Article pubs.acs.org/JPCA
Intramolecular Charge Transfer of Push−Pull Pyridinium Salts in the Triplet Manifold Benedetta Carlotti,† Giuseppe Consiglio,‡ Fausto Elisei,† Cosimo G. Fortuna,‡ Ugo Mazzucato,† and Anna Spalletti*,† †
Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy ‡ Department of Chemical Sciences, University of Catania, viale Andrea Doria 6, 95125 Catania, Italy ABSTRACT: The solvent effect on the triplet state properties of the iodides of three trans (E) isomers of 2-D-vinyl-1-methylpyridinium, where D is a donor group (4-(dimethylamino)phenyl, 3,4,5-trimethoxyphenyl, and 1-pyrenyl), was studied by nanosecond transient absorption techniques. The results obtained allowed us to complete a previous study on the negative solvatochromism and the role of the solvent-controlled intramolecular charge transfer (ICT) relaxation pathways in the excited singlet state, carried out by ultrafast absorption techniques and DFT calculations (J. Phys. Chem. A 2014, 118, 3580−3592). The solvent effect on the intersystem crossing to the triplet manifold gave information on the competition of the relaxation processes in the singlet and triplet states and the extent and mechanism of the photoreactive deactivation, particularly operative for the pyrenyl derivative. For the latter the results showed that the ICT process also occurs in the triplet manifold. In fact, the formation of two triplet states with different nature and affected differently by oxygen was observed for the pyrenyl derivative in water and assigned to the 3LE* and 3ICT* states. The related structure and solvent effects on the trans → cis photoisomerization are briefly discussed.
1. INTRODUCTION
The solvent effect on the photobehavior of compound 1 (usually named by its acronym, o-DASPMI) was previously studied also in other laboratories (for some representative papers see refs 7−11) but the description of the charge movement during the excitation and the lifetime of the first excited singlet state left some open questions. In our previous work1 a reasonable interpretation of the overall ICT process was obtained. The dipole moment of 1 was computed for the ground (μg) and excited FC (μFC) states and for the relaxed twisted (μTICT) state formed after completion of the charge movement in the excited S1 state. A relatively high value (μg = 6.9 in dichloromethane, DCM) was found for the dipole moment of the most stable conformer due to the positive charge largely localized on the pyridinium group, as indicated by the computed Mulliken charges. The computed MOs and charge densities showed the CT character of the transition under light absorption leading to an electron transfer toward the pyridinium group, which reduces the polar character of the FC excited state (μFC = 3.3 D for 1 in DCM). The corresponding shift of the positive charge led to a change of direction of the dipole moment component along the principal molecular axis (μx) upon excitation.1
The long-term interest of our group on the photobehavior of stilbene-like molecules has been recently directed to push−pull compounds bearing electron donor/acceptor (D/A) units and particularly to stilbazolium (A) salts bearing dimethylamino (D) substituents.1 The latter compounds are known to be of interest as fluorescent sensors2 because the introduction of the dialkylamino group generally affects significantly the competition between the emissive and reactive (trans → cis photoisomerization) decays3 and for potential applications in the medical fields, due to their ability to form complexes with DNA.4,5 Moreover, their push−pull character can induce intramolecular charge transfer (ICT) and solvent effects of potential interest for applications in optoelectronics/photonics as nonlinear optical (NLO) materials.6 In the previous work we investigated in depth (by stationary and ultrafast spectrometric techniques and theoretical DFT calculations) the ICT process in the singlet manifold of three compounds with D groups of different efficiency, namely the trans (E) isomers of the iodides of 2-[4-(dimethylamino)styryl]1-methylpyridinium (1, the reference compound), and two analogues where the 4-dimethylanilino group was replaced by 3,4,5-trimethoxyphenyl (2) and 1-pyrenyl (3).1 The possible role of multiple conformers of these compounds was addressed in the previous paper.1 The results indicated that the conformational equilibrium was shifted toward a largely prevalent species. © XXXX American Chemical Society
Received: May 20, 2014 Revised: August 21, 2014
A
dx.doi.org/10.1021/jp504963v | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Scheme 1. Molecular Structures of the Trans (E) Cations of the Iodides of 2-D-ethenyl,1-methylpyridinium (D = 4(Dimethylamino)phenyl (1), 3,4,5-Trimethoxyphenyl (2), and 1-Pyrenyl (3))
Table 1. Triplet Properties of 1−3 in Different Solvents (λexc = 355 nm) solvent 1
DCM MeCN W
2
3
DCM MeCNb W DCM MeCNb W
a
λmax (nm)
τ (μs)
τa (μs)
ϕT × εT (M−1cm−1)
600 860 590 840 570 710 860 450 430 430 710 490 700 550 760 530 740
1.6 1.2 7.9 7.9 14 0.41
0.34
5130
5 19 125 0.57 200 10 21 6.5 14 1.3
160 1.4 0.33
εT (M−1 cm−1)
20100
250
ϕT
kox (109 M−1 s−1)
0.27
1.2
0.008 0.012
930
