Uncovering Structure–Property Relationships in Push–Pull Chromophores: A Promising Route to Large Hyperpolarizability and Two-Photon Absorption

Abstract

In this investigation, we report the first hyperpolarizabilities and two-photon absorption cross sections of a large series of 12 push–pull cationic chromophores. All of these dyes show a dipolar acceptor+–π–donor structure, where the nature of the donor and acceptor units and π-bridge was synthetically tuned to allow insightful comparisons among the molecules. The hyperpolarizability was obtained through a solvatochromic method, by exploiting the rare negative solvatochromism exhibited by the investigated compounds. The two-photon absorption cross sections were determined through two-photon excited fluorescence measurements by means of a tunable nanosecond laser system for sample excitation. The nonlinear optical properties were discussed relatively to the photoinduced intramolecular charge transfer occurring in these donor–acceptor systems, investigated by femtosecond transient absorption experiments. We found a strong increase in hyperpolarizability upon increasing the molecular conjugation. Unexpectedly, the hyperpolarizability is almost unaffected by an increase in donor/acceptor strength and intramolecular charge transfer degree. Differently, the two-photon absorption cross sections of these dyes are enhanced by an increase in both molecular conjugation and intramolecular charge transfer efficiency. Several recent literature works have reported at the same time scattered information about the hyperpolarizability and two-photon absorption of small organic molecules. Our investigation is, to the best of our knowledge, the first attempt to uncover detailed structure–property relationships for these two nonlinear optical properties. Our results represent a promising route to achieve large hyperpolarizability and two-photon absorption in push–pull dyes and may drive the design of new efficient nonlinear optical materials.

Modification of the Second Harmonic Generation and Fluorescence Efficiency of D289 Dye Based on a Donor–Acceptor Structure

“…The pyridinium in TPAP and TPAPM was used as an electron-deficient segment connected to the π-conjugation with an electron donor. In addition, decoration with pyridinium makes these molecules more soluble in water, and it also provides affinity for the cell membrane based on electrostatic interactions with the lipid membrane, which are all essential for an ideal SHG probing material. − The TPAP and TPAPM molecules were synthesized and characterized via the procedures detailed in Figure and the Supporting Information (SI).…”

“…Both experimental and computational efforts were applied in this work to evaluate the hyperpolarizabilities of the three molecules. We used the solvatochromism method, which has been widely used as a simple and available way to obtain the value of β CT based on the Reichardt theory. ,− In the solvatochromism method, wavelength differences between adsorption and fluorescence peaks are measured for push–pull molecules in solvents with different polarities and used to estimate the hyperpolarizabilities of the molecules. The absorption and fluorescence spectra of the three molecules (D289, TPAP, and TPAPM) were measured in nine solvents with different polarities, as shown in Figures S6–S8.…”

“…The resulting wavelength differences (converted to wavenumbers) were plotted versus the polarities of the solvents, as shown in Figure a–c. Based on procedures used in previous reports, , the β CT values of the D289, TPAP, and TPAPM molecules were deduced from the slopes of these linear fits to be 2088, 1671, and 2786 (as shown in Table , all with units of 10 –30 esu –1 cm 5 ), respectively.…”

Using diketopyrrolopyrroles to stabilize double excitation and control internal conversion

“…We stress that our primary goal was not to benchmark vinyl-linked chromophores but to explain the quenching of fluorescence in the aforementioned DPPs derivatives. Many ethylene spaced molecules have been synthesized in the past, [38][39][40] mostly in the context of push-pull chromophores (where the olefinic unit acts in order to decrease the HOMO-LUMO gap), 39 but their particular investigation is outside the scope of this paper.…”

Probing the structural features and the micro-heterogeneity of various deep eutectic solvents and their water dilutions by the photophysical behaviour of two fluorophores

“…In order to shed light on the structural features of a set of differently structured DESs, a spectroscopic femtosecond-resolved study is proposed in this work with the use of suitable fluorescence probes, whose photophysical properties and excited state dynamics have already been deeply investigated in our laboratory [50][51][52][53][54]. Specifically, an N-methylpyridinium iodide (2-4-(diphenylamino)phenyl-ethenyl-1-methylpyridinium iodide, PyI) was chosen as a polarity and viscosity probe, while a phenanthroline derivative (4,4'-((1E,1'E)-(1,10-phenanthroline-2,9diyl)bis(ethene-2,1-diyl))bis(3-methoxy-N,N-dimethylaniline, X5), showing fluorosolvatochromism and bearing protonation sites, was used as polarity and H-bond probe.…”