Natural Born Laser Dyes: Excited-State Intramolecular Proton Transfer (ESIPT) Emitters and Their Use in Random Lasing Studies

A series of five excited-state intramolecular proton transfer (ESIPT) emitters based on a 2-(2′-hydroxyphenyl) benzoxazole (HBO) scaffold, functionalized with a mono-or bis-(trialkylsilyl) acetylene extended spacer are presented. Investigation of their photophysical properties in solution and in the solid-state in different matrix, along with ab initio calculations gave useful insights into their optical behavior. Random lasing studies were conducted on a series of PMMA doped thin films, showing the presence of stimulated emission above the threshold of pumping energy density (ρth ≈ 0.5–2.6 mJ cm−2). In this work, the similarity of four level laser systems is discussed in light of the ESIPT photocycle.


Materials and methods
All reactions were performed under a dry atmosphere of argon. Chemicals were purchased from commercial sources and used without further purification. Reaction solvents were distilled according to common procedures. Thin layer chromatography (TLC) was performed on silica gel or aluminum oxide plates coated with fluorescent indicator. Chromatographic purifications were conducted using 40-63 μm silica gel. All mixtures of solvents are given in v/v ratio. 1 H NMR (400.1 MHz) and 13 C NMR (100.5 MHz) spectra were recorded on a Bruker Advance 400 MHz spectrometer, with perdeuterated solvents containing residual protonated solvent signals as internal references.
Absorption spectra were recorded using a dual-beam grating Schimadzu UV-3000 absorption spectrometer with a 1 cm quartz cell. The steady-state fluorescence emission and excitation spectra were obtained by using a Horiba S2 Jobin Yvon Fluoromax 4. All fluorescence spectra were corrected. Solvents for spectroscopy were spectroscopic grade and were used as received.
All fluorescence spectra were corrected. The fluorescence quantum yield (Φexp) was calculated from Eq (1).
where I denotes the integral of the corrected emission spectrum, OD is the optical density at the excitation wavelength, and η is the refractive index of the medium. The quantum yield was determined in solution by using quinine sulfate as a reference (λ exc = 366 nm, Φ = 0.55 in 1N H2SO4), for dyes emitting below 480 nm, Rhodamine 6G as a reference (λ exc = 488 nm, Φ = 0.88 in ethanol), for dyes emitting between 480 and 570 nm or cresyl violet (λ exc = 546 nm, Φ = 0.55 in ethanol) as a reference for dyes emitting above 570 nm.
Luminescence lifetimes were measured on an Edinburgh Instruments spectrofluorimeter equipped with a R928 photomultiplier and a PicoQuant PDL 800-D pulsed diode connected to a GwInstect GFG-8015G delay generator. No filter was used for the excitation. Emission wavelengths were selected by a monochromator. Lifetimes were deconvoluted with FS-900 software using a light-scattering solution (LUDOX) for instrument response. The excitation source was a laser diode (λ = 320 nm).

S6.1 Methods
Our computational protocol is based on current state-of-the-art for modeling ESIPT-type reactions as well as the emission and absorption spectra of comparable types of molecules. 3 We used a composite approach combining the results of Time-Dependent Density Functional Theory (TD-DFT) and the second-order algebraic diagrammatic construction [ADC (2)  For photodegradation measurements we used following definition: a number of pulses upon which the intensity of emission drops by half (t1/2). Setup was the same, as for RL studies described previously, alas intensity of input power was constant (2mJ cm -2 ).       We report the maximum emission wavelengths of the keto forms (dyes 1-5) and of the aggregated forms (dyes 1, 2), in nm.