Steady State and Time-Resolved Fluorescence Study of Isoquinoline: Reinvestigation of Excited State Proton Transfer

Abstract

In the present work we report some hitherto unnoticed features in the steady state and time-resolved measurements of isoquinoline in water and trifluoroethanol (TFE). Absorption spectra reveal that in water, neutrals as well cationic species are present. Emission spectrum shows structured features at shorter wavelengths accompanied with a broad band around 375 nm, which correspond to neutrals and cations respectively. However, time-resolved data indicate that protonation does not take place in the excited state in water. On the contrary, in stronger hydrogen bonding solvent TFE, distribution of decay components is observed and at longer wavelengths a small rise time is present. This is ascribed to neutral and cation-like species present in the ground as well as in the excited state. The difference in the results is explained in terms of different excited state potential energy surfaces for water and TFE; particularly, the presence of a rather small barrier for protonation in case of TFE.

Fluorescence enhancement of quinolines by protonation

“…Joshi et al observed similar structureless broadband absorption in acidic medium at 330 nm for isoquinoline. 9 The absorption spectra of acridine and benzo[h]quinoline (see ESI †) show similar spectral changes at longer wavelengths due to protonation, but from these spectra, the absorption bands of the neutral and protonated species are difficult to distinguish. The smaller changes in the spectra could indicate that in those compounds, protonation occurs more easily in the excited state than in the ground state.…”

“…These heterocycles possess non-bonding electrons which give rise to (n-p*) excited states resulting in an increased spin-orbit coupling. [9][10][11] This coupling leads to an enhanced intersystem crossing, effectively decreasing the uorescence quantum yield. It is also commonly observed that the protonation of the "aromatic" nitrogen results in a loss of emission ne structure and a notable red-shi in the emission wavelength.…”

“…15 The excited-state proton transfer in N-heterocycles, including isoquinoline and benzo[h]quinoline, has been previously studied in acidic media. 9,[16][17][18] Benzo[h]quinoline and acridine (benzo[b] quinoline), belonging to a group of polycyclic aromatic Nheterocycles, have been discovered to be particularly interesting since they possess different pK a values in the ground and excited state. [19][20][21][22] However, there has been no studies that are based on comparing different acids and the effects of the resulting counter-anion.…”

Hydrogen bonds in quinoline N-oxide derivatives: first-principle molecular dynamics and metadynamics ground state study

“…The dynamical nature of such kind of compounds has not been broadly investigated computationally; therefore, there is a lack of data concerning the hydrogen bridge dynamics at the ground state. Quinoline N-oxides exhibit excited state intramolecular proton transfer (ESIPT), which was an object of many studies, see, e.g., [4][5][6] and references therein.…”

“…Knowing that quinoline derivatives are intensively investigated due to their potential practical applications in medicine and other branches of science, it could be of interest to examine their performance and perspective of chemical skeleton modification using steric and inductive effects. The excited state dynamics of intramolecular enolketo proton-transfer tautomerism in 10-hydroxybenzoquinoline (HBQ) and its deuterated analog (DBQ) was studied by steady state absorption and fluorescence spectroscopy as well as computationally [4,5]. On the basis of atoms in molecules (AIM) [7] and natural bond orbital (NBO) [8] theories, the excited state intramolecular proton transfer (ESIPT) was studied as well [6].…”