Valence-, Dipole- and Quadropole-Bound Electronically Excited States of Closed-Shell Anions Formed by Deprotonation of Cyano- and Ethynyl-Disubstituted Polycyclic Aromatic Hydrocarbons

Marie E Strauss,Taylor J Santaloci,Ryan C Fortenberry

doi:10.3390/chemistry4010004

Marie E Strauss, Taylor J Santaloci + Show 1 more

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https://doi.org/10.3390/chemistry4010004

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Abstract

Dicyano-functionalized benzene and naphthalene anion derivatives exhibit a relatively rich population of electronically excited states in stark contrast to many assumptions regarding the photophysics of anions in general. The present work has quantum chemically analyzed the potential electronically excited states of closed-shell anions created by replacing hydrogen atoms with valence-bound lone pairs in benzene and naphthalene difunctionalized with combinations of -CN and -C2H. Dicyanobenzene anion derivatives can exhibit dipole-bound excited states as long as the cyano groups are not in para position to one another. This also extends to cyanoethynylbenzene anions as well as deprotonated dicyano- and cyanoethynylnaphthalene anion derivatives. Diethynyl functionalization is less consistent. While large dipole moments are created in some cases for deprotonation on the -C2H group itself, the presence of electronically excited states beyond those that are dipole-bound is less consistent. Beyond these general trends, 2-dicyanonaphthalene-34 gives strong indication for exhibiting a quadrupole-bound excited state, and the 1-cyanoethynylnaphthalene-29 and -36 anion derivatives are shown to possess as many as two valence-bound excited states and one dipole-bound excited state. These photophysical properties may have an influence on regions where polycyclic aromatic hydrocarbons are known to exist such as in various astrochemical environments or even in combustion flames.

Highlights

IntroductionPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
The myth that anions cannot posses electronically excited states is abating in the chemical literature [1,2,3,4,5,6]
Each simplified molecular-input line-entry system (SMILES) string is converted to Cartesian coordinates with the Open Babel (OBabel) computational package [67,68]

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. These PAHs and PAH-like molecules are mostly dipolar enough for radioastronomical observation due to their dipole-inducing cyano and ethynyl functional groups While these structures certainly could posses dipole-bound states as radical anions, another interstellar spectral anomoly exists in the electronic region of the electromagnetic spectrum where the closed-shell anion forms of such molecules is more applicable. Cyano-functionalization creates dipole-bound excited states in singly-deprotonated benzene anions in some isomers and in all cases once the number of rings in the structure is two or greater beginning with the case of naphthalene [64]. Ethynyl-functionalization is a bit less consistent, but the largest dipole moments for deprotonated PAH isomers arise when the deprotonation site is on the ethynyl hydrogen itself [65] This promotes valence excited states as the highest occupied molecular orbital is no longer a carbene lone-pair but shifts into the distributed π cloud. The number of possible isomers, increases significantly than single functionalization and gives the opportunity for even more novel photochemistry with possible applications both astrochemically and terrestrially

Computational Details

Relative Energies and Electrostatic Properties

Vertical Excitation Energies

Cyanoethynylnaphthalene and Diethynylnaphthalene

Conclusions