying excited states of long acenes, with an emphasis on the singlet-triplet gap, starting from naphthalene, all the way up to decacene. Methodology adopted in our work is based upon Pariser-Parr-Pople model (PPP) Hamiltonian, along with large-scale multi-reference singles-doubles configuration interaction (MRSDCI) approach. Our results predict that even though the singlet-triplet gap decreases with the increasing conjugation length, nevertheless, it remains finite till decacene, thus providing no evidence of the predicted singlet-triplet crossover. We also analyze the nature of many-particle wavefunction of the correlated singlet ground state and find that the longer acenes exhibit tendency towards a open-shell singlet ground state. Moreover, when we compare the experimental absorption spectra of octacene and nonacene with their calculated singlet and triplet absorption spectra, we observe excellent agreement for the singlet case. Hence, the optical absorption results also confirm the singlet nature of the ground state for longer acenes. Calculated triplet absorption spectra of acenes predict two well separated intense long-axis polarized absorptions, as against one such peak observed for the singlet case. This is an important prediction regarding the triplet optics of acenes, which can be tested in future experiments on oriented samples.
The electronic and optical properties of four different coronene derivatives with lower symmetry,... more The electronic and optical properties of four different coronene derivatives with lower symmetry, namely, benzo[a]coronene (C$_{28}$H$_{14}$), naphtho[2,3a]coronene (C$_{32}$H$_{16}$), anthra[2,3a]coronene (C$_{36}$H$_{18}$) and naphtho[8,1,2abc]coronene (C$_{30}$H$_{14}$) were investigated. For the purpose, we performed electron-correlated calculations using screened and standard parameters in the $\pi$-electron Pariser-Parr-Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using MRSDCI methodology. The PPP model Hamiltonian includes long range Coulomb interactions which increases the accuracy of our calculations. The results of our calculations predict that with the increasing sizes of the coronene derivatives, optical spectra are red shifted as well as the optical gaps decrease. In each spectrum, the first peak represents the optical gap which is moderately intense, while the more intense peaks appear at higher energies. Our c...
The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with low... more The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with lower symmetry, namely, benzo[ghi]perylene (C 22 H 12), benzo[a]coronene (C 28 H 14), naphtho[2,3a]coronene (C 32 H 16), anthra[2,3a]coronene (C 36 H 18) and naphtho[8,1,2-abc]coronene (C 30 H 14) were investigated. For the purpose, we performed electron-correlated calculations using screened, and standard parameters in the πelectron Pariser-Parr-Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using MRSDCI methodology. PPP model Hamiltonian includes long-range Coulomb interactions, which increase the accuracy of our calculations. The results of our calculations predict that, with the increasing sizes of the coronene derivatives, optical spectra are red shifted, and the optical gaps decrease. In each spectrum, the first peak representing the optical gap is of moderate intensity, while the more intense peaks appear at higher energies. Our computed spectra are in good agreement with the available experimental data. For the purpose of comparison, we also performed first-principles timedependent density-functional theory (TDDFT) calculations of the optical gaps of these molecules using Gaussian basis functions, and found that they yielded values lower than our CI results.
In this paper we present a detailed computational study of the electronic structure and optical p... more In this paper we present a detailed computational study of the electronic structure and optical properties of triply-bonded hydrocarbons with linear, and graphyne substructures, with the aim of identifying their potential in opto-electronic device applications. For the purpose, we employed a correlated electron methodology based upon the Pariser-Parr-Pople model Hamiltonian, coupled with the configuration interaction (CI) approach, and studied structures containing up to 42 carbon atoms. Our calculations, based upon large-scale CI expansions, reveal that the linear structures have intense optical absorption at the HOMO-LUMO gap, while the graphyne ones have those at higher energies. Thus, the opto-electronic properties depend on the topology of the graphyne substructures, suggesting that they can be tuned by means of structural modifications. Our results are in very good agreement with the available experimental data.
In this paper, we perform large-scale electron-correlated calculations of optoelectronic properti... more In this paper, we perform large-scale electron-correlated calculations of optoelectronic properties of rectangular graphene-like polycyclic aromatic hydrocarbon molecules. Theoretical methodology employed in this work is based upon Pariser-Parr-Pople (PPP) π-electron model Hamiltonian, which includes long-range electron-electron interactions. Electron-correlation effects were incorporated using multi-reference singles-doubles configurationinteraction (MRSDCI) method, and the ground and excited state wave functions thus obtained were employed to calculate the linear optical absorption spectra of these molecules, within the electric-dipole approximation. As far as the ground state wave functions of these molecules are concerned, we find that with the increasing size, they develop a strong diradical open-shell character. Our results on optical absorption spectra are in very good agreement with the available experimental results, outlining the importance of electron-correlation effects ...
In this paper, we report large-scale configuration interaction (CI) calculations of linear optica... more In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of various isomers of magnesium clusters Mg n (n=2-5), corresponding to valence transitions. Geometry optimization of several low-lying isomers of each cluster was carried out using coupled-cluster singles doubles (CCSD) approach, and these geometries were subsequently employed to perform ground and excited state calculations using either the full-CI (FCI) or the multi-reference singles-doubles configuration interaction (MRSDCI) approach, within the frozen-core approximation. Our calculated photoabsorption spectrum of magnesium dimer (Mg 2) isomer is in excellent agreement with the experiments both for peak positions, and intensities. Owing to the sufficiently inclusive electron-correlation effects, these results can serve as benchmarks against which future experiments, as well as calculations performed using other theoretical approaches, can be tested.
We present systematic and comprehensive correlated-electron calculations of the linear photoabsor... more We present systematic and comprehensive correlated-electron calculations of the linear photoabsorption spectra of small neutral closed-and open-shell sodium clusters (Nan, n=2-6), as well as closed-shell cation clusters (Nan + , n=3, 5). We have employed the configuration interaction (CI) methodology at the full CI (FCI) and quadruple CI (QCI) levels to compute the ground, and the low-lying excited states of the clusters. For most clusters, besides the minimum energy structures, we also consider their energetically close isomers. The photoabsorption spectra were computed under the electric-dipole approximation, employing the dipole-matrix elements connecting the ground state with the excited states of each isomer. Our calculations were tested rigorously for convergence with respect to the basis set, as well as with respect to the size of the active orbital space employed in the CI calculations. These calculations reveal that as far as electron-correlation effects are concerned, core excitations play an important role in determining the optimized ground state geometries of various clusters, thereby requiring all-electron correlated calculations. But, when it comes to low-lying optical excitations, only valence electron correlation effects play an important role, and excellent agreement with the experimental results is obtained within the frozen-core approximation. For the case of Na 6 , the largest cluster studied in this work, we also discuss the possibility of occurrence of plasmonic resonance in the optical absorption spectrum.
In this paper, optical and electronic properties of diamond shaped graphene quantum dots (DQDs) h... more In this paper, optical and electronic properties of diamond shaped graphene quantum dots (DQDs) have been studied by employing large-scale electron-correlated calculations. The computations have been performed using the π-electron Pariser-Parr-Pople model Hamiltonian, which incorporates long-range Coulomb interactions. The influence of electroncorrelation effects on the ground and excited states has been included by means of the configuration-interaction approach, used at various levels. Our calculations have revealed that the absorption spectra are red-shifted with the increasing sizes of quantum dots. It has been observed that the first peak of the linear optical absorption, which represents the optical gap, is not the most intense peak. This result is in excellent agreement with the experimental data, but in stark contrast to the predictions of the tight-binding model, according to which the first peak is the most intense peak, pointing to the importance of electron-correlation effects.
We report quadruple configuration interaction calculations within the extended Pariser−Parr−Pople... more We report quadruple configuration interaction calculations within the extended Pariser−Parr−Pople Hamiltonian on the excited states of aggregates of polyenes, crystalline acenes, and covalently linked dimers of acene molecules. We determine the precise energy orderings and analyze the cluster wave functions in order to arrive at a comprehensive physical understanding of singlet fission in these diverse families of materials. Our computational approach allows us to retain a very large number of basis states and thereby obtain the correct relative energy orderings of one electron−one hole Frenkel and charge-transfer excitons versus intra-and intermolecular two electron−two hole triplet−triplet excited states. We show that from the energy orderings it is possible to understand the occurrence of singlet fission in polyene and acene crystals, as well as its near total absence in the covalently linked acene dimers. As in the acene crystals, singlet fission in the polyenes is a multichromophoric phenomenon, with the well-known 2 1 A − g playing no direct role. Intermolecular charge-transfer is essential for singlet fission in both acenes and polyenes, but because of subtle differences in the natures and orderings of the aggregate excited states, the mechanisms of singlet fission are slightly different in the two classes. We are thus able to give qualitative physical reasoning for the slower singlet fission in the polyenes, relative to that in crystalline pentacene. Our work also gives new insight into the complex exciton dynamics in tetracene crystals, which has been difficult to understand theoretically. Our large-scale many-body calculations provide us with the ability to understand the qualitative differences in the singlet fission yields and rates between different classes of π-conjugated materials.
Frontiers in Optics 2008/Laser Science XXIV/Plasmonics and Metamaterials/Optical Fabrication and Testing, 2008
ABSTRACT We show that in thin films of conjugated polymers with nonnegligible interchain interact... more ABSTRACT We show that in thin films of conjugated polymers with nonnegligible interchain interactions, photoexcitation leads to both optical excitons and excimers. The excimer plays a strong role in both photoluminescence and photoinduced absorptions.
We show that chemical modification of the transpolyacetylene structure that involves substitution... more We show that chemical modification of the transpolyacetylene structure that involves substitution of the backbone hydrogen atoms with conjugated side groups, leads to reduction of the backbone bond alternation as well as screening of the effective Coulomb interaction. Consequently the optical gap of the substituted material is smaller than the parent polyene with the same backbone length, and the excited state ordering is conducive to efficient photoluminescence. The design of organic polymeric infrared lasers, in the ideal long chain limit, thereby becomes possible.
Electron-electron interactions in general lead to both ground state and excited state confinement... more Electron-electron interactions in general lead to both ground state and excited state confinement. We show, however, that in phenyl-substituted polyacetylenes electron-electron interactions cause enhanced delocalization of quasiparticles in the optically excited state from the backbone polyene chain into the phenyl groups, which in turn leads to enhanced confinement in the chain direction. This cooperative delocalization-confinement lowers the energy of the one-photon state and raises the relative energy of the lowest two-photon state. The two-photon state is slightly below the optical state in mono-phenyl substituted polyacetylenes, but above the optical state in di-phenyl substituted polyacetylenes, thereby explaining the strong photoluminescence of the latter class of materials. We present a detailed mechanism of the crossover in the energies of the one-and two-photon states in these systems. In addition, we calculate the optical absorption spectra over a wide wavelength region, and make specific predictions for the polarizations of low and high energy transitions that can be tested on oriented samples. Within existing theories of light emission from π-conjugated polymers, strong photoluminescence should be restricted to materials whose optical gaps are larger than that of trans-polyacetylene. The present work show that conceptually at least, it is possible to have light emission from systems with smaller optical gaps.
In this paper we present a detailed theory of the triplet states of oligoacenes containing up to ... more In this paper we present a detailed theory of the triplet states of oligoacenes containing up to seven rings, i.e., starting from naphthalene all the way up to heptacene. In particular, we present results on the optical absorption from the first triplet excited state 1 3 B + 2u of these oligomers, computed using the Pariser-Parr-Pople (PPP) model Hamiltonian, and a correlated electron approach employing the configuration-interaction (CI) methodology at various levels. Excitation energies of various triplets states obtained by our calculations are in good agreement with the experimental results, where available. The computed triplet spectra of oligoacenes exhibits rich structure dominated by two absorption peaks of high intensities, which are well separated in energy, and are caused by photons polarized along the conjugation direction. This prediction of ours can be tested in future experiments performed on oriented samples of oligoacenes.
Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and pol... more Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that there exist two fundamentally different classes of two-photon Ag states in these systems to which photoinduced absorption (PA) can occur. At relatively lower energies there occur Ag states which are superpositions of one electron-one hole (1e-1h) and two electron-two hole (2e-2h) excitations, that are both comprised of the highest delocalized valence band and the lowest delocalized conduction band states only. The dominant PA is to one specific member of this class of states (the mAg). In addition to the above class of Ag states, PA can also occur to a higher energy kAg state whose 2e-2h component is different and has significant contributions from excitations involving both delocalized and localized bands. Our calculated scaled energies of the mAg and the kAg agree reasonably well to the experimentally observed low and high energy PAs in PPV. The calculated relative intensities of the two PAs are also in qualitative agreement with experiment. In the case of ladder-type PPP and its oligomers, we predict from our theoretical work a new intense PA at an energy considerably lower than the region where PA have been observed currently. Based on earlier work that showed that efficient charge-carrier generation occurs upon excitation to odd-parity states that involve both delocalized and localized bands, we speculate that it is the characteristic electronic nature of the kAg that leads to charge generation subsequent to excitation to this state, as found experimentally.
In this paper we report first-principles calculations on the ground-state electronic structure of... more In this paper we report first-principles calculations on the ground-state electronic structure of two infinite onedimensional systems: (a) a chain of carbon atoms and (b) a chain of alternating boron and nitrogen atoms. Meanfield results were obtained using the restricted Hartree-Fock approach, while the many-body effects were taken into account by second-order Møller-Plesset perturbation theory and the coupled-cluster approach. The calculations were performed using 6-31G * * basis sets, including the d-type polarization functions. Both at the Hartree-Fock (HF) and the correlated levels we find that the infinite carbon chain exhibits bond alternation with alternating single and triple bonds, while the boron-nitrogen chain exhibits equidistant bonds. In addition, we also performed density-functional-theory-based local density approximation (LDA) calculations on the infinite carbon chain using the same basis set. Our LDA results, in contradiction to our HF and correlated results, predict a very small bond alternation. Based upon our LDA results for the carbon chain, which are in agreement with an earlier LDA calculation calculation [ E.J. Bylaska, J.H. Weare, and R. Kawai, Phys. Rev. B 58, R7488 (1998).], we conclude that the LDA significantly underestimates Peierls distortion. This emphasizes that the inclusion of many-particle effects is very important for the correct description of Peierls distortion in one-dimensional systems.
Several years back Angliker et al. [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the... more Several years back Angliker et al. [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the first linear acene with the triplet state 1 3 B 2u as the ground state, instead of the singlet 1 1 A g state. However, contrary to that prediction, in a recent experimental work Tönshoff and Bettinger [Angew. Chem. Int. Ed. 2010, 49, 4125] demonstrated that nonacene has a singlet ground state. Motivated by this experimental finding, we decided to perform a systematic theoretical investigation of the nature of the ground, and the low-lying excited states of long acenes, with an emphasis on the singlet-triplet gap, starting from naphthalene, all the way up to decacene. Methodology adopted in our work is based upon Pariser-Parr-Pople model (PPP) Hamiltonian, along with large-scale multi-reference singles-doubles configuration interaction (MRSDCI) approach. Our results predict that even though the singlet-triplet gap decreases with the increasing conjugation length, nevertheless, it remains finite till decacene, thus providing no evidence of the predicted singlet-triplet crossover. We also analyze the nature of many-particle wavefunction of the correlated singlet ground state and find that the longer acenes exhibit tendency towards a open-shell singlet ground state. Moreover, when we compare the experimental absorption spectra of octacene and nonacene with their calculated singlet and triplet absorption spectra, we observe excellent agreement for the singlet case. Hence, the optical absorption results also confirm the singlet nature of the ground state for longer acenes. Calculated triplet absorption spectra of acenes predict two well separated intense long-axis polarized absorptions, as against one such peak observed for the singlet case. This is an important prediction regarding the triplet optics of acenes, which can be tested in future experiments on oriented samples.
In this paper, we report large-scale configuration interaction (CI) calculations of linear optica... more In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of tetracene and pentacene. For the purpose, we used Pariser-Parr-Pople (P-P-P) model Hamiltonian and quadruple-CI (QCI) and multi-reference single-double CI (MRSDCI) approaches. The role of Coulomb parameters used in P-P-P Hamiltonian was also examined by considering standard parameters as well as screened set of parameters. Results are presented both for the long-and the short-axis-polarized components of the spectra and wave functions of various excited states have also been analysed.
In this paper we present a computational study of linear optical absorption in phenacene class of... more In this paper we present a computational study of linear optical absorption in phenacene class of polyaromatic hydrocarbons. For the purpose, we have employed a correlated-electron methodology based upon configuration-interaction (CI) approach, and the Pariser-Parr-Pople (PPP) π-electron model Hamiltonian. The molecules studied range from the smallest one with three phenyl rings (phenanthrene), to the largest one with nine phenyl rings. These structures can also be seen as finite-sized hydrogen-passivated armchair graphene nanoribbons of increasing lengths. Our CI calculations reveal that the electron-correlation effects have a profound influence not just on the peak locations, but also on the relative intensity profile of the computed spectra. We also compare our phenacene results with isomeric oligo-acenes, and find that in all the cases former have a wider optical gap than the latter. Available experiments based upon optical absorption and electron-energy-loss-spectroscopy (EELS) are in very good agreement with our results.
Several years back Angliker et al. [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the... more Several years back Angliker et al. [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the first linear acene with the triplet state 1 3 B 2u as the ground state, instead of the singlet 1 1 A g state. However, contrary to that prediction, in a recent experimental work Tönshoff and Bettinger [Angew. Chem. Int. Ed. 2010, 49, 4125] demonstrated that nonacene has a singlet ground state. Motivated by this experimental finding, we decided to perform a systematic theoretical investigation of the nature of the ground, and the low-lying excited states of long acenes, with an emphasis on the singlet-triplet gap, starting from naphthalene, all the way up to decacene. Methodology adopted in our work is based upon Pariser-Parr-Pople model (PPP) Hamiltonian, along with large-scale multi-reference singles-doubles configuration interaction (MRSDCI) approach. Our results predict that even though the singlet-triplet gap decreases with the increasing conjugation length, nevertheless, it remains finite till decacene, thus providing no evidence of the predicted singlet-triplet crossover. We also analyze the nature of many-particle wavefunction of the correlated singlet ground state and find that the longer acenes exhibit tendency towards a open-shell singlet ground state. Moreover, when we compare the experimental absorption spectra of octacene and nonacene with their calculated singlet and triplet absorption spectra, we observe excellent agreement for the singlet case. Hence, the optical absorption results also confirm the singlet nature of the ground state for longer acenes. Calculated triplet absorption spectra of acenes predict two well separated intense long-axis polarized absorptions, as against one such peak observed for the singlet case. This is an important prediction regarding the triplet optics of acenes, which can be tested in future experiments on oriented samples.
The electronic and optical properties of four different coronene derivatives with lower symmetry,... more The electronic and optical properties of four different coronene derivatives with lower symmetry, namely, benzo[a]coronene (C$_{28}$H$_{14}$), naphtho[2,3a]coronene (C$_{32}$H$_{16}$), anthra[2,3a]coronene (C$_{36}$H$_{18}$) and naphtho[8,1,2abc]coronene (C$_{30}$H$_{14}$) were investigated. For the purpose, we performed electron-correlated calculations using screened and standard parameters in the $\pi$-electron Pariser-Parr-Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using MRSDCI methodology. The PPP model Hamiltonian includes long range Coulomb interactions which increases the accuracy of our calculations. The results of our calculations predict that with the increasing sizes of the coronene derivatives, optical spectra are red shifted as well as the optical gaps decrease. In each spectrum, the first peak represents the optical gap which is moderately intense, while the more intense peaks appear at higher energies. Our c...
The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with low... more The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with lower symmetry, namely, benzo[ghi]perylene (C 22 H 12), benzo[a]coronene (C 28 H 14), naphtho[2,3a]coronene (C 32 H 16), anthra[2,3a]coronene (C 36 H 18) and naphtho[8,1,2-abc]coronene (C 30 H 14) were investigated. For the purpose, we performed electron-correlated calculations using screened, and standard parameters in the πelectron Pariser-Parr-Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using MRSDCI methodology. PPP model Hamiltonian includes long-range Coulomb interactions, which increase the accuracy of our calculations. The results of our calculations predict that, with the increasing sizes of the coronene derivatives, optical spectra are red shifted, and the optical gaps decrease. In each spectrum, the first peak representing the optical gap is of moderate intensity, while the more intense peaks appear at higher energies. Our computed spectra are in good agreement with the available experimental data. For the purpose of comparison, we also performed first-principles timedependent density-functional theory (TDDFT) calculations of the optical gaps of these molecules using Gaussian basis functions, and found that they yielded values lower than our CI results.
In this paper we present a detailed computational study of the electronic structure and optical p... more In this paper we present a detailed computational study of the electronic structure and optical properties of triply-bonded hydrocarbons with linear, and graphyne substructures, with the aim of identifying their potential in opto-electronic device applications. For the purpose, we employed a correlated electron methodology based upon the Pariser-Parr-Pople model Hamiltonian, coupled with the configuration interaction (CI) approach, and studied structures containing up to 42 carbon atoms. Our calculations, based upon large-scale CI expansions, reveal that the linear structures have intense optical absorption at the HOMO-LUMO gap, while the graphyne ones have those at higher energies. Thus, the opto-electronic properties depend on the topology of the graphyne substructures, suggesting that they can be tuned by means of structural modifications. Our results are in very good agreement with the available experimental data.
In this paper, we perform large-scale electron-correlated calculations of optoelectronic properti... more In this paper, we perform large-scale electron-correlated calculations of optoelectronic properties of rectangular graphene-like polycyclic aromatic hydrocarbon molecules. Theoretical methodology employed in this work is based upon Pariser-Parr-Pople (PPP) π-electron model Hamiltonian, which includes long-range electron-electron interactions. Electron-correlation effects were incorporated using multi-reference singles-doubles configurationinteraction (MRSDCI) method, and the ground and excited state wave functions thus obtained were employed to calculate the linear optical absorption spectra of these molecules, within the electric-dipole approximation. As far as the ground state wave functions of these molecules are concerned, we find that with the increasing size, they develop a strong diradical open-shell character. Our results on optical absorption spectra are in very good agreement with the available experimental results, outlining the importance of electron-correlation effects ...
In this paper, we report large-scale configuration interaction (CI) calculations of linear optica... more In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of various isomers of magnesium clusters Mg n (n=2-5), corresponding to valence transitions. Geometry optimization of several low-lying isomers of each cluster was carried out using coupled-cluster singles doubles (CCSD) approach, and these geometries were subsequently employed to perform ground and excited state calculations using either the full-CI (FCI) or the multi-reference singles-doubles configuration interaction (MRSDCI) approach, within the frozen-core approximation. Our calculated photoabsorption spectrum of magnesium dimer (Mg 2) isomer is in excellent agreement with the experiments both for peak positions, and intensities. Owing to the sufficiently inclusive electron-correlation effects, these results can serve as benchmarks against which future experiments, as well as calculations performed using other theoretical approaches, can be tested.
We present systematic and comprehensive correlated-electron calculations of the linear photoabsor... more We present systematic and comprehensive correlated-electron calculations of the linear photoabsorption spectra of small neutral closed-and open-shell sodium clusters (Nan, n=2-6), as well as closed-shell cation clusters (Nan + , n=3, 5). We have employed the configuration interaction (CI) methodology at the full CI (FCI) and quadruple CI (QCI) levels to compute the ground, and the low-lying excited states of the clusters. For most clusters, besides the minimum energy structures, we also consider their energetically close isomers. The photoabsorption spectra were computed under the electric-dipole approximation, employing the dipole-matrix elements connecting the ground state with the excited states of each isomer. Our calculations were tested rigorously for convergence with respect to the basis set, as well as with respect to the size of the active orbital space employed in the CI calculations. These calculations reveal that as far as electron-correlation effects are concerned, core excitations play an important role in determining the optimized ground state geometries of various clusters, thereby requiring all-electron correlated calculations. But, when it comes to low-lying optical excitations, only valence electron correlation effects play an important role, and excellent agreement with the experimental results is obtained within the frozen-core approximation. For the case of Na 6 , the largest cluster studied in this work, we also discuss the possibility of occurrence of plasmonic resonance in the optical absorption spectrum.
In this paper, optical and electronic properties of diamond shaped graphene quantum dots (DQDs) h... more In this paper, optical and electronic properties of diamond shaped graphene quantum dots (DQDs) have been studied by employing large-scale electron-correlated calculations. The computations have been performed using the π-electron Pariser-Parr-Pople model Hamiltonian, which incorporates long-range Coulomb interactions. The influence of electroncorrelation effects on the ground and excited states has been included by means of the configuration-interaction approach, used at various levels. Our calculations have revealed that the absorption spectra are red-shifted with the increasing sizes of quantum dots. It has been observed that the first peak of the linear optical absorption, which represents the optical gap, is not the most intense peak. This result is in excellent agreement with the experimental data, but in stark contrast to the predictions of the tight-binding model, according to which the first peak is the most intense peak, pointing to the importance of electron-correlation effects.
We report quadruple configuration interaction calculations within the extended Pariser−Parr−Pople... more We report quadruple configuration interaction calculations within the extended Pariser−Parr−Pople Hamiltonian on the excited states of aggregates of polyenes, crystalline acenes, and covalently linked dimers of acene molecules. We determine the precise energy orderings and analyze the cluster wave functions in order to arrive at a comprehensive physical understanding of singlet fission in these diverse families of materials. Our computational approach allows us to retain a very large number of basis states and thereby obtain the correct relative energy orderings of one electron−one hole Frenkel and charge-transfer excitons versus intra-and intermolecular two electron−two hole triplet−triplet excited states. We show that from the energy orderings it is possible to understand the occurrence of singlet fission in polyene and acene crystals, as well as its near total absence in the covalently linked acene dimers. As in the acene crystals, singlet fission in the polyenes is a multichromophoric phenomenon, with the well-known 2 1 A − g playing no direct role. Intermolecular charge-transfer is essential for singlet fission in both acenes and polyenes, but because of subtle differences in the natures and orderings of the aggregate excited states, the mechanisms of singlet fission are slightly different in the two classes. We are thus able to give qualitative physical reasoning for the slower singlet fission in the polyenes, relative to that in crystalline pentacene. Our work also gives new insight into the complex exciton dynamics in tetracene crystals, which has been difficult to understand theoretically. Our large-scale many-body calculations provide us with the ability to understand the qualitative differences in the singlet fission yields and rates between different classes of π-conjugated materials.
Frontiers in Optics 2008/Laser Science XXIV/Plasmonics and Metamaterials/Optical Fabrication and Testing, 2008
ABSTRACT We show that in thin films of conjugated polymers with nonnegligible interchain interact... more ABSTRACT We show that in thin films of conjugated polymers with nonnegligible interchain interactions, photoexcitation leads to both optical excitons and excimers. The excimer plays a strong role in both photoluminescence and photoinduced absorptions.
We show that chemical modification of the transpolyacetylene structure that involves substitution... more We show that chemical modification of the transpolyacetylene structure that involves substitution of the backbone hydrogen atoms with conjugated side groups, leads to reduction of the backbone bond alternation as well as screening of the effective Coulomb interaction. Consequently the optical gap of the substituted material is smaller than the parent polyene with the same backbone length, and the excited state ordering is conducive to efficient photoluminescence. The design of organic polymeric infrared lasers, in the ideal long chain limit, thereby becomes possible.
Electron-electron interactions in general lead to both ground state and excited state confinement... more Electron-electron interactions in general lead to both ground state and excited state confinement. We show, however, that in phenyl-substituted polyacetylenes electron-electron interactions cause enhanced delocalization of quasiparticles in the optically excited state from the backbone polyene chain into the phenyl groups, which in turn leads to enhanced confinement in the chain direction. This cooperative delocalization-confinement lowers the energy of the one-photon state and raises the relative energy of the lowest two-photon state. The two-photon state is slightly below the optical state in mono-phenyl substituted polyacetylenes, but above the optical state in di-phenyl substituted polyacetylenes, thereby explaining the strong photoluminescence of the latter class of materials. We present a detailed mechanism of the crossover in the energies of the one-and two-photon states in these systems. In addition, we calculate the optical absorption spectra over a wide wavelength region, and make specific predictions for the polarizations of low and high energy transitions that can be tested on oriented samples. Within existing theories of light emission from π-conjugated polymers, strong photoluminescence should be restricted to materials whose optical gaps are larger than that of trans-polyacetylene. The present work show that conceptually at least, it is possible to have light emission from systems with smaller optical gaps.
In this paper we present a detailed theory of the triplet states of oligoacenes containing up to ... more In this paper we present a detailed theory of the triplet states of oligoacenes containing up to seven rings, i.e., starting from naphthalene all the way up to heptacene. In particular, we present results on the optical absorption from the first triplet excited state 1 3 B + 2u of these oligomers, computed using the Pariser-Parr-Pople (PPP) model Hamiltonian, and a correlated electron approach employing the configuration-interaction (CI) methodology at various levels. Excitation energies of various triplets states obtained by our calculations are in good agreement with the experimental results, where available. The computed triplet spectra of oligoacenes exhibits rich structure dominated by two absorption peaks of high intensities, which are well separated in energy, and are caused by photons polarized along the conjugation direction. This prediction of ours can be tested in future experiments performed on oriented samples of oligoacenes.
Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and pol... more Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that there exist two fundamentally different classes of two-photon Ag states in these systems to which photoinduced absorption (PA) can occur. At relatively lower energies there occur Ag states which are superpositions of one electron-one hole (1e-1h) and two electron-two hole (2e-2h) excitations, that are both comprised of the highest delocalized valence band and the lowest delocalized conduction band states only. The dominant PA is to one specific member of this class of states (the mAg). In addition to the above class of Ag states, PA can also occur to a higher energy kAg state whose 2e-2h component is different and has significant contributions from excitations involving both delocalized and localized bands. Our calculated scaled energies of the mAg and the kAg agree reasonably well to the experimentally observed low and high energy PAs in PPV. The calculated relative intensities of the two PAs are also in qualitative agreement with experiment. In the case of ladder-type PPP and its oligomers, we predict from our theoretical work a new intense PA at an energy considerably lower than the region where PA have been observed currently. Based on earlier work that showed that efficient charge-carrier generation occurs upon excitation to odd-parity states that involve both delocalized and localized bands, we speculate that it is the characteristic electronic nature of the kAg that leads to charge generation subsequent to excitation to this state, as found experimentally.
In this paper we report first-principles calculations on the ground-state electronic structure of... more In this paper we report first-principles calculations on the ground-state electronic structure of two infinite onedimensional systems: (a) a chain of carbon atoms and (b) a chain of alternating boron and nitrogen atoms. Meanfield results were obtained using the restricted Hartree-Fock approach, while the many-body effects were taken into account by second-order Møller-Plesset perturbation theory and the coupled-cluster approach. The calculations were performed using 6-31G * * basis sets, including the d-type polarization functions. Both at the Hartree-Fock (HF) and the correlated levels we find that the infinite carbon chain exhibits bond alternation with alternating single and triple bonds, while the boron-nitrogen chain exhibits equidistant bonds. In addition, we also performed density-functional-theory-based local density approximation (LDA) calculations on the infinite carbon chain using the same basis set. Our LDA results, in contradiction to our HF and correlated results, predict a very small bond alternation. Based upon our LDA results for the carbon chain, which are in agreement with an earlier LDA calculation calculation [ E.J. Bylaska, J.H. Weare, and R. Kawai, Phys. Rev. B 58, R7488 (1998).], we conclude that the LDA significantly underestimates Peierls distortion. This emphasizes that the inclusion of many-particle effects is very important for the correct description of Peierls distortion in one-dimensional systems.
Several years back Angliker et al. [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the... more Several years back Angliker et al. [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the first linear acene with the triplet state 1 3 B 2u as the ground state, instead of the singlet 1 1 A g state. However, contrary to that prediction, in a recent experimental work Tönshoff and Bettinger [Angew. Chem. Int. Ed. 2010, 49, 4125] demonstrated that nonacene has a singlet ground state. Motivated by this experimental finding, we decided to perform a systematic theoretical investigation of the nature of the ground, and the low-lying excited states of long acenes, with an emphasis on the singlet-triplet gap, starting from naphthalene, all the way up to decacene. Methodology adopted in our work is based upon Pariser-Parr-Pople model (PPP) Hamiltonian, along with large-scale multi-reference singles-doubles configuration interaction (MRSDCI) approach. Our results predict that even though the singlet-triplet gap decreases with the increasing conjugation length, nevertheless, it remains finite till decacene, thus providing no evidence of the predicted singlet-triplet crossover. We also analyze the nature of many-particle wavefunction of the correlated singlet ground state and find that the longer acenes exhibit tendency towards a open-shell singlet ground state. Moreover, when we compare the experimental absorption spectra of octacene and nonacene with their calculated singlet and triplet absorption spectra, we observe excellent agreement for the singlet case. Hence, the optical absorption results also confirm the singlet nature of the ground state for longer acenes. Calculated triplet absorption spectra of acenes predict two well separated intense long-axis polarized absorptions, as against one such peak observed for the singlet case. This is an important prediction regarding the triplet optics of acenes, which can be tested in future experiments on oriented samples.
In this paper, we report large-scale configuration interaction (CI) calculations of linear optica... more In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of tetracene and pentacene. For the purpose, we used Pariser-Parr-Pople (P-P-P) model Hamiltonian and quadruple-CI (QCI) and multi-reference single-double CI (MRSDCI) approaches. The role of Coulomb parameters used in P-P-P Hamiltonian was also examined by considering standard parameters as well as screened set of parameters. Results are presented both for the long-and the short-axis-polarized components of the spectra and wave functions of various excited states have also been analysed.
Uploads
Papers by Alok SHUKLA