This paper is published as part of a PCCP Themed Issue on: Physical Chemistry of Ionic Liquids

Guest Editor: Frank Endres (Technical University of Clausthal, Germany)

Editorial

Physical chemistry of ionic liquids Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/c001176m

Perspectives

Ionicity in ionic liquids: correlation with ionic structure and physicochemical properties Kazuhide Ueno, Hiroyuki Tokuda and Masayoshi Watanabe, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921462n

Design of functional ionic liquids using magneto- and luminescent-active anions Yukihiro Yoshida and Gunzi Saito, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920046k

Accelerating the discovery of biocompatible ionic liquids Nicola Wood and Gill Stephens, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b923429b

Ionic liquids and reactions at the electrochemical interface Douglas R. MacFarlane, Jennifer M. Pringle, Patrick C. Howlett and Maria Forsyth, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b923053j

Photochemical processes in ionic liquids on ultrafast timescales Chandrasekhar Nese and Andreas-Neil Unterreiner, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b916799b

At the interface: solvation and designing ionic liquids Robert Hayes, Gregory G. Warr and Rob Atkin, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920393a

Ionic liquids in surface electrochemistry Hongtao Liu, Yang Liu and Jinghong Li, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921469k

Discussion

Do solvation layers of ionic liquids influence electrochemical reactions? Frank Endres, Oliver Höfft, Natalia Borisenko, Luiz Henrique Gasparotto, Alexandra Prowald, Rihab Al-Salman, Timo Carstens, Rob Atkin, Andreas Bund and Sherif Zein El Abedin, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b923527m

Papers

Plasma electrochemistry in ionic liquids: deposition of copper nanoparticles M. Brettholle, O. Höfft, L. Klarhöfer, S. Mathes, W. Maus-Friedrichs, S. Zein El Abedin, S. Krischok, J. Janek and F. Endres, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b906567a Size control and immobilization of gold nanoparticles stabilized in an ionic liquid on glass substrates for plasmonic applications Tatsuya Kameyama, Yumi Ohno, Takashi Kurimoto, Ken-ichi Okazaki, Taro Uematsu, Susumu Kuwabata and Tsukasa Torimoto, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b914230d Electrostatic properties of liquid 1,3-dimethylimidazolium chloride: role of local polarization and effect of the bulk C. Krekeler, F. Dommert, J. Schmidt, Y. Y. Zhao, C. Holm, R. Berger and L. Delle Site, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b917803c

Selective removal of acetylenes from olefin mixtures through specific physicochemical interactions of ionic liquids with acetylenes Jung Min Lee, Jelliarko Palgunadi, Jin Hyung Kim, Srun Jung, Young-seop Choi, Minserk Cheong and Hoon Sik Kim, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b915989d Screening of pairs of ions dissolved in ionic liquids R. M. Lynden-Bell, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b916987c Double layer, diluent and anode effects upon the electrodeposition of aluminium from chloroaluminate based ionic liquids Andrew P. Abbott, Fulian Qiu, Hadi M. A. Abood, M. Rostom Ali and Karl S. Ryder, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b917351j A comparison of the cyclic voltammetry of the Sn/Sn(II) couple in the room temperature ionic liquids N-butyl-N-methylpyrrolidinium dicyanamide and N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide: solvent induced changes of electrode reaction mechanism Benjamin C. M. Martindale, Sarah E. Ward Jones and Richard G. Compton, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920217j Ionic liquids through the looking glass: theory mirrors experiment and provides further insight into aromatic substitution processes Shon Glyn Jones, Hon Man Yau, Erika Davies, James M. Hook, Tristan G. A. Youngs, Jason B. Harper and Anna K. Croft, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b919831h Nitrile-functionalized pyrrolidinium ionic liquids as solvents for cross-coupling reactions involving in situ generated nanoparticle catalyst reservoirs Yugang Cui, Ilaria Biondi, Manish Chaubey, Xue Yang, Zhaofu Fei, Rosario Scopelliti, Christian G. Hartinger, Yongdan Li, Cinzia Chiappe and Paul J. Dyson, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920025h Ionic liquid as plasticizer for europium(III)-doped luminescent poly(methyl methacrylate) films Kyra Lunstroot, Kris Driesen, Peter Nockemann, Lydie Viau, P. Hubert Mutin, André Vioux and Koen Binnemans, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920145a Ab initio study on SN2 reaction of methyl p-nitrobenzenesulfonate and chloride anion in [mmim][PF6] Seigo Hayaki, Kentaro Kido, Hirofumi Sato and Shigeyoshi Sakaki, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920190b Influence of imidazolium bis(trifluoromethylsulfonylimide)s on the rotation of spin probes comprising ionic and hydrogen bonding groups Veronika Strehmel, Hans Rexhausen and Peter Strauch, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920586a Thermo-solvatochromism in binary mixtures of water and ionic liquids: on the relative importance of solvophobic interactions Bruno M. Sato, Carolina G. de Oliveira, Clarissa T. Martins and Omar A. El Seoud, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921391k

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DView Online / Journal Homepage / Table of Contents for this issue

Patterns of protein unfolding and protein aggregation in ionic liquids Diana Constatinescu, Christian Herrmann and Hermann Weingärtner, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921037g High vacuum distillation of ionic liquids and separation of ionic liquid mixtures Alasdair W. Taylor, Kevin R. J. Lovelock, Alexey Deyko, Peter Licence and Robert G. Jones, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920931j Designer molecular probes for phosphonium ionic liquids Robert Byrne, Simon Coleman, Simon Gallagher and Dermot Diamond, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920580b States and migration of an excess electron in a pyridinium-based, room-temperature ionic liquid: an ab initio molecular dynamics simulation exploration Zhiping Wang, Liang Zhang, Robert I. Cukier and Yuxiang Bu, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921104g J-aggregation of ionic liquid solutions of meso-tetrakis(4-sulfonatophenyl)porphyrin Maroof Ali, Vinod Kumar, Sheila N. Baker, Gary A. Baker and Siddharth Pandey, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920500d Spontaneous product segregation from reactions in ionic liquids: application in Pd-catalyzed aliphatic alcohol oxidation Charlie Van Doorslaer, Yves Schellekens, Pascal Mertens, Koen Binnemans and Dirk De Vos, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920813p Electrostatic interactions in ionic liquids: the dangers of dipole and dielectric descriptions Mark N. Kobrak and Hualin Li, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920080k Insights into the surface composition and enrichment effects of ionic liquids and ionic liquid mixtures F. Maier, T. Cremer, C. Kolbeck, K. R. J. Lovelock, N. Paape, P. S. Schulz, P. Wasserscheid and H.-P. Steinrück, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920804f Ionic liquids and reactive azeotropes: the continuity of the aprotic and protic classes José N. Canongia Lopes and Luís Paulo N. Rebelo, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b922524m A COSMO-RS based guide to analyze/quantify the polarity of ionic liquids and their mixtures with organic cosolvents José Palomar, José S. Torrecilla, Jesús Lemus, Víctor R. Ferro and Francisco Rodríguez, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920651p Solid and liquid charge-transfer complex formation between 1-methylnaphthalene and 1-alkyl-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide ionic liquids Christopher Hardacre, John D. Holbrey, Claire L. Mullan, Mark Nieuwenhuyzen, Tristan G. A. Youngs, Daniel T. Bowron and Simon J. Teat, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921160h Blending ionic liquids: how physico-chemical properties changeF. Castiglione, G. Raos, G. Battista Appetecchi, M. Montanino, S. Passerini, M. Moreno, A. Famulari and A. Mele, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921816e

NMR spectroscopic studies of cellobiose solvation in EmimAc aimed to understand the dissolution mechanism of cellulose in ionic liquids Jinming Zhang, Hao Zhang, Jin Wu, Jun Zhang, Jiasong He and Junfeng Xiang, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920446f Electrochemical carboxylation of -chloroethylbenzene in ionic liquids compressed with carbon dioxide Yusuke Hiejima, Masahiro Hayashi, Akihiro Uda, Seiko Oya, Hiroyuki Kondo, Hisanori Senboku and Kenji Takahashi, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920413j A theoretical study of the copper(I)-catalyzed 1,3-dipolar cycloaddition reaction in dabco-based ionic liquids: the anion effect on regioselectivity Cinzia Chiappe, Benedetta Mennucci, Christian Silvio Pomelli, Angelo Sanzone and Alberto Marra, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921204c Fragility, Stokes–Einstein violation, and correlated local excitations in a coarse-grained model of an ionic liquid Daun Jeong, M. Y. Choi, Hyung J. Kim and YounJoon Jung, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921725h Reactions of excited-state benzophenone ketyl radical in a room-temperature ionic liquid Kenji Takahashi, Hiroaki Tezuka, Shingo Kitamura, Toshifumi Satoh and Ryuzi Katoh, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920131a In search of pure liquid salt forms of aspirin: ionic liquid approaches with acetylsalicylic acid and salicylic acid Katharina Bica, Christiaan Rijksen, Mark Nieuwenhuyzen and Robin D. Rogers, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b923855g Nanocomposites of ionic liquids confined in mesoporous silica gels: preparation, characterization and performance Juan Zhang, Qinghua Zhang, Xueli Li, Shimin Liu, Yubo Ma, Feng Shi and Youquan Deng, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920556j An ultra high vacuum-spectroelectrochemical study of the dissolution of copper in the ionic liquid (N-methylacetate)-4-picolinium bis(trifluoromethylsulfonyl)imide Fulian Qiu, Alasdair W. Taylor, Shuang Men, Ignacio J. Villar-Garcia and Peter Licence, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b924985k Understanding siloxane functionalised ionic liquids Heiko Niedermeyer, Mohd Azri Ab Rani, Paul D. Lickiss, Jason P. Hallett, Tom Welton, Andrew J. P. White and Patricia A. Hunt, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b922011a On the electrodeposition of tantalum from three different ionic liquids with the bis(trifluoromethyl sulfonyl) amide anion Adriana Ispas, Barbara Adolphi, Andreas Bund and Frank Endres, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b922071m Solid-state dye-sensitized solar cells using polymerized ionic liquid electrolyte with platinum-free counter electrode Ryuji Kawano, Toru Katakabe, Hironobu Shimosawa, Md. Khaja Nazeeruddin, Michael Grätzel, Hiroshi Matsui, Takayuki Kitamura, Nobuo Tanabec and Masayoshi Watanabe, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b920633g Dynamics of ionic liquid mediated quantised charging of monolayer-protected clusters Stijn F. L. Mertens, Gábor Mészáros and Thomas Wandlowski, Phys. Chem. Chem. Phys., 2010, DOI: 10.1039/b921368f

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J-aggregation of ionic liquid solutions of

meso-tetrakis(4-sulfonatophenyl)porphyrinw

Maroof Ali,a Vinod Kumar,a Sheila N. Baker,b Gary A. Bakerb and

Siddharth Pandey*a

Received 1st October 2009, Accepted 26th November 2009

First published as an Advance Article on the web 21st December 2009

DOI: 10.1039/b920500d

The title porphyrin was dissolved in the hydrophilic ionic liquid 1-butyl-3-methylimidazolium

tetrafluoroborate, [bmim][BF4], and triggered to assemble into J-aggregates by the addition of

incremental volumes of water containing various amounts of acid (0.1, 0.2, or 1.0 M HCl).

In contrast to recent studies, the current investigation is unique in that it centers on media that

contain a predominant ionic liquid component (2.9–5.4 M [bmim][BF4]), as opposed to an

aqueous electrolyte containing a small fraction of ionic liquid as dissociated solute. Complex

aggregation and underlying photophysical behavior are revealed from absorption spectroscopy,

steady-state fluorescence, and resonance light scattering studies. Upon addition of aqueous

HCl, the efficient formation of H4TPPS2� J-aggregates from the diprotonated form of

meso-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4�) occurs in [bmim][BF4]-rich media in a

manner highly dependent upon the acidity, TPPS concentration, and solvent composition. The

unique features of TPPS aggregation in this ionic liquid were elucidated, including the surprising

disassembly of J-aggregates at higher aqueous contents, and our results are described qualitatively

in terms of the molecular exciton theory. Finally, the potential of this system for the optical

sensing of water at a sensitivity below 0.5 wt% is demonstrated. Overall, our findings accentuate

how little is known about functional self-assembly within ionic liquids and suggest a number of

avenues for exploring this completely untouched research landscape.

Introduction

Throughout the recent decade or so, room temperature ionic

liquids (ILs) as potential environmentally-friendly solvents have

garnered widespread attention and curiosity from the academic

and industrial research communities due to their unusual and

useful properties.1 Almost every named reaction and many

additional organic/inorganic/organometallic reactions have

been reported in ILs.2 Novel analytical applications of ILs are

emerging daily; effective, and in some cases truly unique,

deployment of ILs has been demonstrated within a variety of

analysis modes encompassing electroanalysis, separation,

extraction, mass spectrometry, and sensing.3 Combined with

the fact that ILs are composed entirely of cations and anions

but still exist in the liquid state at ambient conditions, heavy

recent investigation into ILs is also due in part to their potential

for ecologically-benign behavior. Most ILs have negligible

vapor pressure in a practical sense,2c,d and can be recycled

easily. As a consequence, it is logical to employ ILs as sub-

stitutes for volatile organic compounds in many applications.2

The porphyrins, a popular class of tetrapyrrolic dyes, are

useful in the photodynamic therapy (PDT) of cancer4a,b and as

potential fluorescence and magnetic resonance imaging (MRI)

contrast agents as well.4c,d The physical properties of porphyrins

also generate great interest for their potential as sensitizer

molecules in solar energy conversion and storage.5a–g The

study of the excited states of porphyrins is particularly helpful

toward understanding electron transfer in primary photo-

synthetic processes.5i–m Porphyrin dyes are prone to aggregation

in solution and such assemblies have been employed to develop

organic photoconductors,6a as markers for biological and

artificial membrane systems,6b and as device materials for

enhanced non-linear optical (NLO) applications,6c–f among

others. Close-stacked molecular aggregates may possess

properties suitable for superconductivity, optical frequency

conversion, and information processing and storage.6g–l

During the past few decades, the water-soluble 5,10,15,20-

tetrakis(4-sulfonatophenyl)porphyrin (meso-tetrakis(4-sulfonato-

phenyl)porphyrin or TPPS) has received much attention due

to the fact that TPPS yields J- or H-aggregates in aqueous

solutions of low pH and high ionic strength via side-by-side

and face-to-face stacking, respectively.7 Apart from several

reports in acidic solvent media, the formation of both J- and

H-aggregates of TPPS has been reported in many complex and

confining environments.8 Besides aggregation phenomena,

the photophysical behavior of porphyrins highly depends on

many factors, such as the substituent at the para position of

the meso-phenyl group, the metal center in the pyrrole ring,

and the nature of the surface (e.g., polycation films) onto

which it is adsorbed.9 Yoon et al. studied the effects of solvent

polarity on metallated TPPS and demonstrated that the

photophysical properties changed drastically with solvent

aDepartment of Chemistry, Indian Institute of Technology Delhi,Hauz Khas, New Delhi—110016, India.E-mail: sipandey@chemistry.iitd.ac.in; Tel: 91-11-26596503

bChemical Sciences Division, Oak Ridge National Laboratory,Oak Ridge, TN 37831, USA. Tel: 865-241-9361

w Electronic supplementary information (ESI) available: Additionalspectroscopic details (Fig. S1–S6). See DOI: 10.1039/b920500d

1886 | Phys. Chem. Chem. Phys., 2010, 12, 1886–1894 This journal is �c the Owner Societies 2010

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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polarity, even from switching between water and methanol.

Scolaro et al. reported the aggregation of TPPS salt in

dichloromethane and within short-chain alcohols.10 Though

the reports demonstrating the role of solvent milieu on

aggregation and photophysical behavior of tetraaryl-

substituted porphyrins are few,11 the need to understand TPPS

aggregation in nonaqueous media is increasing due to the

rapidly expanding application of these porphyrins. Thus, the

aggregation of porphyrins in novel and alternate media

has emerged as an important topic of research in general

chemistry, with significant implications in energy, medicine,

communications, and the environment.

As demonstrated by several research groups including our

own, in recent reports the behavior of structurally-diverse

probes within IL-based media sometimes exhibits unusual or

unpredicted responses, possibly due to specific solute–solvent

interactions or to the presence of interesting solvent nano-

structuring.12a–d Highly altered physicochemical properties of

IL-based mixtures resulting in anomalous probe behavior

(e.g., hyperpolarity12a,b) is now an accepted consequence of

the unusual properties (such as intrinsic ordering) associated

with these interesting solvents. ILs and IL/water mixtures, in

this context, are subject to keen interest for their potential as

green co-solvent systems.12 The aggregation of surfactants,12e–j

the interaction and dimerization of common probes,12a–j

the behavior and mobility of dyes,12k–n,13 and biomolecular

behavior12o–q are frequently observed to be drastically different

within ILs and IL/water mixtures. As a result, it becomes

compelling to investigate the aggregation behavior of TPPS as

a model porphyrin in neat IL and increasingly hydrated IL

solution. Due to the technological importance of porphyrin

aggregates,6c–l in this work we report a detailed photophysical

study probing the aggregation behavior of TPPS within

the hydrophilic IL 1-butyl-3-methylimidazolium tetrafluoro-

borate ([bmim][BF4]), both in the rigorous absence of

water and within [bmim][BF4] mixtures including incremental

levels of water up to 40 wt%. Our results suggest excep-

tional promise in the exploitation of IL-based systems for

deliberate modulation of the aggregation and photo-behavior

of porphyrin assemblies for a range of future applications.

(Scheme 1)

Experimental

Materials

Sodium salt of tetrakis(4-sulfonatophenyl)porphyrin (TPPS,

high purity) was obtained from Sigma-Aldrich and was used

as received. IL 1-butyl-3-methylimidazolium tetrafluoroborate

([bmim][BF4], Merck, ultra pure, halide content o10 ppm,

water content o10 ppm) was stored in dry conditions and

was also used as received. Doubly-distilled deionized water was

obtained from aMillipore, Milli-Q Academic water purification

system having Z 18 MO cm resistivity. Dimethyl sulfoxide

(DMSO), 1-butanol, cyclohexane and toluene of spectroscopic

or HPLC grade were obtained from Spectrochem Pvt. Ltd.

Ethanol (99.9%) was obtained from SD Fine-Chem. Ltd.

Sodium dihydrogen orthophosphate and disodium hydrogen

orthophosphate, phosphoric acid and hydrochloric acid (HCl)

were purchased from Qualigens with the highest purity possible.

Acetonitrile (ACN) and methanol of spectroscopic grade were

obtained from Sisco Research laboratory, Pvt. Ltd.

Methods.

Stock solutions of TPPS were prepared in ethanol and water,

respectively, and stored under refrigeration at 4 � 1 1C in pre-

cleaned amber glass vials. The required amount of appropriate

stock solution was taken in sample tube and dried with N2.

Neat [bmim][BF4] was added to the sample tube under dry

conditions to attain the desired final concentration. The

sample tube was kept under dry conditions for a few hours

for complete solubilization of TPPS within neat [bmim][BF4].

A pre-calculated amount of deionized water or aqueous HCl

or phosphate buffer was directly added to [bmim][BF4] con-

taining TPPS. Buffers (50 mM) of pH 1.0, 3.0 and 7.0 were

prepared by a proper combination of phosphoric acid, sodium

dihydrogen orthophosphate, and disodium hydrogen ortho-

phosphate. pH adjustment was done with the help of dilute

aqueous HCl and aqueous NaOH. The required amounts of

materials were weighed using Mettler-Toledo AB104-S

balance with a precision of�0.1 mg. A Perkin-Elmer Lambdabio

20 and a Systronics 2201 double beam spectrophotometer with

a variable bandwidth were used for the acquisition of UV-vis

molecular absorbance data. Resonance light scattering

and fluorescence spectra were acquired on model FL 3-11,

Fluorolog-3 modular spectrofluorimeter with single Czerny–

Turner grating excitation and emission monochromators hav-

ing 450 W Xe arc lamp as the excitation source and a PMT as

the detector purchased from Horiba-Jobin Yvon, Inc. All the

data were acquired using 0.5 and 1-cm2 path length quartz

cuvettes. Spectral response from appropriate blanks was

subtracted before data analysis. All data were collected at

least in triplicate starting from sample preparation. All

data analyses were performed using Microsoft Excel and/or

SigmaPlot 10.0 softwares.

Results and discussion

TPPS in neat [bmim][BF4]

Fig. 1 presents concentration-dependent electronic absorbance

behavior of TPPS in neat ionic liquid [bmim][BF4]. TheScheme 1

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 1886–1894 | 1887

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absorbance spectra of TPPS show a sharp and intense Soret

band (B band) in the high energy region at ca. 418 nm which is

bathochromically shifted B4 nm in comparison to its position

in neat water (414 nm).14 This band is accompanied by a

shoulder at around 400 nm. Four weak low energy Q bands at

515, 551, 589 and 645 nm are also clearly visible. These

features are the characteristics of deprotonated TPPS monomer.

In order to compare and contrast the behavior of our ionic

liquid with that of other common solvents in this context,

absorbance spectra of TPPS were also measured in polar

protic (1-butanol and methanol), polar aprotic (dimethyl-

sulfoxide and acetonitrile) and nonpolar (cyclohexane and

toluene) organic solvents. The details of the position of Soret

and Q bands are provided in Table 1. The UV-vis absorbance

characteristics of TPPS in methaol are in good agreement with

those reported by Scolaro et al.10c It is interesting to note that

the position of the Soret and the Q bands of TPPS in

polar solvents are similar to that observed in [bmim][BF4]

re-emphasizing the expected polar nature of ionic liquids. The

TPPS absorbance behavior is different in nonpolar cyclo-

hexane and toluene as the Soret band is broad and significantly

red-shifted and clear Q-bands are not observed. This is in

accordance with what is reported in literature.10b,c

Interesting features were revealed from the concentration-

dependent absorption behavior of TPPS in neat [bmim][BF4]

(Fig. 1). A plot of absorbance of TPPS at 418 nm as a function

of [TPPS] (inset Fig. 1) shows a clear deviation from the

Beer–Lambert law at ca. 15 mM [TPPS], which could be

attributed to the possible formation of aggregates of TPPS

in neat [bmim][BF4]. It is well-documented that J-aggregates of

TPPS in acidic aqueous solutions of high ionic strength feature

themselves as a band in the vicinity of 490 � 10 nm in the

electronic absorbance spectrum. A careful examination of

concentration-dependent absorbance spectra of TPPS in this

region reveals the presence of a weak broad band centered

around 482 nm (inset Fig. 1), which further corroborates the

presence of J-type aggregates of TPPS within neat

[bmim][BF4]. Formation of the aggregates may be assigned

to the presence of a small amount of H+ invariably present

due to the hydrolytic instability associated with ionic

liquid [bmim][BF4].15 The reduction in electrostatic repulsion

between anionic sulfonates due to the presence of the ionic

liquid cation [bmim+] may also facilitate the aggregation

process. Rotomskis et al. and other workers have reported

the aggregation of TPPS at a higher concentration in neutral

aqueous solution but such behavior is rarely reported in neat

organic solvents.16

Fluorescence emission and excitation

Fluorescence emission spectra of TPPS in neat [bmim][BF4]

shows the usual two bands with maxima at 650 and 715 nm

corresponding to a (0–0) transition and its vibronic (0–1)

transition, respectively (inset Fig. 2). The fluorescence emission

spectra are found to be excitation wavelength independent

(Fig. S1 in the ESI).w Further, no significant changes in the

shape and position of the emission bands are observed with

increase in [TPPS] (inset Fig. 2). Again, the fluorescence

emission spectral characteristics of TPPS in neat [bmim][BF4]

are similar to those observed in polar (protic and aprotic)

organic solvents (Table 1). While no distinct features implying

the presence of aggregates are visible in the concentration-

and excitation wavelength-dependent fluorescence emission

behavior of TPPS in neat [bmim][BF4], the concentration-

dependent excitation spectra of TPPS (lem = 715 nm) do

indeed contain an evidence of aggregation within the solution.

A gradual broadening of the Soret band is followed by a

splitting of the band as the [TPPS] is increased. In the

excitation spectra of TPPS and other porphyrins the splitting

of a Soret band is observed in organic solvents due to

aggregation11c–e and this phenomenon has been explained in

terms of the molecular exciton theory.17 No clear relationship

between the physical properties of the solvent (e.g., e, ETN, etc.)

and the behavior of Soret and Q bands was observed.

The presence of aggregates of TPPS in neat [bmim][BF4] is

clearly evident from the concentration-dependent UV-vis

molecular absorbance and fluorescence excitation behavior

of this porphyrin.

TPPS in acidic buffer-added and HCl-added [bmim][BF4]

the presence of small amounts of water within [bmim][BF4] is

observed to result in dramatic changes in the aggregation

behavior of TPPS that are manifested through electronic

absorbance and molecular fluorescence of TPPS.

The electronic absorbance behavior of TPPS in [bmim][BF4]

in the presence of different wt% of pH 1.0 buffer is presented

in Fig. 3. The absorbance spectra of TPPS undergo dramatic

changes as more and more pH 1.0 buffer solution is added to

[bmim][BF4]. A careful examination of Fig. 3 reveals several

interesting outcomes. A rapid decrease in a monomeric Soret

band (B418 nm) is accompanied by the appearance of a

new band at ca. 448 nm in the Soret region and its corresponding

Q band at ca. 671 nm. As the concentration of pH 1.0 buffer is

increased in the solution this band grows, however, it under-

goes B10 nm hypsochromic shift as up to B20 wt% pH 1.0

buffer is added to the solution. This new band in the Soret

region is assigned to the diprotonated form of TPPS in

agreement with the suggestions in the literature where an

appearance of B448 nm band is reported for the protonated

Fig. 1 Absorbance spectra of TPPS at different concentrations (1–20 mM)

in neat [bmim][BF4] at ambient conditions. Inset shows the plot of

absorbance of TPPS at 418 nm (A418) vs. [TPPS].

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species of porphyrins in organic solvents after addition

of acids.11e

Interestingly, an addition of pH 1.0 buffer results in the

appearance of a band at ca. 485 nm in the Soret region and a Q

band at ca. 702 nm. The relatively high absorbance of this

band indicates an efficient formation of the J-aggregates of

TPPS within the solution. This band corresponding to the

J-aggregates grows monotonically with the concentration of

pH 1.0 buffer in the solution. It is clear that the presence of

small amounts of pH 1.0 buffer within the ionic liquid

[bmim][BF4] facilitates J-aggregation.

In order to investigate the effect of well-documented hydrolytic

instability of our ionic liquid,15 we measured the electronic

absorbance spectra of TPPS within [bmim][BF4] in the

presence of different wt% aqueous HCl solutions (Fig. 4).

The results of addition of 0.1 M aqueous HCl are similar to

those obtained when pH 1.0 buffer was added to [bmim][BF4]

(panel A, Fig. 4). However, three distinct differences are

noteworthy. First, the formation of protonated TPPS and

J-aggregates in the presence of 0.1 M aqueous HCl are

more efficient. The ratios of the absorbance values of the

protonated-to-deprotonated monomeric forms clearly indicate

the efficiency of the formation of the protonated form to be

more for pH 1.0 aqueous HCl addition as compared to that

for pH 1.0 buffer addition (inset, Fig. 3). We believe the

hydrolytic instability of [bmim][BF4] results in additional

lowering of the pH of aqueous HCl as compared to that of

aqueous buffer of the same pH as the buffer would resist the

decline in pH of the solution. Second, an interesting outcome

is that the absorbance of 485 nm band corresponding to the

J-aggregates increases more rapidly as compared to that

observed for pH 1.0 buffer addition, however, it starts to

decrease as the concentration of 0.1 M aqueous HCl becomes

greater than 10–12 wt%. At 40 wt% 0.1 M aqueous HCl

addition, the band at 485 nm completely converts to the band

corresponding to the protonated species. Third, contrary to

what was observed for pH 1.0 buffer addition, no clear

isosbestic point before the addition of 10 wt% 0.1 M aqueous

HCl could be attributed to the presence of different non-

interconverting species in the solution; for pH 1.0 aqueous

HCl above 10 wt%, a clear isobestic point around 450 nm is

clearly observed between the species corresponding to 448

and 485 nm, respectively, which confirms the conversion of

J-aggregated TPPS to the protonated one on destabilization.

The highlight of these results is the destabilization of

J-aggregates with 0.1 M aqueous HCl addition after a

Table 1 Absorbance and fluorescence emission maxima of Soret (B) and Q bands of TPPS (20 mM) in various solvents at ambient conditions(e is the static dielectric constant of the solvent). Error in absorbance and emission maxima is �1 nm or less

Solvent ea

Absorbance maxima/nm Emission Maxima/nm

Soret (B) Qy (0–1) Qy (0–0) Qx (0–1) Qx (0–0)

Water 78.5 414 515 551 580 645 650 710Dimethylsulfoxide 48.9 417 515 551 590 646 651 714Acetonitrile 36.6 414 513 546 589 645 648 713Methanol 33.0 411 510 547 588 643 647 7111-Butanol 17.8 417 518 560 590 650 647 711Cyclohexane 2.02 424 b b b b 664 715Toluene 2.38 425 b b b b 657 713[bmim][BF4] 11.7c 418 515 550 589 645 648 714

a Handbook of Chemistry and Physics; Lide, D. R.; Ed.; CRC Press: Boca Raton, 77th edn, 1996. b Absorbance maxima difficult to assign with

precision due to considerable band broadening. c Wakai, C.; Oleinikova, A.; Otto, M.; Weingartner, H. J. Phys. Chem. B 2005, 109, 17028.

Fig. 2 Fluorescence excitation spectra of TPPS (lemission = 715 nm)

at different concentrations (1–20 mM) in neat [bmim][BF4] at ambient

conditions. Inset shows emission spectra (lexcitation = 420 nm).

Fig. 3 Absorbance spectra of TPPS (10 mM) in [bmim][BF4] in the

presence of 0–40 wt% of pH 1.0 aqueous buffer at ambient conditions.

Inset shows plots of the ratio of absorbance values at different

wavelengths vs. wt% of 0.1 M aqueous HCl and pH 1.0 buffer within

[bmim][BF4].

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threshold concentration of B10 wt%, and the bathochromic

and hypsochromic shifts of the 485 nm and the 448 nm bands,

respectively, with increasing the wt% of acidic aqueous solutions

(the shifting of Soret bands in the presence of different wt% of

aqueous HCl is reported in Table S1 of the ESI).w Initially, we

envisioned the destabilization of J-aggregates to be due to high

concentration of H+ (it is reported that the high concentration

of H+ causes destabilization of J-aggregates7d), however,

when a similar investigation was carried out with 0.2 and

1.0 M of aqueous HCl, respectively, the trend in absorbance

corresponding to J-aggregates was observed to be similar

(Fig. 4); the destabilization of J-aggregates started after

addition of >10 wt% aqueous HCl solution. Therefore, the

role of [H+] in the destabilization of J-aggregates of TPPS

within water-added [bmim][BF4] is considered to be minimal.

It is suggested that the concentration of water within

[bmim][BF4] plays a major role in the destabilization of TPPS

aggregates. Initially, at a lower concentration of the aqueous

phase, protonated TPPS molecules are mostly surrounded by

the cations and the anions of [bmim][BF4] which effectively

shield the negative and positive charges of sulfonyl moieties

and the protonated core, respectively, thus reducing the

electrostatic repulsion between them, in the process, facilitating

the formation of aggregated species. As the concentration of

the aqueous phase increases, the [bmim][BF4] molecules start

to form the aggregates/clusters by interacting with each other

in the solution as already reported by several workers.18 Koga

et al. proposed the formation of [bmim][BF4] aggregates/

clusters at a similar concentration of the aqueous phase

(B10 wt%) within [bmim][BF4] based on a thermodynamic

study of this system.19 Due to the formation of such aggregates/

clusters of [bmim][BF4], the shielding of sulfonyl moieties and

the protonated cores are lowered, which results in destabilization

of the J-aggregates. The bathochromic and the hypsochromic

shifts of the bands corresponding to the J-aggregates and the

protonated species, respectively, could be attributed to the

change in the solvation microenvironment. The positions of

the Soret bands for the protonated and the J-aggregates at

436 and 489 nm, respectively, after addition of 40 wt% 0.1 M

aqueous HCl are very near to the values reported for these

species in water at 434 and 491 nm,14 respectively. This

confirms the dominance of water in the solvation micro-sphere

of these species in the presence of higher wt% of aqueous HCl.

While the destabilization of J-aggregates starts to take place

at similar aqueous phase concentrations, the overall absorbance

behavior of TPPS in the presence of 0.2 M and 1.0 M aqueous

HCl is significantly different from that observed in the

presence of 0.1 M aqueous HCl (Fig. 4). A coupled broad

band with two maxima at B448 and B479 nm are observed

with the addition of 2 wt% 0.2 M aqueous HCl (Fig. 4B).

While the band at 448 nm is due to protonated TPPS, the one

at 479 nm is due to J-aggregates in [bmim][BF4]-rich environ-

ment (vide supra). Further addition of 0.2 M aqueous HCl

results in an increased formation of protonated TPPS as well

as J-aggregates where these species are now in a water-rich

environment (indicated by the hypsochromic and bathochromic

shifts, respectively; xwater B 0.58 at 10 wt% water). For both

0.2 M and 1.0 M aqueous HCl addition, the deprotonated

TPPS rather efficiently converts to a protonated form at

2 wt% addition (Fig. 4), the effective protonation appears to

be higher for 1.0 M aqueous HCl—the solution with highest

[H+]. In contrast to the 0.1 M aqueous HCl addition, the

formation of J-aggregates is observed at much lower wt%

0.2 M and 1.0 M aqueous HCl addition.

Effect of TPPS concentration on aggregation

In order to assess the dependence of [TPPS] on its aggregation,

the absorbance spectra were collected for 3 mM r [TPPS] r18 mM within water-added [bmim][BF4] solutions at different

concentrations of 0.2 M aqueous HCl (the most efficient

J-aggregation was observed at this HCl concentration, see

inset Fig. 4A). The results are presented in Fig. S2 of the

ESI.w While the features depicting deprotonated and

protonated species are usual, clearly more J-aggregates

form at higher [TPPS] (Fig. 5). It is important to mention

here that the decrease in J-aggregation after B10 wt%

aqueous HCl is significantly less or does not happen at higher

[TPPS]. This could be due to the presence of enough

protonated TPPS in the solution which favors the formation

of J-aggregates.

Fig. 4 Absorbance spectra of TPPS (10 mM) in [bmim][BF4] in

presence of 0–40 wt% of 0.1 M (panel A), 0.2 M (panel B) and

1.0 M (panel C) aqueous HCl at ambient conditions. Inset in panel A

shows plots of absorbance corresponding to J-aggregates (AJ-aggregate)

vs. wt% 0.1, 0.2 and 1.0 M of aqueous HCl within [bmim][BF4]

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No remarkable changes in the absorbance spectra are

observed with addition of pH 2.0 and pH 3.0 aqueous HCl,

and neat water (pH 6.5) up to 40.0 wt% (Fig. S3w). This clearlyindicates that low [H+] within water-added [bmim][BF4]

solutions disfavors the J-aggregation irrespective of aqueous

phase concentration. The data clearly show the absence of

significant amount of protonated TPPS which is essential for

J-aggregation. In other words, addition of aqueous acidic

solution having pH 2 and higher does not result in J-aggregation

within water-added [bmim][BF4].

Molecular fluorescence of TPPS

Since the maximum J-aggregation was observed with addition

of 0.2 M aqueous HCl, fluorescence emission and excitation

spectra of TPPS within water-added [bmim][BF4] were collected

at this HCl concentration (Fig. 6). In neat [bmim][BF4], TPPS

shows emission spectra with maxima at 650 and 714 nm which

is excitation wavelength independent (vide supra, Fig. 1).

Interestingly, after addition of 0.2 M aqueous HCl to

[bmim][BF4], the TPPS emission becomes excitation wavelength

dependent. This indicates aggregation of TPPS in its ground

state. The emission spectra for lex = 490 nm (corresponding

to the J-aggregate band) show sharp decrease in the fluores-

cence intensity accompanied by a distinct change in the

spectral band shape with increase in wt% of 0.2 M aqueous

HCl in the solution (panel A Fig. 6). The emission band at

650 nm vanishes and a new band grows in the vicinity of

680 � 5 nm. Due to the enhanced rate of nonradiative

deactivation pathways, J-aggregates similar to several other

aggregates, have significantly low fluorescence quantum

yields.20 We believe the band at 680 � 5 nm is due to the

protonated TPPS. In order to corroborate this proposition, we

collected emission spectra at lexcitation = 440 nm (correspond-

ing to the Soret band of protonated TPPS, panel B Fig. 6).

While the fluorescence maxima shift from 650 to 687 nm, a

dramatic enhancement in the fluorescence intensity of the

band centered at 680 � 5 nm is clearly visible as more

and more 0.2 M aqueous HCl is added to [bmim][BF4]

implying increase in the concentration of protonated TPPS

in the solution. As expected, usually for the protonated

species, this band is fairly broad. Fluorescence emission

spectra at lexcitation= 420 nm (corresponding to the deprotonated

TPPS, panel C Fig. 6) further confirms our observation as the

band at 650 and 715 nm merge into a broad band centered at

680 � 5 nm as more and more 0.2 M aqueous HCl is added to

[bmim][BF4].

Fluorescence excitation spectra of TPPS at lemission = 690 nm

within water-added [bmim][BF4] (Fig. S4w) show the bands

at similar positions to those present in UV-vis absorbance

spectra except for the bands characterizing J-aggregates as

these aggregates are non-fluorescent or have extremely low

fluorescence quantum yields (vide supra). It is worthy of

mention that in all fluorescence plots the most dramatic

changes, either in the band shift or in the fluorescence intensity

change, occur close to 10 wt% 0.2 M aqueous HCl addition

(Table S1).w All-in-all, the fluorescence behavior of TPPS

within water-added [bmim][BF4] fully supports the absorbance

results.

In the presence of 1.0 M aqueous HCl, TPPS shows similar

fluorescence behavior to the case of 0.2 M aqueous HCl

addition to [bmim][BF4] (Fig. S5).w However, the formation

of protonated and aggregated TPPS species is observed to be

way more efficient even with the addition of only B2 wt%

1.0 M aqueous HCl as compared to the addition of the same

concentration of 0.2 M aqueous HCl due to the much higher

Fig. 5 Plot of absorbance corresponding to J-aggregates (AJ-aggregate)

at different [TPPS] vs. wt% of 0.2 M aqueous HCl within [bmim][BF4]

at ambient conditions.

Fig. 6 Fluorescence emission spectra of TPPS (5 mM), lexcitation= 490 nm

(Panel A), lexcitation = 440 nm (Panel B), and lexcitation = 420 nm

(Panel C) within [bmim][BF4] in the presence of 0–40 wt% of 0.2 M

aqueous HCl at ambient conditions.

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strength of H+ in the former. The behavior of the maxima of

the Soret and emission bands within [bmim][BF4] in the

presence of 1.0 M aqueous HCl is provided in Table S1.w

Resonance light scattering. Resonance light scattering (RLS)

is used to further investigate the aggregation of TPPS within

aqueous HCl-added [bmim][BF4]. Fig. 7 shows the RLS

spectra of TPPS in [bmim][BF4] in the presence of differing

amounts of 0.2 M aqueous HCl. A sharp peak in RLS spectra

at ca. 491 nm is observed with addition of 5 wt% 0.2 M

aqueous HCl to [bmim][BF4] which confirms the presence of

J-aggregates of TPPS as reported in the literature as well.7d

The increase in the intensity of this peak is observed up to the

addition of 10 wt% 0.2 M aqueous HCl, further addition

causes decrease in intensity similar to what was observed from

the absorbance behavior (vide supra) indicating destabilization

of TPPS J-aggregates at higher aqueous HCl concentrations.

For 1.0 M aqueous HCl addition to [bmim][BF4] (inset

Fig. S5B, ESIw), the results of RLS investigations are similar

except for the fact that addition of even a smaller amount

of 1.0 M aqueous HCl, i.e., 2 wt%, results in appreciable

formation of J-aggregates. Higher concentration of 1.0 M

aqueous HCl again destabilizes the aggregates. The RLS

results nicely corroborate our earlier observations.

Change in TPPS fluorescence with trace water within

[bmim][BF4]

As mentioned earlier, addition of up to 40 wt% doubly-

distilled deionized water to [bmim][BF4] results in no change

in the absorbance behavior of TPPS (Fig. S3).w The fluorescenceexcitation behavior of TPPS, however, shows interesting

changes as rather small amounts of deionized water are added

to [bmim][BF4] (Fig. 8). When a very small amount of water

(B0.08 wt%) is added to [bmim][BF4], an unusual broadening

of the Soret band accompanied by an increase in the intensity

of Q bands (especially at 515 nm) is observed in the normalized

excitation spectra with lemission = 715 nm. On further increasing

the wt% of water, the Soret band at 418 nm splits into two

bands (Fig. 8). The separation between the two Soret bands

becomes bigger with increasing wt% of deionized water. These

effects diminish after addition of B2 wt% deionized water

with the two Soret band maxima now locating at 407 and

427 nm (separated by B1150 cm�1). A relative change in the

intensity of the Q band at 515 nm upon addition of deionized

water is presented in the inset of Fig. 8. It is inferred that this

feature of TPPS may be used to detect the presence of trace

amount of water within the ionic liquid [bmim][BF4]. Similar

behavior of Soret and Q bands was observed in the case

of metalloporphyrin array and free base porphyrins.11e,21

Surprisingly, fluorescence emission behavior of TPPS within

[bmim][BF4] does not change upon addition of up to 2 wt%

deionized water (TPPS concentration-dependent fluorescence

emission spectra within [bmim][BF4] in the presence of

0–2 wt% deionized water are shown in Fig. S6).wThe broadening and splitting of the Soret band and the

increase in the intensity of the Q bands depend highly on the

concentration of TPPS, although they appear to be independent

of the emission wavelength at which the excitation is

monitored (Fig. S6).w The excitation spectra ofB19 mMTPPS

in the presence of 0–2 wt% deionized water within

[bmim][BF4] at lem = 650 nm show similar changes as those

observed at lem = 715 nm (lower panel, Fig. S6).w However,

at lower TPPS concentration (B6 mM) only a broadening of

the Soret band is observed with addition of deionized water

and the increase in the intensity of the band at 515 nm is

minimal. The splitting of the Soret band combined with a

relatively larger increase in the intensity of the Q band at

515 nm is observed for B12.5 mM TPPS. It is proposed that

the hydrolytic instability of [bmim][BF4]15 results in the

formation of fluoride ions as deionized water is added to

the solution. These fluoride ions interact with the hydrogens

of the pyrrole ring of TPPS, resulting in distortion of the

porphyrin ring from planarity and thus changing the symmetry

of the TPPS molecule. The redistribution and inter-mixing of

energy states of the porphyrin molecules also take place due to

excitonic interaction.22 As a result of the splitting of the Bx

component of the Soret band, two energy states Bx0 and Bx0 0

arise which lie very near to the energy states of the Q bands.

Due to the narrowness of the Soret and the Q band energy

states, the transfer of intensity from the Soret to the low lying

Q bands takes place resulting in an increase in the intensity of

the Q band at 515 nm.23

Fig. 7 Resonance light scattering (RLS) spectra of TPPS (5 mM)

within [bmim][BF4] in the presence of 0–40 wt% of 0.2 M aqueous HCl

at ambient conditions.

Fig. 8 Fluorescence excitation spectra of TPPS (20 mM, lem= 715 nm)

within [bmim][BF4] in the presence of 0–2 wt% of deionized water at

ambient conditions. Inset shows the plot of relative increase in the

intensity of the Q band (515 nm) vs. wt% of water within [bmim][BF4].

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Acknowledgements

This work was generously funded by a grant to SP from the

Department of Science and Technology (DST), Government

of India (grant no. SR/S1/PC-16/2008). MA and VK would

like to thank UGC, India and CSIR, India, respectively, for

fellowships.

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