Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

Abstract

Three (donor-π-acceptor)(+) systems with a methyl pyridinium or quinolinium as the electron-deficient group, a dimethyl amino as the electron-donor group, and an ethylene or butadiene group as the spacer have been investigated in a joint spectroscopic and TD-DFT computational study. A negative solvatochromism has been revealed in the absorption spectra, which implies a solution color change, and interpreted by considering the variation in the permanent dipole moment modulus and orientation upon photoexcitation. The fluorescence efficiency decreases upon increasing solvent polarity, in agreement with the excited-state optimized geometries (planar in low-polarity media and twisted in high-polarity media). Femtosecond transient absorption has revealed the occurrence of a fast photoinduced intramolecular charge transfer (ICT) and the molecular factors that determine an efficient ICT. Considering the crucial role of the ICT in tuning the nonlinear optical (NLO) properties, these compounds can be considered promising NLO materials.

Efficient Excited‐State Symmetry Breaking in a Cationic Quadrupolar System Bearing Diphenylamino Donors

“…In view of the very similar dielectric constant of both solvents, the differencesi nt he spectra and fluorescenceq uantum yields recorded in this mediumw ith respect to those measured in MeOH should be due solelyt oi ts increasedv iscosity.N oi mportant variation was observed on the absorption spectra (see SI, Figure S4), whereas ab lue shift of the emissionb and and an increase of af actor of 5i nt he fluorescenceq uantum yield were found for both systemsu pon increasingt he solventv iscosity (see Ta ble 3). This result is rather interesting ands ignificantly different from what previouslyf ound for other push-pull dipolar dimethylamino substituted and cationic dyes bearing different electron acceptorp ortions, [36] forwhich achange in the excited state geometry (from planar in DCM to twisted in MeCN and W) was predicted upon increasing the solvent polarity.T his theoretical finding could convincingly explain the substantial fluorescenceq uenching experimentally revealed for those chromophores in the more polar media. Ac omprehensive theoreticali nvestigation about the structural and electronic properties of 1 and 2 allowed gaining further insight into the factorsg overning their spectral and photophysical behaviour.T he optimised geometries for the ground and first excited singlet state of both the dipolara nd the quadrupolar molecules in MeOH are showni nF igure 3.…”

“…On the contrary,i ti sn oteworthy that in both solvents the emission bands of compounds 1 and 2 roughlyo verlap [ Figure 2(a)].T his peculiar behaviour was pre-viously found in highly polar media for similar dipolar and quadrupolarcompounds where the electron donor was adimethylamino group, [26] but it is here surprisingly observed in all the investigated solvents regardless of their polarity.W hen significantly strong electron donor groups are present in the quadrupolarD -p-A + -p-D structure (such as diphenylamino or also dimethylamino groups), that is, in systems showing as trongq uadrupolar character,t his surprising coincidence between the emission bands of dipolara nd quadrupolara nalogues is observed. f F = 0.0023 for 1 and 0.0013 for 2 in MeOH), the decrease being strikingly slightly more important in the case of the symmetrical quadrupolar methyl pyridinium derivativew ith respectt ot he asymmetrical compound 1.T hese resultsa re substantially different from those previously retrieved for dimethylamino substituted analogous systems, [30,36,41] for whichasignificant fluorescence was detectedi nl ow polar solvents together with somet riplet productiona nd photoreactivity.F or the dimethylamino derivatives am ore important quenching of the emission was revealed upon increasing the solvent polarity (two/three orders of magnitude). [40] Not only the fluorescences pectral properties of 1 and 2 are very close in all the studied media, but an analogous trend with solvent polarity was retrieved also considering their fluorescence efficiencies.…”

“…The reasons behind the negative solvatochromic behaviour of the absorption spectra of both compounds 1 and 2 were deeply investigated with the help of ac omputational study about the permanent dipole and quadrupole moments of these molecules in their ground and first excited singlet state. [30,36] On the other hand, ah igh permanent dipole moment is predicted (ac-tually higher than in the ground state) for the relaxed lowest excited singlet state S 1r (m = 20.76 Df or 1 in MeOH). However,i n QM computation, a" working definition" is alwayse mployed, which refers to the centre of nuclear charge of the system.…”

An ultrafast spectroscopic and quantum mechanical investigation of multiple emissions in push–pull pyridinium derivatives bearing different electron donors

“…B3LYP 46 and M06-2X) 47 and range-separated (viz. 21,27,37 For the sake of brevity, only the results obtained at the CAM-B3LYP/SNSD level of theory are shown in order to have a direct comparison with those shown in the previous references. Calculations performed by coupling different functionals and basis sets confirm what emerged in previous studies.…”

“…[3][4][5][6][7][8][9] It has been observed that the extent of the NLO response and the two photon absorption cross-sections are strongly dependent upon the molecular structure of the pushpull molecules and are particularly affected by the strengths of the electron donor/acceptor groups. [14][15][16][17][18] This effect was observed not only in widely studied neutral donor-acceptor systems, but also in cationic chromophores bearing a methyl pyridinium or a methyl quinolinium as an electron deficient moiety, [19][20][21][22][23][24] whose photobehaviour has been less investigated so far and, therefore, not yet deeply understood. [14][15][16][17][18] This effect was observed not only in widely studied neutral donor-acceptor systems, but also in cationic chromophores bearing a methyl pyridinium or a methyl quinolinium as an electron deficient moiety, [19][20][21][22][23][24] whose photobehaviour has been less investigated so far and, therefore, not yet deeply understood.…”