58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Introduction: Organic light emitting diodes (OLEDs) have gained attention as one of the most appealing solutions for low energy consumption in solid-state lighting.i To date organic white light-emitting devices (WOLEDs) are obtained by combining the emission from red, green and blue or sky-blue and orange emitters. Contrary to polymers, small molecules can be evaporated and therefore very complex multi-layer structures can be constructed. This high flexibility in layer design is the main reason for the high efficiencies of the small-molecule (SM)-OLEDS. The new direction we have taken with these versatile fluorescent platforms leads toward extending the dual fluorescent features of these systems to more panchromatic or white light emission. Plans to accomplish this centered on the transition from our 1,8-naphthalimide fluorescent system to the structurally and symmetry related 2,3-naphthalimide class of fluorophores. and finally (3) Based on our results for the 2,3-naphthalimide dyes, this report outlines:

(Goal 1) Results from solution phase spectroscopy coupled with Gaussian DFT calculations to identify the excited state species involved in dual fluorescence. A 32 page review provides an abstract of our review in press on single emitter WOLED systems

(Goal 2) Current results from our study of the electroluminescent properties of these dyes via fabrication of solid-state ITO based systems.

Summary of results obtained during funding period are briefly listed along with their corresponding publications below:

(Achieved goal 1) Resulting publications

[1] Bao, L.; Heagy, M.D. "The Quest for Picture Perfect Dyes in the Next Generation of Single Layer WOLED Displays: A Review of Single Chromophore White Light Emitters" Current Organic Chemistry, in press, 2013.

Abstract: Organic fluorophores have at times been criticized for their wide band emission. In the rapidly developing field of white light-emitting devices (WOLEDs) however, such features have become exalted in the search for dyes that display panchromatic emission. To achieve white light emission, mixing of emitters that emit red, green and blue (RGB) or complementary color mixing of bluish/green with orange/red colors represent the most common methods. Among the various design approaches, single white-light emitters feature improved stability, better reproducibility, and true single-chromophore layer fabrication. The intent of this review is to inform promising researchers of the fundamental requirements for single-emitter chromophore design and present the latest developments obtained through different research directions. Thus, molecular systems with the unique capacity to emit white-light are categorized into small organic molecules, dendrimers, polymeric molecules, and organometallic complexes. These single white-light emitters groups are further classified as either photoluminescent (in solution or thin films) or electro-luminescent (EL). In addition to providing examples of the authors' contributions to the field, the review provides a brief outline of the synthetic routes for certain white-light emitters. Where possible, a description of the photophysical properties is given as well as commentary on the white-light emitter features such as excitation and emission wavelength maxima within each class of compounds. Therefore, strategies to obtain new white light emitters including thin film requirements, CIE index values and turn-on voltage for electroluminescent systems are summarized for those seeking to contribute to the leading emerging technology in display devices.

[2] 'Electronic properties of N-aryl-2,3-naphthalimides', Bao, L.; Heagy, Michael D. manuscript in preparation

Abstract: This paper describes a study of excited-state properties of four dual-fluorescent 2,3-naphthalimide derivatives. Both absorption and fluorescence spectra were collected in solvents of varying polarity, and Lippert-Mataga equations were used to determine the excited-state character of these 2,3-NI derivatives. The lifetime of the excited states was also measured. Furthermore, the experimental findings were compared to calculated data obtained using time-dependent density functional (TD-DFT) methods. We found that light absorption by all 2,3-NI derivatives leads to the production of two excited states: S1 and S2, and the characters of the excited states (n?p* and p?p*) as well as the relative concentration of these two excited states is sensitive to the polarity of the solvents. We also found that the excited state of short wavelength (SW) peak of these dual-fluorescent 2,3-NIs has both n?p* and p?p* characters, and the relative concentration is sensitive to the varying polarity of solvents. The excited state of long wavelength (LW) peak of these dual-fluorescent 2,3-NIs has a p?p* character. Figure 1: Fluorescence decays of F-NI-OCH3 in hexane

Resulting publication:

(Achieved goal 3) Having selected a suitable panchromatic dye based on its HOMO/LUMO values via cyclic voltammetry, 3-chloro-5-nitronaphthalic imide was thermally deposited onto an ITO coated glass slide. The electrochromic material was characterized via its color emission via the CIE protocol.

Resulting publications:

[1] 'WOLED fabrication using single emitter dye with N-aryl-2,3-napthalimides', manuscript in preparation.

Concluding statements/ Impacts of the grant:

Funding provided by the ACS-PRF New Directions award has resulted in preliminary results that allowed our group to submit to the NSF Chemical Dynamics program in the Chemistry Division. The proposal was not funded, but the reviewer comments provide helpful in directing project goals for future resubmission. In closing, each of the three aims proposed within the proposal were achieved in varying degrees of success as indicated by the number of publications listed under each goal.

Students supported under this grant are listed below:

Lili Bao Ph.D. candidate. Dissertation Title: 'A Comparative Study into Two Dual Fluorescent Ranjith Kumar, Ph.D. candidate, Dissertation Title: 'Design and Synthesis of Panchromatic Dyes for Electrochromic Materials'

Undergraduate

Fred Hanson Thesis title: 'Synthesis of substituted 2,3-naphthalic imides'

i Gustafsson, G.; Gao, T.; Treacy, G.M.; Klavetter, F.; Colaneri, N.; Heeger, A.J. "Flexible light-emittin-diodes made from soluble conducting polymers" Nature, 1992, 357, 477-479. (a) Bulovic, V.; Gu, G.; Burrows, P.E.; Forrest, S.R.; Thompson, M.E. "Transparent light emitting diodes", Nature, 1996, 380, 28-29.