© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 984 wileyonlinelibrary.com FULL PAPER www.MaterialsViews.com www.advopticalmat.de Naturally Assembled Excimers in Xanthenes as Singular and Highly Efficient Laser Dyes in Liquid and Solid Media Luis Cerdán, Virginia Martínez-Martínez,* Inmaculada García-Moreno, Angel Costela, María E. Pérez-Ojeda, Iñigo López Arbeloa, Liangxing Wu, and Kevin Burgess* Dr. L. Cerdán, Prof. I. García-Moreno, Prof. A. Costela, M. E. Pérez-Ojeda Departamento de Sistemas de Baja Dimensionalidad Superficies y Materia Condensada Instituto Quimica-Fisica “Rocasolano” C.S.I.C., Serrano 119, 28006, Madrid, Spain Dr. V. Martínez-Martínez, Prof. I. López Arbeloa Departamento de Química Física Universidad País Vasco (UPV-EHU) Aptdo., 644, 48080, Bilbao, Spain Phone: (+34) 946015384); Fax: (+34) 946013500 E-mail: virginia.martinez@ehu.es Prof. L. Wu, Prof. K. Burgess Department of Chemistry Texas A & M University Box 30012, College Station, TX, 77842, USA Phone: (979) 845–4345; Fax: (979) 845–8839 E-mail: burgess@tamu.edu DOI: 10.1002/adom.201300383 1. Introduction Self-aggregation of organic dyes profoundly influences their emission properties and real-world applications as optical mate- rials. [1–7] Dissolved in concentrated solutions or doped into solids, dye molecules tend to aggregate, both in the ground and excited states. Formation of dimers, excimers, and exciplexes tends to suppress laser action as well as fluorescence, [8–11] so steps are often taken to reduce aggregation-induced fluores- cence quenching. [12–15] However, dyes that form highly fluores- cent excimers and/or exciplexes [16–18] should have better lasing properties than their non-aggregating forms; this is because their ground states are unstable and consequently virtually depleted, relaxing the pumping condi- tions needed to reach the critical effective population inversion for laser action. As far as we are aware, efficient and photo- stable laser action from naturally occur- ring excimers of organic dyes has not yet been reported to date. The challenge of finding systems of this type is attractive with respect to advanced applications of organic optical materials. Like rhodamines, rosamines (e.g., 1– 4; Scheme 1) have high fluorescence quantum yields in the solution state, but they are not vulnerable to cyclize to non- fluorescent lactones since they do not have carboxylic acid groups. Rosamine libraries have been designed and synthesized, and their photo- physical properties and their potential as cell imaging probes and bio-analyte sensors have been reported. [19–23] However, the lasing properties of these dyes had never been analyzed. This paper describes the laser action of four rosamine derivatives, previously synthesized by Burgess et al., [22,23] as a function of the environmental properties (dye concentration, polarity, and viscosity of the solvent), and pumping conditions (cavity con- figuration, pump geometry, and fluence). As a result of these studies, high laser action from naturally occurring organic dye excimers is reported here for the first time. 2. Results and Discussion 2.1. Lasing and Photophysical Properties Ethanolic solutions of rosamines (4 × 10 -4 M) in a 1 cm optical- path quartz cell were transversely pumped, in a simple plane parallel non-tunable resonator, at 532 nm with 5 mJ and 6 ns full width at half-maximun (FWHM) pulses focused onto the cell as a line of about 0.3 × 10 mm 2 . Under these pumping condi- tions, rosamines 2– 4 emit broad-line width laser signals peaked at 605 nm with an oscillation bandwidth of 6 nm, beam diver- gence of 4 mrad, pulse duration of 9 ns, and laser efficiencies (percentage of the excitation energy converted into laser emis- sion) of 29, 38, and 32%, respectively. As expected, these laser efficiencies depend on the dye concentration; they increase with the dye concentration until a maximum efficiency is reached Herein, for the first time, outstanding laser performance is demonstrated in liquid solution and solid state by naturally assembling excimers of organic fluorophores (rosamine dyes). Highly efficient and photostable laser dye properties, with broadened tunability covering 80 nm in the red spectral region (590–670 nm), is attributed to the coexistence of monomers and excimers induced under high optical gain conditions. Amplified spontaneous emission measurements in rosamine-doped polymer thin films show that the excimer exhibits a threshold lower and a gain higher than those corre- sponding to the monomer species. These laser properties make rosamines excellent candidates for biophotonic and spectroscopic applications, over- coming the main drawbacks exhibited by other long-wavelength (>600 nm) laser dyes, including low absorption at the standard pump wavelength (532 nm), low laser efficiency, and poor chemo- and photostability. Adv. Optical Mater. 2013, 1, 984–990