Path-selective photoinduced electron transfer (PET) in a membrane-associated system studied by pH-dependent fluorescence

Inorganica Chimica Acta 381 (2012) 243–246

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Path-selective photoinduced electron transfer (PET) in a membrane-associated system studied by pH-dependent fluorescence Jialong Liu, A. Prasanna de Silva ⇑ School of Chemistry and Chemical Engineering, Queen’s University, Belfast BT9 5AG, Northern Ireland, United Kingdom

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Article history: Available online 18 October 2011 Fluorescence Spectroscopy: from Single Chemosensors to Nanoparticles Science – Special Issue Keywords: Fluorescent sensors Photoinduced electron transfer pH sensors Path-selectivity Photosynthetic reaction center

a b s t r a c t The pH-dependent fluorescence behavior of two regioisomeric ‘receptor1–spacer1–fluorophore–spacer2– receptor2’ systems 1 and 2 in micellar solutions of sodium dodecyl sulfate show that photoinduced electron transfer (PET) only occurs from the amine group connected to the 4-amino position of the aminonaphthalimide fluorophore in both cases. This demonstrates the directing influence of the photogenerated electric field within the aminonaphthalimide excited state on the electron transfer process. Since path-selectivity of PET is also known within the membrane-bound photosynthetic reaction center in bacteria, its origins may be illuminated by the simple experiments described here. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction When combined with suitable molecular sensors and switches [1–6], fluorescence spectroscopy [7–9] becomes a powerful tool for medical/environmental diagnostics [10,11], intracellular monitoring [11,12] and molecular information processing [13–22]. However, the same combination can be fruitful in examining aspects of the photosynthesis process. We now show how a membrane -associated fluorescent molecular switch system can emulate the path-selective photoinduced electron transfer occurring in the membrane-bound photosynthetic reaction center (PRC). The resolution of the X-ray structure of the bacterial PRC arose following the hitherto unknown crystallization of a membranebound protein complex [23]. Fast laser experiments on the PRC showed that the photoinduced electron transfer chose one of two nearly identical paths [24]. Since several explanations have been offered for this intriguing phenomenon [25], it is important to demonstrate similar effects in structurally and mechanistically simpler systems where these explanations can be tested. The fluorescent PET [26–28] sensor/switch system [1,29–36] arranges competition between PET and fluorescence in rather small molecules on their own or in association with larger structures [37,38]. Conse-

⇑ Corresponding author. Tel.: +44 2890 974422; fax: +44 2890 974890. E-mail address: [email protected] (A.P. de Silva). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.09.059

quently, the mere observation of fluorescence as a function of a chemical stimulus can shed light on PET processes and their idiosyncracies. Cation- [39,40] and anion-driven [41] cases of this kind are known but these do not involve membrane media. Membraneassociated fluorescent PET systems are available [42–45] but these are not suitable for testing the path-selectivity phenomenon. In order to test path-selectivity, the molecule needs to possess a minimum of two paths for PET. This can be arranged by building ‘receptor1–spacer1–fluorophore–spacer2–receptor2’ systems, which are higher-generation versions of the basic ‘fluorophore–spacer–receptor’ format [46–48]. Another requirement for path-selectivity is a directing influence on PET within the molecule. Electrostatic influences, albeit existing for nanoseconds, can be arranged by choosing a fluorophore possessing an internal charge transfer (ICT) excited state [49]. 4-Aminonaphthalimides are particularly suitable for this purpose [50–59]. Amine receptors are thermodynamically able to transfer an electron to a photoexcited 4-aminonaphthalimide [60], with/without kinetic barriers arising from electrostatics or nodes in frontier orbitals [61,62]. This is the rationale for choosing the regioisomers 1 and 2 for our study. In particular, dimethylaminopropyl and morpholinoethyl side-chains are chosen because the basicities of the amine units are so different as to be easily distinguishable, while preserving ease of synthesis of the compounds. While these PET switches 1 and 2 are not very hydrophobic, their mono-/di-protonated forms will be electrostatically associated with anionic micelles of the detergent, sodium dodecyl sulfate (SDS) in water. Hence we have a membraneassociated system carrying several essential features of the PRC.

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2. Materials and methods Compounds 1 and 2 were synthesized and characterized as described in the experimental section. UV–Vis absorption and fluorescence emission spectra were recorded on Perkin-Elmer Lambda 9 and Perkin-Elmer LS-55 spectrometers, respectively. Nuclear magnetic resonance spectra were obtained with a Bruker AC250 spectrometer. Mass spectra were obtained with a VG MS902 instrument. pH measurements employed a Jenway 3310 pH meter with a glass electrode. 3. Experimental 3.1. N-(20 -Morpholinoethyl)-4-[300 (dimethylamino)propylamino]naphthalene-1,8-dicarboximide (1) A mixture of N-(20 -morpholinoethyl)-4-chloronaphthalene-1,8dicarboximide [63] (1.10 g, 3.2 mmol) and 3-(dimethylamino)propylamine (4.0 mL, 32 mmol) was heated at 100 °C for 4 h. The resulting mixture was evaporated to dryness under reduced pressure (0.1 mm Hg). The red solid was crystallized from ether–hexane. Yield 31%, m.p. 142–143 °C. 1H NMR (CDCl3); d 1.97 (m, 2H, NCH2CH2CH2N), 2.42 (s, 6H, CH3N), 2.64 (m, 6H, NCH2CH2O and NCH2CH2CH2NH), 2.71 (t, 2H, CH2N(CH2CH2)2O), 3.48 (t, 2H, NCH2CH2CH2NH), 3.71 (t, 4H, NCH2CH2O), 4.34 (t, 2H, CH2CH2N(CH2CH2)2O), 7.60-8.61 (m, 5H, ArH), 6.60 (m, 1H, NH); m/z; 412 (M+H)+. The title compound C23H30N4O3 requires 411. 3.2. N-(300 -(Dimethylamino)propyl)-4-[20 morpholinoethylamino]naphthalene-1,8-dicarboximide (2) 3-(Dimethylamino)propylamine (1.57 mL, 12.5 mmol) was added to a suspension of 4-chloronaphthalene-1,8-dicarboxylic acid anhydride (2.91 g, 12.5 mmol) in toluene (30 mL). The mixture was then refluxed for 4 h, after which the solvent was evaporated under vacuum. The residue was then dissolved in hydrochloric acid (1.0 M), washed with dichloromethane, basified with sodium carbonate (1.0 M) and extracted with dichloromethane. The solvent was evaporated under vacuum and the initial product crystallized from ethanol. A portion of the initial product (1.02 g, 3.2 mmol) and 2-morpholinoethylamine (3.3 mL, 32 mmol) was subjected to the procedure used for the preparation of 1. This gave a red solid which was crystallized from ether. Yield 37%, m.p. 136–137 °C. 1 H NMR (CDCl3); d 1.93 (m, 2H, NCH2CH2CH2N), 2.27 (s, 6H, CH3N), 2.44 (t, 2H, CH2N(CH2CH2)2O), 2.60 (brs, 4H, NCH2CH2O), 2.86 (t, 2H, CH2N(CH3)2), 3.45 (t, 2H, NCH2CH2NH), 3.81 (t, 4H,

0.7 0.6 0.5

φF

0.4 0.3 0.2 0.1 0

2

4

6

8

10

12

pH Fig. 1. Fluorescence quantum yields of 1 (diamonds) and 2 (squares) as a function of pH in 1:4 methanol:water (v/v).

NCH2CH2O), 4.21 (t, 2H, NCH2CH2CH2NCH3), 6.65-8.60 (m, 5H, ArH), 6.70 (m, 1H, NH).; m/z; 412 (M+H)+. The title compound C23H30N4O3 requires 411. 4. Results and discussion Starting with the data in detergent-free solution, the results for 1 in Fig. 1 can be dissected as follows. As we move from pH 11.5 towards lower values, a fluorescence enhancement (FE) factor of 3.0 is seen as neutral pH values are approached. This variation can be analyzed according to Eq. (1) [64], to produce a pKa value of 9.0. This relatively high value is characteristic of the dimethylaminopropyl side-chain. Thus, protonation of the dimethylamino moiety prevents it from transferring an electron to the excited fluorophore [60]. The relatively low FE value reflects the trimethylene spacer. Shorter, e.g. dimethylene, spacers result in FE values as high as 25 [60]. As we proceed beyond neutral pH values towards the acidic region, a modest drop in fluorescence quantum yield (FE = 0.83) can be seen. Analysis of this change with an appropriate version of Eq. (1) gives rise to a pKa value of 5.7. This relatively low value can be assigned to the morpholinoethyl side-chain with lower electron density on the amine unit. In this instance, protonation of the amine within the morpholine heterocycle allows hydrogenbonding to the carbonyl oxygen to form a 7-membered ring [65]. The electronic energy of the excited fluorophore can thus be converted to vibrational energy [66], via a Born-Oppenheimer hole [67,68], which can be passed onto solvent molecules. It is therefore clear that PET occurs to the fluorophore of 1 only from the dimethylaminopropyl side-chain. The thermodynamics for PET to occur from the morpholinoethyl side-chain is only slightly less favoured [60], but it does not take place kinetically in the present instance.

log ½ðIFmax  IF Þ=ðIF  IFmin Þ ¼ pH  pK a

ð1Þ

Examination of the results for the regioisomer 2 in Fig. 1 provides a nice contrast with the paragraph above. Only a small decrease of fluorescence (FE = 0.89) can be observed as we move from pH 10.8 to 8.9. A pKa value of 9.5 can be extracted from this phenomenon, and can be assigned to the dimethylaminopropyl side-chain. The protonation of the dimethylamino unit leads to a N–H. . .O hydrogen-bond contained in a 8-membered ring. Only a weak Born-Oppenheimer hole emerges owing to the less-favoured larger ring, hence a near-unity FE value is found. Traversing the pH region of 4–8 shows a large H+-induced increase of fluorescence (FE = 8.5), centered at a pKa value of 5.7. Clearly, the morpholinoethyl side-chain can be held responsible. Protonation of the amine within the morpholine ring prevents a fast PET process, occurring across the dimethylene spacer. So we see that PET occurs to the fluorophore of 2 only from the morpholinoethyl side-chain, in spite of its slight thermodynamic handicap when compared to the dimethylamino unit on the other side. We can infer from the two previous paragraphs that only one of two possible, and thermodynamically allowed, PET paths to the fluorophore is used in regioisomers 1 and 2. Neither the nature of the electron donor amine nor the length of the intervening spacer determines the chosen path. The electron enters the fluorophore from the 4-amino side in each case. Understanding of this path-selectivity has two origins. Firstly, the excited state of the 4-aminonaphthalimide fluorophore has substantial internal charge transfer (ICT) character with electron deficiency near the 4-amino unit and electron excess at the carbonyl oxygens. This photogenerated electric field forms a kinetic barrier to electron entry to the fluorophore from the imide side, while facilitating electron entry from the 4-amino side [60]. Secondly, the imide nitrogen is a node in both frontier orbitals of 4-aminonaphthalimide. Hence, an electron arriving from the imide side has to traverse an extra atom

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0.9

Table 1 Various parameters for 1 and 2 in two media, determined mainly via pH-dependent fluorescence spectroscopy.a

0.8 0.7

Solvent

1:4 Methanol: water (v/v)

1 in micellar medium 2  104 M SDS in water

2 in micellar medium 2  104 M SDS in water

5.7 9.5 8.5 0.89 432d 540 0.61

7.8 10.7 0.81 2.5 442 535 0.56

7.9 10.8 5.0 0.94 431 530 0.67

1

2

1:4 Methanol: water (v/v) 5.7 9.0 0.83 3.0 444c 548 0.40

0.6

φF

0.5 0.4 0.3

pKa1 pKa2 FE1 FE2 kAbs (nm)b kFlu (nm)b

0.2 0.1 0

2

4

6

8

10

12

uFlub

pH a

Fig. 2. Fluorescence quantum yields of 1 (diamonds) and 2 (squares) as a function of pH in water containing 2  104 M SDS.

b c d

before reaching the fluorophore p-electron system [61]. Adding an extra atom to a spacer chain will decrease PET rates by at least an order of magnitude in this situation [69–71]. While the path-selectivity of PET in 1 and 2 is intrinsic to these 4-aminonaphthalimide-based systems, it is important to see if this behavior can be preserved in membrane systems. Gratifyingly, the results in Fig. 2 largely mirror what was seen in Fig. 1. Significant perturbations are notable, however. In the case of 1, the notable H+-induced fluorescence enhancement (FE = 2.5) is associated with a pKa value of 10.7 which can be referenced to the suppression of PET from the dimethylaminopropyl side-chain. SDS micelles have induced a pKa shift (DpKa) of +1.7 and a 17% reduction in FE, besides a general increase in fluorescence quantum yields across the pH range examined. The smaller H+-induced fluorescence variation (FE = 0.81) is centered at a pKa value of 7.8 and arises from the Born-Oppenheimer hole created by intramolecular hydrogen bonding between the carbonyl oxygen and the protonated amine of the morpholine unit. In this instance, SDS micelles cause a (DpKa) of +2.1 and a 2% reduction in FE. The PET process in 2 in SDS shows up in the substantial H+-induced FE value of 5.0 with a corresponding pKa value of 7.9 which is assignable to the morpholinoethyl side-chain. A DpKa value of +2.2 and a 41% reduction in FE is caused by SDS micelles. Again, a general rise in fluorescence quantum yields across the pH range is seen. The phenomenon related to the Born-Oppenheimer hole (FE = 0.94) occurs at the pKa value of 10.8, which can be associated with the protonation of the dimethylamino unit. SDS causes a DpKa value of +1.3 and a 50% increase in FE in this case, though the latter increase has a large uncertainty due to the weakness of the phenomenon. Anionic SDS micelles associate with, and stabilize, the monoand di-protonated forms of 1 and 2. Hence, we find substantially positive DpKa values [72,73,42]. The usual reduction in FE values which are observed can be attributed to the drop in polarity in the immediate environment of 1 and 2 when they are associated with micelles. When compared with 1:4 methanol: water, the regions near the micelle surface are less polar [73]. Indeed, this can be seen in the SDS-induced blueshifts in the fluorescence spectra of 1 (DkFlu = 13 nm) and 2 (DkFlu = 10 nm) (Table 1) which are typical effects of lower polarity on ICT fluorophores [1,4,7,8]. PET processes in neutral compounds like 1 and 2 will separate charge and therefore be retarded in less polar environments [74,75]. To complete the observations concerning Fig. 2, we note the beginning of quantum yield loss at extreme acidities. The SDS micelles electrostatically concentrate H+ which begins to protonate the ICT excited state of the aminonapthalimide fluorophore at the carbonyl oxygen in order to quench it [76].

105 M 1 or 2. Fluorescence emission spectra are obtained by excitation at kAbs. At pH 3. log e = 4.38 (e in M1 cm1). log e = 4.44 (e in M1 cm1).

To conclude, the simple experiment of studying the pH-dependent fluorescence of two carefully chosen regioisomers 1 and 2 of the ‘receptor1–spacer1–fluorophore–spacer2–receptor2’ format in SDS solution demonstrates the phenomenon of path-selective PET in a membrane-associated system.

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