Triple Emission of 5′-(para-R-Phenylene)vinylene-2-(2′-hydroxyphenyl)benzoxazole (PVHBO). Part I: Dual Emission from the Neutral Species

Abstract

The effects of 5′-(para-R-phenylene)­vinylene (PV) substituents on the emission properties of 2-(2′-hydroxyphenyl)­benzoxazole (HBO) are analyzed using steady-state and time-resolved absorption and emission spectroscopies in addition to quantum chemical calculations. All members in the series of PVHBOs are capable of excited-state intramolecular proton transfer (ESIPT) with a solvent sensitivity that is typical of a HBO derivative to produce a normal (aka enol) emission and an excited-state tautomer (aka keto) emission. These two emission bands of the neutral dyes are discussed in the current paper. The intermolecular proton transfer, i.e., the deprotonation, of a PVHBO results in the third band of the triple emission, which is described in the succeeding paper. The placement of an electron-withdrawing substituent R on the PVHBO scaffold increases the intensity of the keto emission relative to the enol emission in hydrogen-bonding solvents. The R substituents do not significantly alter the wavelengths of the enol and keto emission bands, which are located in the blue and green regions, respectively, of the visible spectrum. The ultrafast time-resolved spectroscopies and quantum chemical calculations offer explanations on how the R group and the solvent affect the enol and keto emission properties (i.e., wavelength, lifetime, fluorescence quantum yield, and relative ratio of their emissions). The key findings include the following: (1) the emission energies of both enol and keto forms are not sensitively dependent on the R substituent and (2) the solvent-engaged enol excited state is quenched more efficiently as the R substituent becomes more electron-withdrawing. A PVHBO acts as a fusion of HBO and stilbenoid that intersect at the hydroxyphenyl moiety. Depending on the solvent and other environmental conditions, PVHBOs may exhibit the ESIPT property of HBO or the substituent-dependent emission of stilbenoid. This paper and the succeeding article provide a photophysical model of PVHBOs to explain the wavelengths and relative abundances of the three emission bands (enol, keto, and anion) that these compounds are able to produce. Judicial selection of the environmental factors may drive the emission of a PVHBO into the spectral regions of blue, green, and, in a couple of cases, orange or red.

Triple Emission of 5′-(para-R-Phenylene)vinylene-2-(2′-hydroxyphenyl)benzoxazole (PVHBO). Part II: Emission from Anions

“…1−5 The emissions of the normal and tautomer forms can be tuned to blue and green primary color regions of the visible spectrum, respectively, when an ESIPT core is properly substituted. One class of such compounds is reported in part I of this work (preceding paper), 6 which are ESIPT-capable compounds (2′-hydroxyphenyl)benzoxazoles (HBOs) 7,8 substituted at the 5′-position with para-R-phenylenevinylene (PVHBOs, Figure 1). PVHBOs are fusions of the HBO component and an R-substituted stilbenoid dye.…”

“…In part I, 6 we reasoned that in order to develop a dye that emits in three primary colors, a lead fluorophore that is capable of producing three emission bands ought to be identified, based on which structural modifications could be carried out to tune the three bands to primary colors. The lead compound that we selected was HBO, while the structural modification that we elected to execute was the installation of an Rsubstituted phenylenevinylene (PV) group at the 5′-position (Figure 1).…”

Excitation Energy-Dependent, Excited-State Intramolecular Proton Transfer-Based Dual Emission in Poor Hydrogen-Bonding Solvents

“…The B3LYP functional is widely used in (TD)­DFT calculations of organic molecules under a relatively lean budget of computation . The B3LYP is also a popular choice in (TD)­DFT calculations of ESIPT-capable compounds. ,− However, the limitations of B3LYP have been extensively reported, which we have briefly summarized in a previous paper from our group . In particular, the excitation energy values of charge-transfer type molecules can be severely underestimated by the B3LYP functional.…”

“…In a hydrogen-bonding solvent (“S” in Scheme ), the solvation process becomes considerable, which leads to the formation of the solvated enol (SE) in the ground state that affords its normal (aka the enol) emission. Upon thoughtful selection of solvent and/or the introduction of additives (e.g., a base, which could trigger the formation of the anion in Scheme , ), some HBO derivatives − and other fluorophores may afford white emissiona sought-after consequence that raises prospects of practical uses …”

Sterically Hindered 2‐(2’‐Hydroxyphenyl)benzoxazole (HBO) Emitters: Synthesis, Spectroscopic Studies, and Theoretical Calculations

“…They are notably famous for displaying excited-state intramolecular proton transfer (ESIPT) process, owing to the presence of a strong sixmembered intramolecular hydrogen bond in their structure. [1][2][3][4] ESIPT is a phototautomerization reaction which turns, in most cases, an excited enol (E*) derivative into the corresponding keto (K*) upon proton migration and structural reorganization. The radiative relaxation of this excited keto species to the ground-state leads to a strongly red-shifted fluorescence emission (Figure 1).…”