Optical Properties of 4‑Bromobenzaldehyde Derivatives in Chloroform Solution Cla ̀ udia Climent, † Pere Alemany, † Dongwook Lee, ‡ Jinsang Kim, ‡ and David Casanova* ,§,¶ † Departament de Química Física and Institut de Química Teò rica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franque ̀ s, 1-11, 08028 Barcelona, Spain ‡ Materials Science and Engineering, Macromolecular Science and Engineering, Chemical Engineering, and Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States § Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU), Donostia International Physics Center (DIPC), P.K. 1072, 2080 Donostia, Spain ¶ IKERBASQUE, Basque Foundation for Science, Bilbao, 48011, Spain * S Supporting Information ABSTRACT: In this work we give a deeper insight into the electronic structure of a series of purely organic molecules that were recently employed as building blocks in crystals with very efficient phosphorescent emission. With this purpose, the low- lying excited states of a series of 4-bromobenzaldehyde derivatives in chloroform solution are explored by means of time-dependent density functional theory (TDDFT) calculations, together with the absorption, fluorescence, and phosphor- escence experimental spectra. The optical properties of the studied molecular models are extensively discussed, in terms of the frontier molecular orbitals involved in the relevant electronic transitions, the recorded and simulated absorption profiles, and the molecular geometries and transition energies of the emitting states. The calculations eventually help in the assignment of the character of the lowest lying singlet and triplet emitting states for these compounds. 1. INTRODUCTION The importance of organic electronic devices 1−6 has been steadily increasing over the last two decades, evolving from a research field with great promise for new materials and applications to a real industry with commercial products on the market. In an emerging era of flexible, rollable, or foldable high performance displays, the search for new low-cost, mechanically tolerant functional materials that can be easily processed has become one of the major focuses of interest in material sciences. Organic materials that can be precisely printed, stamped, sprayed, drop-cast, or spin-coated into predefined patterns offer a competitive alternative to their conventional inorganic homologues for applications in thin-film transistors (TFTs), photovoltaic cells, radio frequency identi- fication (RFID) tags, sensors, memories, or light-emitting diodes (LEDs). Despite the obvious advantage of purely organic materials for these applications, in some cases, the chemical nature of these compounds poses some serious drawbacks for the development of new functional materials for a specific application. This is, for instance, the case of the development of purely organic LEDs (OLEDs) since it is well-known that competitive organic phosphorescent materials are very scarce due to inefficient spin−orbit coupling in compounds containing only light elements, which prevents singlet and triplet mixing. For this reason, during the past decade, much attention has been paid mainly to organometallic materials, such as iridium(III) complexes, 7 because of their enhanced emission capabilities. The presence of a heavy atom, which facilitates intensity borrowing of the lowest triplet state from bright singlet states, has given these organometallic complexes a central role in the field of OLEDs. 8 Despite the fact that organometallic compounds possess the appropriate physical properties as far as light emission is concerned, purely organic compounds present a series of very advantageous features, which make them very attractive for OLED technology. Purely organic molecules are indeed much cheaper and easier to synthesize than their organometallic counterparts and, by fine-tuning their structure, different emission properties can be readily achieved. However, the quantum yield for phosphorescent emission must be highly improved in order for purely organic materials to be able to compete with organometallics in practical applications. The photophysical properties of a wide family of aromatic carbonyls, such as benzaldehyde and naphthalene deriva- tives, 9−11 were largely studied decades ago, and different strategies were conducted in order to improve their phosphorescence. 12,13 One of these strategies, known as the heavy atom effect, 14,15 consists in introducing an atom of a Received: June 2, 2014 Revised: August 11, 2014 Published: August 11, 2014 Article pubs.acs.org/JPCA © 2014 American Chemical Society 6914 dx.doi.org/10.1021/jp505411r | J. Phys. Chem. A 2014, 118, 6914−6921