Theoretical analysis of an all-photonic multifunctional molecular logic device: Using TD-DFT//DFT to assess photochromic activity of multimeric photochrome Kellon A.A. Belfon, Jonathan D. Gough ⇑ Department of Chemistry and Biochemistry, Long Island University, One University Plaza, Brooklyn, NY 11201, United States article info Article history: Received 10 August 2013 In final form 26 August 2013 Available online 3 September 2013 abstract The structures and properties of a single-molecule photochromic switch consisting of 3 photochromic moieties is investigated. Using time-dependent density functional theory (TD-DFT) we calculated the k max within ± 30 nm (± 0.18 eV) and produced spectra that were similar. The charge-transfer (CT) charac- ter of the molecular orbitals (MO) was assessed via the overlap between the occupied and virtual orbitals (K diagnostic) and did not suffer from CT failure. The MOs were consistent with photochemically produc- tive photochromes. The MO and their contribution to different excited states paralleled both the observed activity and observed inactivity of the photochrome. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Molecular switches are substrates that are interconverted be- tween two or more states via a chemical reaction. The functionality of these systems has increased from simple binary systems [1–3] to complex molecules [4–8]. The major categories include photo- chromic switches, host–guest switches and mechanically inter- locked switches. Functionally, an input switches a system between two or more stable states and an output reads each state. Writing inputs include stimuli such as pH, light, electricity or tem- perature where reading outputs include absorbance, luminescence, infrared absorption or refractive index. Recently, interest in photo- chromic switches has increased because of their fast inter-conver- sion rate, reasonable quantum yield and fatigue resistance. Photon-controlled systems are attractive, as light does not re- quire physical access, thereby allowing remote access. A single molecule with two or more isomeric forms that can be read/writ- ten to, using actinic light, has the potential to store data. Increasing the number of photo-accessible isomers (>2) in a single molecule (a multimeric photoswitch) extends their potential to being used as optical encoders/decoders, [8,9] multiplexer/demultiplexer, [10,11] and logic gates [12]. The most promising photochromic switching molecules include spiropyrans/spirooxazines, [13] azobenzenes, [14–17] phenoxy- naphthacenequinones, [18] fulgides, [3,19] fulgimides, [19] diary- lethenes [2,6] and dihydropyrenes [20]. These molecules utilize photocyclization–cycloreversion reactions for isomerization. Their functionality both in solution and in uniform or combined poly- mers provides many potential advantages. The dithenylethenes (DTE), a subset of diarylethenes, are one of the most promising candidates due to their high thermal irreversibility and fatigue resistance. However, their incorporation into a multi-DTE photo- switch has proved challenging [6]. As a result, mixed DTE photos- witches (where DTE is covalently linked to other photochromes) have produced functional optical storage devices [21,22]. One of the most promising multimeric photoswitches is a mul- tifunctional molecular logic molecule that can function as thirteen logic devices [4,5,9,23]. The single molecule utilizes 2 indolylfulgi- mides (FG) linked to a DTE, thereby creating a trimeric photo- switch FG-DTE (Scheme 1). The success of this trimer lies in its capacity to be interconverted between four isomeric species chemically. In this report we present calculations and analysis that provide insight into FG-DTE photochromic activity and efficiency. Analysis of the molecular orbitals (MO) explains the observed intra-molec- ular processes. These methods can facilitate understanding the functionality of photochromic molecular logic devices, serve as a foundation for studying and assessing novel photochromes pre- and post-synthesis, and provide a blueprint for designing more effective molecular logic devices. FG-DTE contains three photochromic-units that are opened with visible light and closed with UV light. Functionally, the logic device is controlled by its transitions through 4 isomeric states, using the fully open state (FGoo-DTEo) as the initial state (Figure 1). Irradiation at 397, 366 or 302 nm is used as the photoisomerization input; fluorescent emission (624 nm) or the change in absorption 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.08.099 ⇑ Corresponding author. Fax: +1 718 488 1027. E-mail addresses: kellonbelfon@gmail.com (K.A.A. Belfon), Jonathan.gough@ liu.edu (J.D. Gough). Chemical Physics Letters 585 (2013) 63–68 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett