Electron correlations and two-photon states in polycyclic aromatic hydrocarbon molecules: A peculiar role of geometry

Aryanpour, Karan; Shukla, Alok; Mazumdar, S.

doi:10.1063/1.4867363

Cited by 28 publications

(54 citation statements)

References 49 publications

(123 reference statements)

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“…The 2 1 A − g plays a weak role in the photophysics of most extended systems [5], where the phenomena of interest are exciton formation [6,7] and the consequence thereof on nonlinear optical spectroscopy [8]. Understanding the photophysics of discrete but large molecular systems of intermediate size poses new challenges [9][10][11][12]. We consider members of one such family of large π-conjugated molecules here, dimers of bis(triisopropylsilylethynyl) (TIPS) pentacene, covalently linked by 0, 1 and 2 phenyl groups (see Fig.…”

Section: Pacs Numbersmentioning

confidence: 99%

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Khan

Mazumdar

2017

J. Phys. Chem. Lett.

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Covalently linked acene dimers are of interest as candidates for intramolecular singlet fission. We report many-electron calculations of the energies and wavefunctions of the optical singlets, the lowest triplet exciton and the triplet-triplet biexciton, as well as the final states of excited state absorptions from these states in a family of phenyl-linked pentacene dimers. While it is difficult to distinguish between the triplet and the triplet-triplet from their transient absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against published and unpublished experimental transient absorptions in the near and mid infrared we conclude that the end product of photoexcitation in these particular bipentacenes is the bound triplet-triplet and not free triplets. We predict additional transient absorptions at even longer wavelengths, beyond 1500 nm, to the equivalent of the classic 2 1 A − g in linear polyenes. PACS numbers:The consequences of strong electron correlations in π-conjugated systems have been investigated most intensively at the two extremes of system sizes, (a) small molecules such as linear polyenes, and (b) extended systems, such as π-conjugated polymers and single-walled carbon nanotubes. The occurrence of the lowest twophoton 2 , where the phenomena of interest are exciton formation [6,7] and the consequence thereof on nonlinear optical spectroscopy [8]. Understanding the photophysics of discrete but large molecular systems of intermediate size poses new challenges [9][10][11][12]. We consider members of one such family of large π-conjugated molecules here, dimers of bis(triisopropylsilylethynyl) (TIPS) pentacene, covalently linked by 0, 1 and 2 phenyl groups (see Fig. 1) [13]. Following the original investigators we will refer to these molecules as bipentacenes BPn, with n = 0, 1 and 2, respectively. The photophysics of these and other bipentacenes [13][14][15][16][17] and related covalently linked dimeric molecular systems [18][19][20][21] are of strong current interest as candidates for intramolecular singlet fission (iSF), as we briefly discuss below.SF is the process [22] in which an optical spin-singlet exciton S 1 dissociates into two spin triplet excitons T 1 . If each photogenerated triplet dissociates with 100% efficiency at the donor-acceptor interface of an organic solar cell, the photoconductivity is doubled. Enhanced external quantum efficiencies using SF have been reported for pentacene/C 60 solar cells [23,24]. The bulk of the theoretical [25][26][27][28][29][30][31][32][33][34] and experimental [35][36][37][38][39][40][41][42][43][44] literature until now had focused on intermolecular SF (xSF), in which the two triplets are generated on neighboring weakly coupled monomer molecules in a thin film or crystal. Recent investigations of dimer molecules [13][14][15][16][17][18][19][20][21], in which the monomers are linked by covalent bonds, is driven by the belief that the stronger coupling between the monomer components will give higher SF efficienc...

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Section: Pacs Numbersmentioning

confidence: 99%

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Khan

Mazumdar

2017

J. Phys. Chem. Lett.

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“…[82] We have recently performed theoretical work on excited state orderings in large PAH molecules. [83] The approach we have developed here can in principle be extended to clusters of such PAH molecules. This is the goal of future work.…”

Section: %mentioning

confidence: 99%

Theory of Singlet Fission in Polyenes, Acene Crystals, and Covalently Linked Acene Dimers

2015

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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 wavefunctions 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 to 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.

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“…Nevertheless, several polycyclic aromatic hydrocarbons (PAHs) have been synthesized which are nothing but graphene quantum dots of high symmetry, but with edges passivated by hydrogen atoms, 35 a few of which we had studied in earlier works. 30,31 Of the quantum dots considered here, hydrogen passivated counterpart of DQD with 16 carbon atoms (DQD-16, henceforth) is called pyrene, while that of DQD with 30 carbon atoms (DQD-30) is known as dibenzo[bc,kl]coronene, both of which have been well-studied in the chemical literature. 35 A large number of experimental measurements of optical absorption of pyrene in vapor, 36 solution, [37][38][39][40] and matrix isolated phases [41][42][43][44] have been performed, and our results on DQD-16 are in excellent agreement with them.…”

Section: Motivated By Aforementioned Theoretical and Experimental Stumentioning

confidence: 99%

“…The matrix elements t ij depicts one-electron hops, which in our calculations, have been restricted to nearest neighbours, with the value t 0 = 2.4 eV, consistent with our earlier calculations on conjugated polymers, [23][24][25][26][27][28][29] and polyaromatic hydrocarbons. 30,31 Parameterization of the Coulomb interactions is done according to the Ohno relationship 54…”

Section: Computational Detailsmentioning

confidence: 99%

Theory of linear optical absorption in diamond-shaped graphene quantum dots

2015

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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. 1 arXiv:1501.06041v3 [cond-mat.mes-hall]

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Electron correlations and two-photon states in polycyclic aromatic hydrocarbon molecules: A peculiar role of geometry

Cited by 28 publications

References 49 publications

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Theory of Singlet Fission in Polyenes, Acene Crystals, and Covalently Linked Acene Dimers

Theory of linear optical absorption in diamond-shaped graphene quantum dots

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