pH dependent one-/two-photon fluorescence emission properties and mechanism of the dendrimer PAMAM triphenylamine imine

Abstract

Dendrimer PTS-G0, based on poly-amidoamine (PAMAM) functionalized by triphenylamine imine (C=N), and reference compound ETS, based on ethylenediamine triphenylamine imine, were both synthesized. PTS-G0's one-/two-photon (2PA σ₂ = 75 GM) fluorescence emission increased at low pH and remained high at high pH, showing emission ranges that were different from those of NH₂- or HO-terminated PAMAM as previously reported. The pH related fluorescence emissions of PTS-G0 and ETS were compared. The emission properties of the independent chemical groups were calculated by quantum chemical theory methods. The imidic acid (HO–C=N), imine (C=N), and tertiary ammonium groups fluoresce in different pH ranges. The possible fluorescence emission mechanisms of PTS-G0 were discussed, which can help to explain the fluorescence emission of PAMAM and benefit the understanding of intrinsic fluorescence phenomena.

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Introduction

Poly-amidoamine (PAMAM) dendrimers (Scheme 1) were first synthesized by Tomalia¹ in 1985. Since then, these dendrimers have caused interest due to their solubility in water, inside holes,² functional or modifiable chemical groups,³ biocompatibility,⁴ packaging into nano particles,⁵ and abilities to deliver small molecules or drugs.⁶ In recent years, the fluorescence emission phenomena of PAMAM have been observed.⁷ PAMAM dendrimers have amide, primary amine, and tertiary amine groups, which are not traditionally typical fluorescence emission groups. Many PAMAM emission-related influencing factors have been considered, such as pH,⁸ oxidation,⁹ behavior in air,¹⁰ and other influences including sizes, shapes and distances.¹¹ The fluorescence emission phenomena and mechanism of PAMAM have been of interest.

Scheme 1 The structure of PAMAM.

The pH-dependent fluorescence emissions of PAMAM were researched by several groups.¹² It was shown that NH₂-terminated and HO-terminated PAMAM can give strong fluorescence emission at low pH values¹³ although the mechanisms need to be determined. The PAMAM fluorescence emission phenomena were attributed to intrinsic fluorescence. Although intrinsically fluorescent macromolecules have been paid attention,¹⁴ so far the intrinsic fluorescence mechanism has not been studied systematically.¹⁵ Our group has studied the fluorescence emission phenomena of PAMAM,¹⁶ to find that imidic acid (HO–C=N) and ammonium structures exist in PAMAM dendrimers and take part in the fluorescence emissions of PAMAM. The study of PAMAM dendrimers and their modified derivatives will help in finding the mechanism of intrinsic fluorescence emissions.

In order to understand the pH dependent fluorescence emission of PAMAM dendrimers, the dendrimer PTS-G0 (PAMAM_Triphenylamine_Schiff-base imine Generation 0) (Scheme 2) was synthesized from PAMAM-G0 by modification with triphenylamine aldehyde using imine (Schiff base C=N) links. The PTS-G0 has tertiary amine, amide, imine, and aromatic tertiary amine groups. The pH related one-photon and two-photon fluorescence emissions of PTS-G0 were tested. The results show that the PTS-G0 gave strong fluorescence at pH > 2 and weak fluorescence at pH < 2. The two-photon excited fluorescence (2PEF) spectra of PTS-G0 show a similar situation. The PTS-G0 pH related fluorescence phenomena were different from those of NH₂-terminated PAMAM and HO-terminated PAMAM as previously reported.¹² PAMAM shows strong fluorescence at low pH values and weak fluorescence at high pH values. In contrast, the PTS-G0 including the PAMAM core gave strong fluorescence emission at neutral or high pH values, and weak fluorescence at low pH. These differences are discussed and some related mechanisms are given.

Scheme 2 The structures of PTS-G0 and ETS.

The ethylenediamine triphenylamine Schiff base (ETS) was synthesized as a reference compound (Scheme 2), by ethylenediamine modification with triphenylamine aldehyde using imine (Schiff base C=N) links. ETS has imine and triphenylamine groups. Therefore the pH related fluorescence emissions of ETS are a useful reference for PTS-G0.

Two-photon absorption (2PA)¹⁷ is a nonlinear optical process wherein a molecule simultaneously absorbs two photons of energy hν (or of energy hν1 and hν2) via a middle virtual state to access an excited state in the presence of an intense laser pulse. 2PA corresponds to a third-order nonlinear optical process. The 2PA was excited by a near-infrared lamp-house that has longer wavelength. Therefore, the scatter of the light decreased and the light's penetration in the material increased, which allows deep observation of the material. The two-photon process is in a direct ratio with the square of the incidence light intensity, so better modulated properties can be obtained. In recent years, a considerable amount of effort has been devoted to 2PA materials and devices. A variety of compounds including donor–bridge–acceptor (D–π–A) dipoles, donor–bridge–donor (D–π–D) quadrupoles, multi-branched compounds, dendrimers, and octupoles have been synthesized and researched. A number of factors influence the 2PA magnitude, which include electronic delocalization and intramolecular charge-transfer (ICT) phenomena, and conjugated system length.¹⁸

This paper tests one/two photon pH dependent fluorescence emission properties of PTS-G0. The pH dependent fluorescence properties of PTS-G0 and ETS are compared, to get some information about intrinsic fluorescent emission phenomena.

Results and discussion

Structure characterization

The structure of PTS-G0 was determined by ¹H NMR, ¹³C NMR, IR, and MS, and connected with the fluorescence properties. The characterized structure shows the existence of imidic acid (HO–C=N) and imine (C=N) groups in PTS-G0. The ¹H NMR spectrum (Fig. S4†) of PTS-G0 has a chemical shift (δ) of 3.53 ppm, similar to that of water, but there was DMSO-d₆ solvent and not water in the sample. Accordingly this peak was attributed to the hydroxyl group of imidic acid (HO–C=N). The ¹³C NMR (Fig. S5†) shows a δ peak at 171.87 ppm, which should be attributed to the C atom of imidic acid (HO–C=N). There was no amide ¹³C NMR δ peak at about 175.6 ppm. The ¹³C NMR peak at 161.46 ppm was the C atom signal of the C=N. The IR spectrum (Fig. S6†) shows a 1640 cm⁻¹ peak that corresponds to absorption from the C=N of the imine end group or the C=N from imidic acid. The amide carbonyl peak should be at about 1700 cm⁻¹, but this 1700 cm⁻¹ peak does not appear in the IR spectra, which shows that the amides in PTS-G0 have changed. The MS (Fig. S7†) shows molecular ion peaks (1537.02 [M], 1537.91 [M + H]⁺) and some fragment peaks. The MS shows that there were no other new chemical structures produced, which cannot exclude the possibility that imidic acid exists in PTS-G0. The ¹H NMR, IR, and MS show that the imidic acid (HO–C=N) structure exists in PTS-G0 (Scheme 2). The TLC of PTS-G0 (Fig. S13†) shows the long tails of the sample, which means that different ratios of amide/imidic acid exist in PTS-G0 to form tailing bands. The imidic acid (HO–C=N) and imine (C=N) influence the fluorescence of PTS-G0 together.

pH related fluorescence properties

Fig. 1 gives the pH dependent fluorescence emission spectra of PTS-G0, and the pH relationship to peak intensity is shown in Fig. 2. These figures show that the fluorescence of PTS-G0 increased with increasing pH value. The (fluorescence) photographs of PTS-G0 at low/neutral/high pH values are given in Fig. S14 (in the ESI†), and show that the yellow color of the low pH PTS-G0 solution gave very weak fluorescence, and the transparent neutral or high pH PTS-G0 solution gave strong fluorescence under UV light.

Fig. 1 Fluorescence emission spectra of (1 × 10⁻⁵ mol L⁻¹) PTS-G0 (ethanol/water = 1/9 volume ratio) at different pH values (λ_ex = 360 nm).

Fig. 2 The relationship curve of fluorescence emission peak intensity vs. pH of PTS-G0 (inset shows the linear fit: log[(IF_max − IF)/(IF − IF_min)] vs. pH). (Solvent ethanol/water = 1/9 volume ratio).

The pH-dependent fluorescence emission relationship (Fig. 2) shows that the fluorescence intensity increased from pH 1.5 to 3, and remained high at pH > 3. The pH related fluorescence spectra (Fig. 1) have blue shifts at pH ≈ 4; and the pH related UV-vis absorption spectra (Fig. S3†) also show that the wavelength changed at pH ≈ 4 (λ_abs wavelength shifted from 480 to 450 nm). This shows that pH ≈ 4 was a transformation point for the structure centres responsible for the fluorescence emission. In DMSO, the pH related fluorescence spectra show a wavelength change at pH ≈ 6 (Fig. 4; λ_abs shifts from 500 to 450 nm), which shows that PTS-G0's fluorescence structure centres change at pH ≈ 6 in DMSO.

PTS-G0 in the water/ethanol system displays aggregation phenomena: the TEM photograph (Fig. 3) shows the nano liquid balls formed in the water/ethanol mixed solvent. Thus, the pH related fluorescence in the water/ethanol system involved aggregation induced emission (AIE).¹⁹ Therefore DMSO solvent was used to test the pH related fluorescence of PTS-G0 without AIE. Fig. 4 gives the PTS-G0 pH dependent fluorescence in DMSO, which excludes the possibility of AIE or J-aggregation. Fig. 5 shows that the PTS-G0 gives low fluorescence at low pH and high fluorescence at high pH, which is similar to the properties shown in Fig. 2.

Fig. 3 TEM of PTS-G0 (test dry sample in ethanol/water = 1/9 volume ratio).

Fig. 4 Fluorescence emission spectra of (5 × 10⁻⁶ mol L⁻¹) PTS-G0 (in DMSO) at different pH values (λ_ex = 350 nm).

Fig. 5 The relationship curve of fluorescence emission peak intensity vs. pH of PTS-G0 in DMSO.

The pH dependent fluorescence emission spectra (Fig. 6) and pH relationships (Fig. 7) of ETS show high fluorescence emission at low pH values and weak fluorescence at high pH values. The peak is at about pH = 3. The pH dependent fluorescence then reduces and is low in the range pH = 5–13. Comparing the PTS-G0 and ETS pH dependent behavior, it can be seen that they both display high emission at about pH = 3, but are different in the range of high pH values. From pH = 6 to pH = 13, PTS-G0 kept its high fluorescence, but the ETS fluorescence reduced to a low intensity. ETS has triphenylamine and imine groups while PTS-G0 has imidic acid and tertiary amine groups in addition. The different structures make the pH dependent fluorescence different.

Fig. 6 Fluorescence emission spectra of (1 × 10⁻⁵ mol L⁻¹) ETS (ethanol/water = 1/9 volume ratio) at different pH values (λ_ex = 400 nm).

Fig. 7 The relationship curve of fluorescence emission intensity vs. pH of ETS (in ethanol/water = 1/9 volume ratio).

Fig. 8 gives the TEM of the ETS test dry sample in the ethanol/water mixed solvent. The nano particles show that the ETS was aggregated in the solvent. It can be concluded that the ETS pH dependent fluorescence emission shows an AIE effect in this ethanol/water mixed solvent. Fig. 3 and 8 show that both PTS-G0 and ETS show AIE effects in the ethanol/water mixed solvent. DMSO was used as a solvent to test the pH dependent fluorescence without AIE effects.

Fig. 8 TEM of ETS (test dry sample in ethanol/water = 1/9 volume ratio).

Fig. 9 gives the pH dependent fluorescence emission spectra of ETS in DMSO. Fig. 10 gives the relationship of the ETS fluorescence peak intensity versus pH, which displays the same properties as shown by PTS-G0 in DMSO in Fig. 5. Both Fig. 5 and 10 show low fluorescence in the range pH 1–5, high fluorescence in the range pH 6–13, and give low fluorescence points at pH = 8 and pH = 12.

Fig. 9 Fluorescence emission spectra of (1 × 10⁻⁵ mol L⁻¹) PTS-G0 (in DMSO) at different pH values (λ_ex = 350 nm).

Fig. 10 The relationship curve of fluorescence emission intensity vs. pH of PTS-G0 in DMSO.

Fig. 5 and 10 show pH 8 and pH 12 structure change points. The reasons need to be determined; they possibly connect to the structure changes of chemical groups at these two pH values. At pH = 8, the solution is a weak base, which might be fit for the C–N isomerization phenomena. The imine C=N displays isomerization phenomena, in which double bonds produce isomerisation resonance to absorb the energy of the system and lower the fluorescence emission intensity of PTS-G0 or ETS at pH = 8. The pH = 12 low point might be a consequence of the imine connected with TPA. The ⁻OH acts on the N atom of triphenylamine (TPA), to form a new imine N=C–TPA isomerisation system to consume the energy of PTS-G0 or ETS fluorescence. Thus the pH = 12 value gives low fluorescence intensity. Scheme 3 shows how hydroxide ion (⁻OH) of base can act on the N atom of TPA to form –N=C-TPA-OH, to give high fluorescence at pH > 13. The PTS-G0 and ETS both have C=N and TPA, which might give rise to the similar fluorescence properties in DMSO shown in Fig. 5 and 10.

Scheme 3 The possible mechanisms of pH dependent fluorescence emission of chemical groups.

Two-photon fluorescence properties

The PTS-G0 modified dendrimer has pH related two-photon absorption (2PA) properties, which are shown in Fig. 11. The two-photon absorption cross sections (σ₂) were tested by the two-photon induced fluorescence (TPIF) method. The two-photon excited fluorescence (2PEF) of PTS-G0 was tested at three pH values (pH = 7.54; 11.71; 2.53) of PTS-G0 solutions. σ₂ = 75 GM at pH 11.71, σ₂ = 38 GM at pH 7.54, but it was not tested at pH 2.53. The two-photon excited fluorescence spectra of PTS-G0 show that the PTS-G0 gave weak 2PEF intensity at low pH values, moderate 2PEF intensity at neutral pH values and strong 2PEF intensity at high pH values. The 2PEF behavior of PTS-G0 was similar to that of the one-photon fluorescence. The parameters of PTS-G0 are listed in Table S2.†

Fig. 11 Two-photon excited fluorescence (2PEF) emission spectra of PTS-G0 at different pH values. λ_ex = 800 nm by femtosecond Ti: sapphire laser (solvent ethanol/water = 1/9 volume ratio).

Quantum chemical calculations

MOPAC²⁰ software was used to calculate the pK_a of imidic acids in PAMAM-G0 and in PTS-G0 (structures are shown in Schemes 1 and 2 respectively). The PTS-G0 pK_a values for the four hydroxyl hydrogens of HO–C=N are: 1.434; 3.128; 4.529; 6.083 and the values for the PAMAM-G0 four hydrogens of HO–C=N are: 2.289; 2.310; 2.499; 2.555. These pK_a data show that the imidic acid structure is forming in the range of about pH 1.5 to 6, which indicates that the fluorescence centre of PTS-G0 was imidic acid (HO–C=N) (Fig. 12). In this range (pH 1.5–6), the key structure turning points were about pH ≈ 4, corresponding to the UV spectra (Fig. S3†). The PTS-G0 pH related fluorescence ranges (pH 1.5–6) were similar to those of PAMAM.¹² At the low pH values, the ammonium formed, which may influence the fluorescence emission. In the range pH > 6, the PTS-G0 kept its strong fluorescence emission due to imidic acid emissions, which differed from the fluorescence properties of PAMAM as previously reported.¹² From Fig. S10(a–c) and Table S2,† the fluorescence life times were 1.61 ns (at pH = 7.54) and 1.80 ns (at pH = 11.71), which shows that the fluorescent centres of PTS-G0 were different at neutral and high pH. The fluorescence life time was 0.99 ns at pH = 2.53, which shows that the fluorescent centres were different at low pH than at neutral and high pH values. These phenomena can be explained by the amide/imidic-acid transfer mechanism in Scheme 3.

Fig. 12 pH related fluorescence emission ranges of PTS-G0 chemical groups.

In order to test the mechanism of imidic acid and imine (in Scheme 3), the quantum chemical calculations of parts from PTS-G0 (Fig. S15†) were undertaken. The calculation used the time-dependent density functional theory (TD_DFT) b3lyp/6-31g methods in the Gaussian 09 software package.²² The parts from PTS-G0 were small key chemical groups to make the calculations efficient.

Table S3† lists emission spectra of P-01, P-02, P-03, P-03′, P-04, P-04′, P-05, P-05′, P-06, and P-06′ (Fig. S15†) in a gas phase vacuum calculated by the TD_DFT b3lyp/6-31g method. From calculated emission spectra data in Table S3,† the main emission groups were P-01: imidic acid; P-03: imine; P-03′: iminium; P-04′: tertiary ammonium; P-05′: triphenyl-ammonium; P-06′: triphenyl ammonium-iminium. The parts P-02: amide; P-04: tertiaryamine; P-05: triphenylamine; P-06: triphenylamine-imine gave no emissions. The theory calculation data in Table S3† give the emission data of the fluorescence emission groups of PTS-G0 in Fig. S15† distributed by pH ranges. The imidic acid and ammonium groups were the fluorescence emission groups at high pH values. The imine was the fluorescence emission group at low pH values.

Analysis of possible mechanisms

PTS-G0 and ETS have several fluorescence groups. The possible pH related fluorescence emission ranges are given in Fig. 12. The PTS-G0 main fluorescence centres were imidic acid and ammonium groups at 1.5 < pH < 6; the PTS-G0 main fluorescence centres were imidic acid salts at pH > 6. The imine gave fluorescence emission at pH < 6 and at 8 < pH < 12. Triphenylamine gave fluorescence emission at pH > 12. Fig. S16 and S17† indicate from theory calculations that the mechanisms in Scheme 3 are reasonable and may explain the pH dependent one-/two-photon fluorescence emission of PTS-G0.

Imine groups gave emission at pH > 6, and 8 < pH < 12. The end group imine of PTS-G0 connected with triphenylamine formed the fluorescence centre. The isomerization phenomena of imine made it fluoresce weakly at about pH = 8. In conditions that inhibit the imine isomerization, the imine can give stronger fluorescence.²¹ PTS-G0 and ETS both have imines and triphenylamine. In acid conditions, pH < 6, the ETS gave strong fluorescence emission, while in basic conditions, pH > 8, the ETS gave weak fluorescence. The H⁺ and OH⁻ acted on the imine of ETS, and inhibited the isomerization of imine, which made the ETS fluoresce at pH < 6 (Fig. 12). This may be explained by the imine isomerization inhibition mechanism shown in Scheme 3.

Imidic acid is present in PTS-G0. The mechanisms in Scheme 3 show the transformation of amide (and the amide resonance structure) and imidic acid under low pH or high pH conditions. The imidic acid formed by addition of H⁺ can enhance the fluorescence emission of PTS-G0 in acid conditions. The imidic acid rigid co-plane structure has C=N double bonds, a p–π conjugated structure, and the hydroxyl donor electronic group, which can enhance fluorescence in acid conditions. In basic conditions, the imidic acid forms salts, which also give fluorescence emission.

Tertiary ammonium can give fluorescence in acid conditions. The triphenylamine-imine may exhibit isomerization phenomena at some pH points, to consume the energy of the system and lower the fluorescence intensity. The triphenylamine–imine conjugated structures display intramolecular charge transfer (ICT) that can enhance nonlinear optic (two-photon) properties.

The HOMO and LUMO molecular orbits of PTS-G0 are listed in Fig. S16 (imidic acid structure) and S17 (amide structure).† In Fig. S16,† the HOMO ground state electronic clouds are gathered at triphenylamine, and the LUMO excitation electronic clouds are gathered mainly at imine, which shows that the imine connected with triphenylamine takes part in the fluorescence emission. In Fig. S17,† the HOMO ground state electronic clouds are gathered at the centre of PTS-G0, and the LUMO excitation electronic clouds are gathered close to imine and imidic acid parts. The HOMO and LUMO show that imidic acid takes part in the fluorescence emission.

This paper displays interesting data on the pH dependent fluorescence emission phenomena of PTS-G0 and ETS. The mechanisms might not be accurately determined by analysis of these data alone. Further experiments need to be designed to find the mechanism of the intrinsic fluorescence phenomena. The PAMAM dendrimers and their modified compounds will find wide applications through future research.

Conclusions

In conclusion, the pH related one/two photon fluorescence emission of PTS-G0 increased with increasing pH values. These phenomena were different to the pH related fluorescence emission phenomena of NH₂-terminated or HO-terminated PAMAM as reported previously. The pH related fluorescence of PTS-G0 increased in the range 1.5 < pH < 13 (in ethanol/water) and 6 < pH < 13 (in DMSO). The pH related fluorescence of ETS increased in the range 1 < pH < 6 (in ethanol/water) and 6 < pH < 13 (in DMSO). The pH related fluorescence emission of PTS-G0 and ETS showed similar behavior in the solvent DMSO, but different behavior in the ethanol/water mixed solvent. It can be determined that imidic acid (HO–C=N), imine (C=N), and tertiary ammonium groups took part in the fluorescence emission. The possible mechanisms explain these pH dependent fluorescence phenomena. These data on the PTS-G0 dendrimer and ETS can help to explain the fluorescence emission phenomena of PAMAM, and might help in the understanding of intrinsic fluorescence phenomena of dendrimers.

Acknowledgements

Thanks to Dr Zhiqiang Zhou, Prof. Changgui Lv, and Prof. Yiping Cui of the Advanced Photonics Center of Southeast University for the two-photon experiment.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: UV, fluorescence, ¹H NMR, IR, MS spectra, details of experimental syntheses and theoretical calculation data. See DOI: 10.1039/c5ra13046h

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