Fluorescence Spectroscopy and Amplified Spontaneous Emission (ASE) of Phenylimidazoles: Predicted Vibronic Coupling Along the Excited-State Intramolecular Proton Transfer in 2-(2′-Hydroxyphenyl)imidazoles

Abstract

Methylation at the 1N position of 2-phenylimidazole provides the shortest wavelength for a liquid-state laser dye reported to date; that is, the 1-methyl-2-phenylimidazole molecule in cyclohexane solution yields amplified spontaneous emission (ASE) with a peak wavelength at 314.5 nm and a constant laser gain value of 5 cm(-1) from 310 to 317 nm. Methyl substitution in this case favors the appearance of laser action (owing to a torsion-vibrational mechanism) in cyclohexane as compared with the nonmethylated species which does not exhibit ASE in this solvent. The 2-(2'-hydroxyphenyl)imidazole molecules give rise to ASE with high gain values (ca. 9 cm(-1)) at 450 and 466 nm. The mechanism of population inversion is understood in terms of a vibronic coupling between the hydroxyl stretching motion and the torsional vibration of the phenyl and imidazole rings. The proton-transfer spectroscopy of 2-(2'-hydroxyphenyl)imidazoles is studied in dioxane, cyclohexane, dimethyl sulfoxide, methanol, and water. The greater the acidity of the solvent the greater the disruption of the intramolecular hydrogen bond; solvent acidity is the main parameter which favors formation of the open-form species in the ground electronic state. Methyl substitution at the 1N position favors formation of the open species for 2-hydroxyphenylimidazoles in the ground electronic state, which decreases their own capacity to undergo ASE. Low-temperature absorption spectroscopy confirms aggregation processes for 2-(2'-hydroxyphenyl)imidazoles in solution. In accordance with X-ray analyses in the solid phase, these molecules form associations through intermolecular chains of the type N-H...O or O-H...N.

Steady‐State Spectroscopy of the 2‐(N‐methylacetimidoyl)‐1‐naphthol Molecule

“…The gas phase electronic transitions are in good accord with the TD/DFT calculations reported for the NH tautomer of MAN. The calculations reported for the isolated molecules resemble the spectroscopic evidence obtained using inert solvents such as cyclohexane and hexane as it is reported elsewhere for the 5‐hydroxyflavone molecule , and 2‐(2′‐hydroxyphenyl)imidazoles .…”

“…The use of only the CIS methodology without constraints provides very unreliable results for this type of molecular systems with intramolecular hydrogen bonds . The TD/CIS/DFT methodology with constraints has given good results when studying the excited state intramolecular proton transfer in 2‐(2′‐hydroxyphenyl)imindazoles . The calculations leading to relative energies and geometrical parameters for the normal and tautomeric forms of the lumichrome–formic acid complex are also reported to be successful for the first excited state when “calculating at the TD‐B3LYP/6‐31G(d,p) for geometries along the reaction path in the first excited state calculated at CIS/6‐31G(d,p) level.” Also, the ESIPT process for the first excited state of HAN calculated at the TD/CIS/DFT level is reasonably good— from a qualitative point of view —when compared with the results obtained at the TD‐DFT/B3LYP//TZVP level with excited state full geometry optimization (see Figure S1, Supplementary Material).…”

“…The use of only the CIS methodology without constraints provides very unreliable results for this type of molecular systems with intramolecular hydrogen bonds (23)(24)(25). The TD/CIS/DFT methodology with constraints has given good results when studying the excited state intramolecular proton transfer in 2-(2 0 -hydroxyphenyl)imindazoles (26). The calculations leading to relative energies and geometrical parameters for the normal and tautomeric forms of the lumichrome-formic acid complex (27) are also reported to be successful for the first excited state when "calculating at the TD-B3LYP/6-31G(d,p) for geometries along the reaction path in the first excited state calculated at CIS/6-31G(d,p) level."…”

Advanced Organic Optoelectronic Materials: Harnessing Excited‐State Intramolecular Proton Transfer (ESIPT) Process

“…[ 48 ] The shortest wavelength ASE emission from ESIPT fl uorophore was demonstrated for the 2-(2′-hydroxyphenyl)imidazole derivatives by Catalán and colleagues. [ 87 ] A slightly red-shifted ASE to the respective fl uorescence emission maximum at 450 nm and 466 nm for 2HOPI and 1M2HOPI ( Figure 18 ), respectively, was obtained in dioxane solution with a high gain value ( ∼ 9 cm − 1 ).…”

Excited-State Intramolecular Proton Transfer of Naphthalene-Fused 2-(2′-Hydroxyaryl)benzazole Family

“…1,2 ESIPT molecules have attracted particular attention from the viewpoint of developing new functional molecules such as fluorescent probes, [3][4][5][6] laser dyes, [7][8][9] and photostabilizers. [10][11][12][13][14][15][16] The ESIPT process involves a phototautomerization reaction to yield an excited keto form (K*) in the subpicosecond time region, with the molecule changing from the original enol form (E) to the keto form on photoirradiation ( Figure 1).…”