DOI: 10.1002/chem.201500044

Communication

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Restriction of Molecular Twisting on a Gold Nanoparticle Surface Tushar Debnath,[a] Jayanta Dana,[a] Partha Maity,[a] Hyacintha Lobo,[b] Ganapati S. Shankarling,*[b] and Hirendra N. Ghosh*[a] sometimes facilitates charge stabilization in the TICT state by maintaining a large twist angle between a donor and acceptor group.[11] Kim et al. have observed enhancement of TICT emission band for p-N,N-dimethylaminobenzoic acid (DMABA) in the zeolite moiety[12] and on the SiO2 nanoparticle surface,[13] which they have ascribed to be due to hydrogen bonding between dye molecule and zeolite surface or OH group of a SiO2 nanoparticle, respectively. In our earlier investigation,[14, 15] excited-state properties of the TICT molecule (7-diethylaminocoumarin 3-carboxylic acid, D-1421) has been reported on TiO2 and ZrO2 surface, where we have demonstrated that charge separation in D-1421 molecule facilitated on TiO2 surface. Earlier Nag and Bhattacharyya[16] reported enhanced emission from the TICT state of DMABN in an a-cyclodextrin cavity owing to complex formation. It is clear from earlier reports[12–16] that excited properties of TICT states of different probe molecules are well-studied in different kinds of micro-heterogeneous media, such as a zeolite moiety, cyclodextrin cavity, or semiconductor nanoparticle surface. At this juncture it will be interesting to elucidate the photophysical properties of the TICT molecule on metal nanoparticle surface. The interaction between molecular adsorbate and metal nanoparticles has attracted tremendous attention owing to wide applications in optical materials, biosensing, scanning probe microscopes,[17–22] and plasmonic solar cells.[23–25] Energy[26–30] and electron[31, 32] transfer from different organic dye molecules to Au NPs has been widely investigated. However to date no reports are available on the excited state properties of TICT molecule on metal nanoparticle surface. To elucidate the above-mentioned behavior, in the present investigation we have designed and synthesized a new coumarin molecule, C3 (Scheme 1; see the Experimental Section),

Abstract: To understand the photophysical properties of intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states on a gold nanoparticle (Au NP) surface, we have designed and synthesized a new coumarin molecule (C3) that exists both as ICT and TICT states in its excited state in a polar environment. On a Au NP surface, an excited C3 molecule only exists as an ICT state owing to restricted molecular rotation of a diethylamino group; as a result, no conversion from the ICT to TICT state was observed. Selection of the preferential state of a molecule with dual emitting states can be helpful for selected biological applications.

Photoinduced intramolecular charge transfer (ICT) within the donor–acceptor group of a molecular structure for some molecules, which arises from an initially photoexcited locally excited (LE) state, has been widely investigated in the last few decades to understand basic energy conversion for chemical and biological processes.[1–7] Photoinduced charge separation is maximized for such molecules by conformational change of the donor group in such a way that both donor and acceptor group are arranged in mutually perpendicular situation, which results the formation of a so-called twisted intramolecular charge transfer (TICT) state.[8] The polarity of a solvent has a pronounced effect on the formation of the TICT state. With an increase in polarity of the solvent, the formation energy barrier of TICT state decreases; as a result, TICT formation is facilitated in a highly polar solvent. Occasionally a red-shifted separate emission band from TICT state can appear along with ICT state in high polar solvent as detected by Rettig and coworkers for p-(N,N-dimethylamino) benzonitrile (DMABN) due to charge localization in donor–acceptor group.[9] A separate emission band that is due to the TICT state along with the ICT state was also observed by us in a coumarin 2 (C2) molecule in our earlier investigation.[10] Intermolecular hydrogen bonding

[a] T. Debnath, J. Dana, P. Maity, Prof. H. N. Ghosh Radiation and Photochemistry Division Bhabha Atomic Research Centre, Mumbai 400085 (India) E-mail: [email protected]

Scheme 1. Synthetic routes to the coumarin 3 sensitizer.

[b] Dr. H. Lobo, Prof. G. S. Shankarling Department of Dyestuff Technology Institute of Chemical Technology, Mumbai (India) E-mail: [email protected]

which can exhibit both ICT and TICT states in its excited state in polar solvent. Au NPs were synthesized in water after following a modified Turkevich method.[33, 34] Photophysical and excited-state properties of the C3 molecule on the Au NP surface

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500044. Chem. Eur. J. 2015, 21, 1 – 6

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Communication by Rettig and co-workers.[9] In our earlier investigation in the case of coumarin 2 in water[10] and 7-diethylaminocoumarin 3 carboxylic acid (D-1421) on a ZrO2 nanoparticle surface,[14] we have also observed a dual emission band where the emission band in the blue region was attributed owing to the ICT state and in the red region owing to the TICT state. Similarly in the present investigation the molecular structure of C3 is such that it has a free diethylamino group that can easily rotate in the excited state in polar solvent to give a twisted structure (Scheme 1). Rotation of a free amino substituent results in different excited state conformers, such as near-planar and nearperpendicular twisted conformers that give ICT and TICT states, respectively, on photoexcitation. Thus in the present investigation, the separate emission band at about 595 nm that was observed in water can be ascribed to the TICT emission. To confirm the second red-shifted emission was due to the TICT state, pH-dependent emission measurements of C3 in water was carried out. Interestingly the TICT emission completely vanishes owing to protonation of the diethylamino group at lower pH (Supporting Information, Figure S3). The Stokes shift for C3 in cyclohexane is much less compared to that in water (Figure 1 A), which confirms C3 exclusively exists in the LE state in cyclohexane whereas in a polar solvent it exists as both ICT and TICT states (Supporting Information, Table S1). To monitor the involvement of different charge-transfer state of C3, timeresolved emission studies were also carried out in both cyclohexane and in water, monitoring at different wavelengths (Figure 1 B). The emission kinetic trace at 420 nm in cyclohexane (e) can be fitted single exponentially with time constant of 2.8 ns. The emission decay trace at 485 nm in water (f) can be fitted bi-exponentially with time constants of t1 = 190 ps (96 %) and t2 = 2.9 ns (4 %) with tav = 290 ps. However, emission kinetics at 595 nm in water can be fitted single-exponentially with time constants of 2.9 ns. Thus it is clear that in a high-polarity solvent such as water, most of the emission decays with pulsewidth-limited time constants while the emission is monitored at the ICT position. However at the TICT position (595 nm), the emission lifetime increases drastically. In our earlier investigation,[14] a similar fast non-exponential decay was observed in water for D-1421 while monitoring the emission at ICT position. This fast decay of emission at the ICT position was attributed to fast relaxation to the TICT state from the LE/ICT state through non-radiative relaxation and was expected to be barrierless.[9] Similarly in the present investigation, we observed bi-exponential time constants where the shorter component (190 ps) can be attributed to a fast non-radiative relaxation time from ICT to TICT state and the longer component (2.9 ns) can be attributed to the ICT emission lifetime. Interestingly at the TICT position, a longer emission lifetime was detected. This one of the very few examples where the probe molecule (C3) has a very clear and separated emission band for both ICT and TICT emission states. The main intention of the present investigation is to monitor the excited-state properties of the C3 molecule on a Au NP surface. To demonstrate excited-state dynamics on a NP surface, it is important to monitor ground-state interaction between the C3 and a Au NP. Figure 2 A shows optical absorption spectra of

have been investigated through steady-state optical absorption and steady-state and time-resolved emission spectroscopy. The main aim of the present investigation is to study excited-state optical properties of C3 on a Au NP surface. Previously, we monitored photophysical properties of the pure C3 molecule in both the ground and excited states. The basic structure of the coumarin 3 (C3) molecule suggests that C3 can potentially be a molecule that can exist as a TICT state in a high-polarity solvent. To monitor the solvatochromic behavior of C3, we carried out steady-state absorption and emission studies in different solvents with changing polarity (Figure 1 A and Sup-

Figure 1. A) Absorption (a, b) and emission spectra (c, d) of coumarin 3 (C3) in cyclohexane and in water, respectively. B) Time-resolved emission decay traces of C3 at e) 420 nm in cyclohexane and f) 485 nm in water, and g) 595 nm in water after exciting at 374 nm. L stands for the lamp profile of the 374 nm laser excitation source.

porting Information). Figure 1 A shows optical absorption and emission spectra of C3 in cyclohexane and in water. Figure 1 shows the absorption maxima at 404 nm in cyclohexane (a) and at 427 nm in water (b). Optical absorption spectra of C3 in all of the solvents consists of two bands: one in the UV region and another in the visible region (Supporting Information, Figure S1). The emission spectra of C3 in cyclohexane (c) display an emission band having a maximum at 420 nm. The emission spectra of C3 in water (d) contains two distinct emission bands, one at about 486 nm and another at about 595 nm. The emission band at 420 nm in cyclohexane can be attributed to the emission owing to a locally excited (LE) state. It is wellknown that 7-aminocoumarin molecules exist in an intramolecular charge-transfer (ICT) state in their excited states in highpolarity solvent,[10, 14, 15] so the band at 486 nm can be attributed to the emission band due to the ICT state. We have shown (Supporting Information) that with increasing polarity, the emission band becomes broader and red-shifted; in solvents such as acetonitrile and ethyl acetate, along with the ICT band another red-shifted hump appears in the red region of the spectrum (Supporting Information, Figure S2). However in water, two distinct emission peaks are observed for the C3 molecule. The presence of a separate red-shifted TICT band was observed from dialkylamino benzonitriles and benzoesters &

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Communication state on a Au NP surface. Recently we have reported a similar kind of observation for the D-1421 molecule where emission intensity of ICT state increases drastically on a Au NP surface owing to restricted molecular rotation.[42] Now to understand the excited-state dynamics of C3 molecule on the Au NP surface, we have carried out time-resolved emission studies of the C3 molecule after changing the Au NP concentration and monitoring the emission at 485 nm (Figure 2 C) and 595 nm (Supporting Information). It is interesting to see that with increasing Au NP concentration, the contribution of the shorter component decreases and simultaneously the contribution of the longer component increases at 485 nm (Table 1). However at 595 nm (TICT state), there is not much change of emission lifeFigure 2. Steady-state A) optical absorption, B) emission spectra of C3, and C) time-resolved emission decay traces of C3 at 485 nm after 374 nm excitation in the absence and presence of different gold nanoparticle concentrations (a, a’, a’’) 0.0 nm Au NP, (b, b’, b’’) 2.5 nm Au NP, (c, c’, c’’) 5.0 nm Au NP, and (d, d’, d’’) 10.0 nm Au NP in 70 mm C3 dye in water. Trace e in (A): Optical absorption spectrum of Au NPs. In (C), L stands for the lamp profile of the 374 nm laser excitation source.

Table 1. Multi-exponential lifetimes of the C3 molecule for different concentrations of a Au NP surface after 374 nm laser excitation.

the C3 molecule with changing Au NP concentration. Concentrations of Au NP used are 0 nm (a), 2.5 nm (b), 5 nm (c), and 10 nm (d). Optical absorption spectra of 10 nm pure Au NP (e) shows a plasmon band at 518 nm. It is interesting to see that for a composite mixture of C3 and Au NP, the signals become broad and in particular the plasmon band of Au NP becomes broad and red-shifted. Wang and co-workers[38] have reported that the plasmonic absorption of gold nanorods becomes broad and red-shifted in the presence of a dye molecule owing to the strong coupling nature of the dye molecule. Such a strong coupling nature of a dye is found to depend on spectral overlap of dye and plasmonic resonance.[39–41] Although there is no significant spectral overlap between optical absorption of C3 and plasmon absorption of the Au NP, the plasmon band was still found to become broad and red-shifted. This observation clearly suggests that there might be strong molecular interaction of C3 on Au NP where the nitrogen atom of the diethylamino group plays an important role. TEM images (Supporting Information, Figure S2) of Au NP suggest that no aggregation is formed. Thus a red-shifted and broad optical absorption band cannot be attributed to the aggregate formation of Au NPs. Ultrafast transient absorption (TA) studies on the above systems also reveal that C3/Au NP composite materials have a strong plasmon–molecular-resonance interaction. Ultrafast plasmon–molecular-resonance interaction has been demonstrated through femtosecond transient absorption spectroscopy and is described in the Supporting Information. To monitor the excited state properties of C3 on the Au NP surface, we carried out emission spectroscopy of C3 with changing Au NP concentration (Figure 2 B). It can be clearly seen that with increasing Au NP concentration, emission owing to the TICT state decreases and simultanously emission owing to the ICT state increases. Emission owing to the TICT state completely vanishes with increasing ICT emission at higher Au NP concentrations. This is a remarkable observation of a change of conformation of the C3 molecule in the excited Chem. Eur. J. 2015, 21, 1 – 6

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System/lprobe

Lifetime

C3/485 nm C3/Au NP/2.5 nm/470 nm C3/Au NP/5.0 nm/470 nm C3/Au NP/10.0 nm/470 nm

t1 = 190 ps t1 = 190 ps t1 = 180 ps t1 = 200 ps

(96 %), (93 %), (88 %), (55 %),

t2 = 2.8 ns (4 %) t2 = 3 ns (7 %) t2 = 3.1 ns (12 %) t2 = 3.1 ns (45 %)

[a] The monitoring wavelength has been incorporated as lprobe.

time up to a Au NP concentration of 5 nm (Supporting Information, Figure S6). Thus from both steady-state and time-resolved emission studies, we can conclude that emission of the photoexcited C3 molecule on the Au NP surface is purely due to ICT states. Absence of the TICT state on the Au NP surface clearly suggests that relaxation from the ICT state to the TICT state does not take place. In steady-state absorption studies it has been observed that C3 and Au NP interact quite strongly. The interaction with Au NP takes place through the nitrogen atom owing to its affinity to the Au surface (Scheme 2). From the above experimental results it is clear that on the Au NP surface, rotation of diethylamino group in the excited state is restricted, and as a result relaxation from the ICT state to the TICT state is completely blocked (Scheme 2). It is well-established

Scheme 2. Diagram showing that the C3 molecule is unable to form a TICT state on a Au NP surface.

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Communication 128.8, 123.7, 117.8, 109.5, 108.8, 97.0, 45.1, 12.6; MS: m/z 422 (M Cl,OH).

that in dye–Au NP composite materials, on photoexcitation, energy transfer can take place from photoexcited dye to the Au NP, where the emission band of the dye molecule overlap with the plasmon band of the Au NP. However, in the present investigation no energy transfer process is found to take place in the C3/Au NP composite material, which is due to very negligible overlap between the ICT emission band or the TICT emission band of C3 and the Au plasmon band (Supporting Information, Figure S5). In summary, we have synthesized and characterized a new coumarin dye molecule (C3), which exhibits both an ICT and a TICT state in the excited state in high-polarity solvents, while in low-polarity solvents it exclusively exists as a LE state, where synthesis of a dual emitting (ICT and TICT emission) molecule is always a challenging task. To understand the optical and excited-state properties of such a coumarin dye on a Au NP surface, we carried out steady-state and time-resolved luminescence spectroscopic measurement of C3/Au NP composite materials. To our surprise it was observed that the TICT emission band of C3 completely vanishes on the Au NP surface. This observation suggests that molecular twisting of diethylamino group of C3 on the Au NP surface is completely restricted; as a result, relaxation from the ICT to the TICT state is absolutely forbidden. To the best of our knowledge, for the first time we report the restriction of amino rotation in the excited state of a molecular probe on a Au NP surface. Our fundamental observation on photophysical properties of such a dual emitting molecule on a Au NP surface will be helpful for selected biological applications.

Synthetic procedures for the other intermediates are described in detail in the Supporting Information.

Acknowledgements T.D. and J.D. acknowledge CSIR, and P.M. acknowledges DAE for research fellowship. We also acknowledge Dr. D. K. Palit and Dr. B. N. Jagatap for their encouragement. This work was supported by “DAE-SRC Outstanding Research Investigator Award” (Project/Scheme No.: DAE-SRC/2012/21/13-BRNS) granted to Dr. H. N. Ghosh. Keywords: coumarin dye · dual emission · gold nanoparticles · ICT and TICT states · molecular rotation [1] W. Rettig, Angew. Chem. Int. Ed. Engl. 1986, 25, 971 – 988; Angew. Chem. 1986, 98, 969 – 986. [2] W. Rettig, M. Gleiter, J. Phys. Chem. 1985, 89, 4676 – 4680. [3] S. A. Jenekhe, L. Lu, M. M. Alam, Macromolecules 2001, 34, 7315 – 7324. [4] Z. R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 2003, 103, 3899 – 4031. [5] Z. Guo, S. Park, J. Yoon, I. Shin, Chem. Soc. Rev. 2014, 43, 16 – 29. [6] Z. H. Guo, Z. X. Jin, J. Y. Wang, J. Pei, Chem. Commun. 2014, 50, 6088 – 6090. [7] E. Wang, J. W. Y. Lam, R. Hu, C. Zhang, Y. S. Zhaoc, B. Z. Tang, J. Mater. Chem. C 2014, 2, 1801 – 1807. [8] K.-F. Chen, C.-W. Chang, J.-L. Lin, Y.-C. Hsu, M.-C. P. Yeh, C.-P. Hsu, S.-S. Sun, Chem. Eur. J. 2010, 16, 12873 – 12882. [9] W. Rettig, G. Wermuth, J. Photochem. 1985, 28, 351 – 366. [10] T. Debnath, P. Maity, H. Lobo, B. Singh, G. S. Shankarling, H. N. Ghosh, Chem. Eur. J. 2014, 20, 3510 – 3519. [11] N. Dash, F. A. S. Chipem, R. Swaminathan, G. Krishnamoorthy, Chem. Phys. Lett. 2008, 460, 119 – 124. [12] Y. Kim, B. I. Lee, M. Yoon, Chem. Phys. Lett. 1998, 286, 466 – 472. [13] Y. Kim, H. W. Cheon, M. Yoon, N. W. Song, D. Kim, Chem. Phys. Lett. 1997, 264, 673 – 679. [14] G. Ramakrishna, H. N. Ghosh, J. Phys. Chem. A 2002, 106, 2545 – 2553. [15] S. Verma, H. N. Ghosh, J. Phys. Chem. C. 2014, 118, 10661 – 10669. [16] A. Nag, K. Bhattacharya, Chem. Phys. Lett. 1988, 151, 474 – 476. [17] K. E. Sapsford, L. Berti, I. L. Medintz, Angew. Chem. Int. Ed. 2006, 45, 4562 – 4588; Angew. Chem. 2006, 118, 4676 – 4704. [18] P. C. Ray, G. K. Darbha, A. Ray, J. Walker, W. Hardy, Plasmonics 2007, 2, 173 – 183. [19] M.-C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293 – 346. [20] M. De, P. S. Ghosh, V. M. Rotello, Adv. Mater. 2008, 20, 4225 – 4241. [21] J. N. Anker, W. P. Hall, O. N. Lyandres, C. Shah, J. Zhao, R. P. Van Duyne, Nat. Mater. 2008, 7, 442 – 453. [22] N. Fang, H. Lee, C. Sun, X. Zhang, Science 2005, 308, 534 – 537. [23] H. A. Atwater, A. Polman, Nat. Mater. 2010, 9, 205 – 213. [24] V. E. Ferry, L. A. Sweatlock, D. Pacifici, H. A. Atwater, Nano Lett. 2008, 8, 4391 – 4397. [25] A. P. Kulkarni, K. M. Noone, K. Munechika, S. R. Guyer, D. S. Ginger, Nano Lett. 2010, 10, 1501 – 1505. [26] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer, NewYork, 2006. [27] E. Dulkeith, A. C. Morteani, T. Niedereichholz, T. A. Klar, J. Feldmann, S. A. Levi, F. C. J. M. van Veggel, D. N. Reinhoudt, M. Mçller, D. I. Gittins, Phys. Rev. Lett. 2002, 89, 203002. [28] E. Dulkeith, M. Ringler, T. A. Klar, J. Feldmann, A. MuÇoz Javier, W. J. Parak, Nano Lett. 2005, 5, 585 – 589. [29] C. H. Fan, S. Wang, J. W. Hong, G. C. Bazan, K. W. Plaxco, A. J. Heeger, Proc. Natl. Acad. Sci. USA 2003, 100, 6297 – 6301. [30] T. Sen, A. Patra, J. Phys. Chem. C 2012, 116, 17307 – 17317. [31] K. G. Thomas, P. V. Kamat, Acc. Chem. Res. 2003, 36, 888 – 898.

Experimental Section Coumarin 3 was synthesized by condensation of a formylcoumarin compound 4 with 2-cyano-3-(p-tolyl) acrylic acid 5 in ethanol using piperidine (Scheme 1). The formylcoumarin compound 4 was prepared in two steps starting from DEMAP aldehyde 1 and ethyl cyanoacetate 2 in ethanol using piperidine to obtain acetyl coumarin intermediate 3.[35, 36] This was followed by reaction with DMF/POCl3 that introduces a chloro group in conjugation with the aldehyde group 4.[10] The chromophore and intermediates were characterized by 1H NMR, 13C NMR, mass spectrometry. In a three-necked 100 mL round-bottomed flask, 3-chloro-3-(7-(diethyl amino)-2-oxo2H-chromen-3-yl)acrylaldehyde 4 (1 g, 3.2 mmol) was taken in absolute ethanol (10 mL, 10 vol). This was followed by the addition of 2-cyano-3-(p-tolyl) acrylic acid[37] 5 (0.76 g, 4.0 mmol) and piperidine (3–4 drops) and the reaction mixture was vigorously stirred at reflux temperature for 4 h. The progress of the reaction was monitored by TLC. After completion of the reaction, the reaction mass was added to cold water and product was extracted using ethyl acetate. The ethyl acetate layer was washed with water and then subjected to evaporation under vacuum using rotary evaporator to obtain the product. The crude product was further purified by silica gel column chromatography using a toluene/ethyl acetate system (6:4) as eluent. Yield = 0.91 g (59 %); 1H NMR (CDCl3, 300 MHz): d (ppm) 8.30 (s, 1 H, CH); 7.70 (s, 1 H, CH); 7.32–7.20 (m, 4 H, CH); 6.75–6.68 (m, 3 H, CH); 6.50 (d, 1 H, CH); 5.92–5.90 (m, 2 H, CH); 3.52–3.40 (m, 4 H, CH2); 1.26–1.20 (t, 6 H, CH3); 13C NMR (CDCl3, 300 MHz): d (ppm) 161.1, 156.8, 151.5, 146.8, 142.7, 142.3, 129.7,

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Received: January 6, 2015 Published online on && &&, 0000

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Communication

COMMUNICATION & Molecular Rotation

TICT off: The coumarin 3 (C3) molecule is found to have dual emitting intramolecular charge transfer (ICT) and twisted ICT (TICT) states. Rotation of the free 7amino substituent in the excited state generates the twisted structure, which gives the TICT state. Au NPs restrict the rotation of the 7-amino substituent of C3; dual emitting C3 on the Au NP surface gives emission that is only due to the ICT state.

T. Debnath, J. Dana, P. Maity, H. Lobo, G. S. Shankarling,* H. N. Ghosh* && – && Restriction of Molecular Twisting on a Gold Nanoparticle Surface

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Restriction of molecular twisting on a gold nanoparticle surface.

To understand the photophysical properties of intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states on a gold ...
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