Small molecules based on diphenylamine and carbazole with large two-photon absorption cross sections and extraordinary AIEE properties

Abstract

Two simple small molecules based on diphenylamine (T1) and carbazole (T2) have been synthesized for two-photon absorption materials. Both dyes exhibit extraordinary aggregation-induced emission enhancement (AIEE) properties and high corresponding quantum efficiencies of 11.5% and 16.0%. The value of the two-photon absorption cross-section for T1 is 1240 GM.

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The emergence of two-photon absorption (2 PA) materials has opened up a new avenue for scientists to research progressive fields such as two-photon dynamic therapy, up-converted lasing,¹ optical power limiting materials,² two-photon microscopy and bioimaging. Recently, a series of novel organic fluorophores with large 2 PA cross-sections (σ) and good processabilities has emerged.³ The structures of organic fluorophores include asymmetrical donor–π-bridge–acceptor (D–π–A), symmetrical donor–π-bridge–donor (D–π–D), and donor–π-bridge–acceptor–π-bridge–acceptor–π-bridge–donor (D–π–A–π–A–π–D). The 2 PA cross section is connected with the intramolecular charge transfer from the donor to the π-bridge. Moreover, the solvent polarity, molecular coplanarity, and hydrogen-bonding can also intensify the 2 PA cross-section.⁴

However, most 2 PA dyes are soluble in organic solvent, but their fluorescence will be lost in aqueous media, due to self-aggregation driven by limited solubility as well as π–π interactions.⁵ It is restricted that their biophotonic applications in aqueous media for those highly efficient 2 PA materials. So to investigate the 2 PA dyes that can be soluble or dispersible in water and remain highly fluorescent in aqueous media is considerable. Recently, Tang and co-workers observed that some fluorophores have enhanced emission in aggregation, namely aggregation-induced emission (AIE). They deduced that the mechanism of AIE was restricted intramolecular vibrational and rotational motions (RIR) in the aggregated solid.⁶ This phenomenon offered a new path for the materials with large 2 PA cross sections. Prasad and co-workers studied some dyes with aggregation-enhanced fluorescence and two-photon absorption properties.⁷ Tang and co-workers also investigated some organic molecules and polymers displaying 2 PA and AIE properties.⁶

It is well known that triphenylamine and carbazole with excellent electron-donating and transporting capabilities have been widely used in optical and electroactive materials.⁸ Owing to the special propeller starburst molecular structure of triphenylamine, which has aroused great interest and become the focus of progressive research in 2 PA field. Furthermore, cyano-substituted materials exhibit excellent optical and electrical properties due to their favorable electron affinities.⁹ In this work, considering to extend π-conjugation length largely and facilitate the effect of intramolecular charge transfer efficiently would enlarge the 2 PA cross section,¹⁰ we design two simple small molecule with diphenylamine and carbazole as the donor and two-cyano group as the accepter. In view of the effective intramolecular charge transfer (ICT) effect and the molecule structure of this two dyes, excellent 2 PA values and favorable AIEE properties can be anticipated.

The one-photon absorption spectra of T1–T2 in THF were shown in Fig. S2 (ESI†). The absorption maxima (λ_max) of T1–T2 located at 438 nm, 404 nm, respectively. It was obviously shown that the λ_max of T1 was red shifted by 34 nm than that of T2, which was coincide with that the electron-donating ability of diphenylamine was stronger than that of carbazole. To investigate the effect of the solvent polarity on this two compounds, Fig. S3 (ESI†) showed the solvent effect behaviour of T1–T2. The emission spectra of T1–T2 were made by increasing the polarity from cyclohexane to ethanol. It was a positive solvent effect based on the emission spectra, for example, the emission peak of T1 located at 494 nm in cyclohexane undergo a red shift to 631 nm in ethanol. This phenomenon was similar for T2 that from 481 nm in cyclohexane to 600 nm in ethanol. So the intramolecular charge transfer (ICT) effect of T1 was stronger than that of T2. From the above, an obvious ICT effect lied in this two compounds (Fig. 1).

Fig. 1 Molecular structures of T1–T2.

The compounds T1–T2 can be dissolved by the common organic solvent such as THF and dichloromethane, but not water. To explore the AIEE attributes of T1–T2, we use anhydrous THF and water as the good solvent and the poor solvent, respectively. The method to disperse T1–T2 by the primary way, adding different amounts of water to the anhydrous THF solution and making the water fraction (f_w) at 0–90%. Fig. 2a and d showed the corresponding emission spectra of T1–T2 at a concentration of 1.0 × 10⁻⁵ M. As gradual addition of water amounts into the anhydrous THF solution, the fluorescence intensities of T1 in the THF–water mixtures were obviously quenched at the initial stage from f_w = 0% to f_w = 50%. Meanwhile, the λ_max of emission spectrum of T1 in the solvent mixture was red shifted by 27 nm from f_w = 0% to f_w = 50%. Since T1 was a donor–acceptor fluorogen, this phenomena indicated a typical intramolecular charge transfer (ICT) effect, as a consequence with the increase in solvent polarity. Upon further addition of water amounts into the solvent mixture, the T1 molecules become nano-aggregated, and their fluorescence intensity dramatically recovered and heightened with increasing f_w from 50% to 90%. However, the λ_max of emission spectrum of T1 in the solvent mixture was blue shifted by 25 nm from f_w = 50% to f_w = 90%. Apparently, the emission of T1 was induced by aggregation, and displaying the AIEE feature (Fig. 2a). As a result, both the effect of ICT and AIEE could be suited to T1.¹¹

Fig. 2 (a and d) The PL spectra of T1–T2, in different water fractions (f_w); excitation wavelength: 443 nm (T1), 409 nm (T2). (b and e) Plots of (I/I₀) values versus water content of the solvent mixture for T1–T2, where I₀ is the PL intensity in f_w = 50%. (c and f) Absorptions of T1–T2 in THF and water where f_w = 0%, f_w = 50% and f_w = 90% at a concentration of 1.0 × 10⁻⁵ M.

Moreover, firstly the reduced and red-shifted phenomena and then become enhanced and blue-shifted emissions along with aggregation for the emission spectra of T1, compared to the dyes in the solution, the possible reason lied in the dye crystallization in the solid film.¹² Fig. 2c showed the absorption of T1 when f_w = 0%, f_w = 50% and f_w = 90%. Compared to the fluorescence intensity, the absorption spectra of T1 that from the molecule's nano-aggregated state was red-shifted and widened. A quantitative graph of the AIEE process could be work out by measuring the change of the PL spectra with different f_w. As to T1, the fluorescence reached to its maximum when the f_w at 90%, while the weakest fluorescence at 50% with a 10.7-fold reduction and the intensity was increasing at 0% with an 8.5-fold increase in the I/I₀ ratio (Fig. 2b), respectively. Fig. 3b (T1) showed the scanning electron microscopy (SEM) image of T1. It showed that a nano-aggregated structure has formed in the mixture with a high water content.

Fig. 3 (a) The one-photon fluorescence (OPF) emission photos of 1.0 × 10⁻⁵ M of T1–T2, respectively, in different ratios of water and THF; Excitation wavelength: 365 nm. (b) The SEM images of T1–T2 nano-aggregates prepared in f_w = 90% at concentrations of 1.0 × 10⁻³ M.

The compounds T1–T2 emit different one-photon fluorescence with different ratios of THF and water. Fig. 3a showed the one-photon emission images in different water fractions (f_w) of solution. Slight emission was observed at f_w = 50%, but intensified dramatically when f_w = 0% and f_w = 90%, which was coincide with the AIEE phenomenon. The different pH values in the solution had no effect on the one-photon fluorescence Fig. S9 (ESI†).

Similar phenomenon was observed for T2. The PL intensity was weakest at f_w = 50%, and when the f_w = 90% and f_w = 0%, which gave a strong luminescence with a 19-fold and 18.6-fold increase, respectively (Fig. 2e). Fig. 3b (T2) showed the scanning electron microscopy (SEM) image of T2.

The lively picture of the solid powders could also prove the effect of AIEE property, where they show yellow and deep yellow fluorescence emission (Fig. 4). To quantitatively evaluate the AIEE property of the fluorescent materials, the emission spectra and quantum efficiencies of the solid powders were investigated. The dyes (T1–T2) exhibited emission peak at 586 nm and 545 nm and the quantum efficiencies were pitched on 11.5% and 16.0%, respectively. Such high quantum efficiencies of the solids indicated that they had great prospects in OLED applications.¹³

Fig. 4 (a) The solid powder emission spectra of T1–T2; excitation wavelength: 443 nm (T1), 409 nm (T2). (b) The images of T1–T2 as solid powders under 365 nm UV light illumination.

The 2 PA cross sections of T1–T2 were measured by a previously described method that the femtosecond open-aperture Z-scan technique, Fig. 5a and b showed the data fitting of the open-aperture Z-scan. The 2 PA cross section (σ) can be calculated by the equation of σ = hνβ/N₀, in which N₀ = N_AC is the number density of the absorption centers, N_A is the Avogadro constant and C represents the solute molar concentration.¹⁴ The values of the 2 PA at the wavelength of 800 nm for T1–T2 were 1420 GM and 802 GM in chloroform, respectively. While in dichloromethane, the values of 2 PA for both dyes become small in Fig. S8 (ESI†), which were 457 GM and 210 GM respectively. It was noteworthy that solvent effect had a significant influence on the 2 PA cross section, the value in chloroform was bigger than that in dichloromethane. As the solvent polarity increased, consistent with more pronounced ICT in the excited state, larger changes were produced by solvent stabilization. The correlation between the solvent effect and the 2 PA cross section value was complicated and had not made very clear by far, which may related to the solute–solvent interactions, solubility and the solvent polarity.¹⁵

Fig. 5 (a and b) The open-aperture Z-scan trace of T1–T2 (scattered circles = experimental data, straight line = theoretical fitted data) in CHCl₃. (c and d) At 800 nm excited, the TPF emission spectra of T1–T2 in the solution 50 : 50 (v/v water–THF) and 90 : 10 (v/v water–THF) at a concentration of 1.0 × 10⁻⁵ M.

The 2 PA cross section of T1 was bigger than that of T2, due to the ability of electron-donating group based on diphenylamine was stronger than that of carbazole. The cross sections of T1–T2 in chloroform were quite large for 2 PA dyes based on small molecules compared to those reported dyes,^10a,b due to its large π-conjugated system and the effective intramolecular charge transfer (ICT) from the donor to the acceptor, which was also in agreement with the general conclusion that enhancing the π-conjugated system could enlarge the value of 2 PA.¹⁶ So we could control the value of 2 PA cross section by simply introducing different donors or acceptors, and taking the molecular structure into account.

The two-photon fluorescence (TPF) intensity could examined the aggregation effect on the two-photon activity. Under the excitation of an 80 fs, 800 nm pulse, T1–T2 emit yellow and deep yellow fluorescence with peaks located at 624 nm and 606 nm, respectively (Fig. 5c and d), in a mixture of THF and water (f_w = 50% and f_w = 90%). Compared with the one- and two-photon excitation fluorescence, the good overlap between them indicated that the emissions resulted from the same excited state, regardless of the different mode of excitation. The two-photon emission images in different water fractions (f_w) of solution were showed in the inside picture of Fig. 5c and d, slightly emission was observed at the f_w = 50%, While at the f_w = 90% the dyes irradiated yellow and deep yellow emission, when excited at 800 nm. The same phenomenon had also been seen in the one-photon emission images (Fig. 3a). Due to the limitation of our laser apparatus, just irradiation at the wavelength of 800 nm was used to carry out the two-photon excitation experiment. The 2 PA cross sections of both final products would be better in other wavelengths.¹⁷

In summary, we have synthesized two simple small molecules T1–T2 with large 2 PA cross sections and extraordinary AIEE properties. The 2 PA cross sections of T1–T2 were 1420 GM and 802 GM in chloroform, respectively. To the best of our knowledge, the values were quite large for 2 PA materials based on simple small molecules. Moreover, this two dyes exhibited particular AIEE properties with the fluorescence reached to their peak when the f_w at 90%, while the weakest fluorescence at 50% and the intensity recovered increasing to 0%. This extraordinary AIEE properties could be used in two-photon microscopy and bioimaging, which was under our further investigation. What's more, the solid fluorescence of this two compounds located at 586 nm and 545 nm and the corresponding quantum efficiencies were 11.5% and 16.0%. From the above, these excellent performances could promote them as promising materials for biophotonic applications.

Acknowledgements

This work was supported by NSFC/China (20772031), the National Basic Research 973 Program (2006CB806200), the Fundamental Research Funds for the Central Universities (WJ0913001), and the Scientific Committee of Shanghai (10520709700).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The one-photon absorption of T1–T2 in THF; the emission spectra of T1–T2 in different solvents; The 2 PA cross sections of T1–T2 in CH₂Cl₂; Experimental procedures for synthesis; ¹H NMR, ¹³C NMR, EI-TOF; the effect of pH on the one-photon fluorescence. See DOI: 10.1039/c4ra06314g

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