Excited-state proton transfer of 4-hydroxyl-1, 8-naphthalimide derivatives: A combined experimental and theoretical investigation

Abstract

The photophysical properties of N-butyl-4-hydroxyl-1, 8-naphthalimide (BOH) and N-(morpholinoethyl)-4-hydroxy-1, 8-naphthalimide (MOH) in various solvents are presented and the density functional theory (DFT) / time-dependent density functional theory (TDDFT) methods at the B3LYP/TZVP theoretical level are adopted to investigate the UV-visible absorption and emission data. An efficient intermolecular excited-state proton transfer (ESPT) reaction occurs for both compounds in DMSO, methanol and water. In aqueous solution, both BOH and MOH can be used as ratiometric pH probes and perform as strong photoacids with pKa* = −2.2, −2.4, respectively. Most interestingly, in the steady-state fluorescence spectra of BOH and MOH in concentrated HCl, an unexpected blue-shifted band is observed and assumed to originate from the contact ion pair (CIP) formed by hydronium ion and the anionic form of the photoacid resulted from ESPT. Theoretical calculations are used to simulate the CIP in the case of BOH, which afford reasonable results compared with the experimental data. Keywords: Naphthalimide Excited-state proton transfer Photoacid Contact ion pair Time-dependent density functional theory 1. Introduction Upon photoexcitation, aromatic alcohols (ROH) such as naphthols, hydroxyquinolines, and hydroxypyrenes may increase the acidity dramatically. Research on this photoinduced deprotonation is first described by Fὅrster in 1949 [1] and still attracting intensive attention in the field of biology and physical chemistry [2-11]. The Eigen-Weller model for proton transfer is used to depict this process, where the initial short-range proton transfer forms a contact ion pair (CIP) as a reaction intermediate followed by diffusion controlled separation of the ions [12-13]. © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/Recently, Brouwer and coworkers synthesized and characterized some hydroxy-substituted 1, 8-naphthalimide derived "super" photoacids, where the hydroxyl group is located at the 3 or 6 position of the naphthalimide ring [6,14]. However, for 4-hydroxyl-1, 8-naphthalimide derivatives, which are extensively used for designing turn-on or rationmetric fluorescent indicators for bioimaging [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], the photophysical properties and the excited-state proton transfer (ESPT) reactions are rarely studied [30].In this contribution, the steady-state absorption and emission spectra of two 4-hydroxyl-1, 8-naphthalimide derivatives (BOH and MOH, Scheme 1) in organic solvents and water with various pH values are recorded. The density functional theory (DFT) / time-dependent density functional theory (TDDFT) methods at B3LYP/TZVP theoretical level are adopted to investigate the UV-visible absorption and emission data. In the study of the ESPT of 5-cyano-2-naphthol in sub-and supercritical water, Kobayashi and coworkers [31] found that the band of their excited neutral (ROH*) and anionic (RO − *) species...

Unraveling the Detailed Mechanism of Excited-State Proton Transfer

“…For example, when new ESPT chromophores are developed, the design strategies and possible mechanisms are proposed based on the experimental results. − However, the suggested mechanism is usually not completely corrected and sometimes problematic, thereby requiring further theoretical studies to validate or invalidate; − this can provide guidance for developing new ESPT chromophores . When multiple protons are involved, such as in excited-state double PT (ESDPT), whether the mechanism is stepwise or concerted is usually under debate among experimental scientists; , extensive theoretical studies are necessary to resolve this controversy. − When solvent molecules participate in ESPT as proton acceptor or catalyst, the size of the solute·(solvent) n cluster is difficult to determine experimentally, and theoretical studies are necessary. − …”

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

“…[18][19][20][21][22] However, examples of a clear identication of the CIP* spectral signatures are scarce. [23][24][25][26][27][28] Few studies reported an intermediate uorescence between the ROH* and RO À * bands that was attributed to the contact ion pairs but this was observed only in supercritical, 23 frozen or strongly acidic aqueous solutions 24,25 and aprotic organic solvents. 26,27 Therefore, it remains unclear whether the CIP* emission would be detectable in protic solvents at a moderate pH range.…”

Photoacidity of the 7‐Hydroxyflavylium Cation

“…Taken as a whole, the present results provide a coherent theoretical description of the photodissociation of the 7-hydroxyflavylium cation, promoted by a significant shift of charge away from the OH group in the first singlet excited state, leading smoothly to the excited conjugate base and a protonated water cluster. The consistency of the results of the present study can be attributed to a combination of several factors: (1) our theoretical approach (TD-DFT with the B3-LYP functional and def2-TZVP basis set utilizing Grimme's D3 dispersion correction), which was found to give results similar to high-level ab initio methods for anthocyanins (25); (2) the modeling of the solvent effect by immersing the photoacid, hydrogen-bonded to a cluster of discrete water molecules (12)(13)(14)(15)(16)(17)(18)(19), into a water-like continuum solvent (COSMO); (3) the fact that the S 1 state of the flavylium cation (and of anthocyanins in general) (33) is dominated by HOMO-LUMO contributions and is well separated in energy from S 2 and higher excited states; and (4) unlike a neutral or anionic photoacid, the proton is transferred from the excited cationic photoacid to water to form a neutral excited conjugate base, avoiding Coulombic interactions between the proton and the conjugate base. As such, the present approach should serve as a useful framework for the theoretical treatment of intermolecular ESPT in other molecular systems.…”

“…1), agree quite well with the corresponding experimental values (3) of 2.90 eV (427 nm) and 2.61 eV (475 nm) for these two species. As shown by a number of recent studies, proper theoretical treatment of intermolecular ESPT requires the use of a discrete proton acceptor such as a solvent cluster (12)(13)(14)(15)(16)(17)(18)(19). Indeed, upon hydrogen-bonding AH + to a discrete water molecule or to clusters of three or five discrete water molecules (optimized in a COSMO aqueous environment), the S 1 energy decreased by about 0.07-0.08 eV (Fig.…”

“…Although theoretical and experimental studies have reported that, in some cases, the enhanced photoacidity of the S 1 state can be ascribed mainly to electronic effects in the excited conjugate base rather than in the excited acid (5,6), theoretical descriptors based on calculated properties of the excited acid have been employed (7,8) to correlate the photoacidity trends of hydroxyaromatics. At the same time, as is known from both experiment (9)(10)(11)(12) and recent theoretical work (12)(13)(14)(15)(16)(17)(18)(19), quantum chemical treatments of intermolecular excited state proton transfer (ESPT) must adequately take into account explicit solvent molecules in order to reproduce adequately their role in modulating the photoacidity.…”