Raymo, Francisco M.

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The Spectrum Volume 17 Issue 2 Article 5 2004 Intermolecular Fluorescence Modulation: A Mechanism to Process Optical Signals With Molecules Francisco M. Raymo University of Miami Follow this and additional works at: https://scholarworks.bgsu.edu/spectrum How does access to this work benefit you? Let us know! Recommended Citation Raymo, Francisco M. (2004) "Intermolecular Fluorescence Modulation: A Mechanism to Process Optical Signals With Molecules," The Spectrum: Vol. 17: Iss. 2, Article 5. Available at: https://scholarworks.bgsu.edu/spectrum/vol17/iss2/5 This In This Issue is brought to you for free and open access by ScholarWorks@BGSU. It has been accepted for inclusion in The Spectrum by an authorized editor of ScholarWorks@BGSU. intermolecular fluorescence modulation: a mechanism to process optical signals with molecules ---===::=::::a--=::::::w--.c:===:z:: The central nervous system collects, elaborates and scores information relying on the synergism of multiple and concomitant chemical proccsses.1 The mechanism of vision, for example, involves a concarcnation of chemical transformations to transducc visual information into nerve impulses. First, the light reflected from the objects around us propagates through space to our eyes. Then, the crystal-line lenses at the front of our eyes focus the incoming light on the retinas ar the back. In the retinal tissue, a rel;itivcly simple chromophore (1 L-cis-rctinal) absorbs the focused light and switches to the corresponding trans isomer. The photoinduced isomcrization triggers a cascade of conforma-tiona I changes culminating in the generation of an action potential. The resulting electrical signal travels from the retinal tissue through the optic nerve to the brain. There, information is communicated from one neuron to the next in chemical form. Indeed, pre-synaptic neurons release sim-ple organic molecules ( the neurotransmitter ) in the synap-tic cleft. These chemical messengers travel to post-synaptic neurons delivering information. The combination of these fascinating processes ensures the efficient acquisition of information from the surrounding environment and its ef-fective propagation from the eye to the brain, after multiple signal transduction steps at the molecular level. The complexity of ncurochcmical processes is beyond reach of any artificial mimic ar the present stage. However, their operating principles can certc1inly serve as viable models for the design and development of chemical systems able to martipulate information.z ln fact, clever examples of molecules that can process signals, 1 an<l even add digits" and play gamcs,5 have been demonstrated over the past few years. Although most o( these intriguing chemical proces-sors remain at the prototypical stage, some of them have evolved into practical applications, especially, in the con-text of chemical sensing. The mechanisms responsible for the manipularion of information in these artificial systems arc generally based on acid/base reactions, moleculc1r rec-ognition, phocoinduced isomeriwtions or redox processes. 1 These transformations arc accompanied by detectable changes in spectroscopic or voltammctric outputs c1nd en-able the interplay of chemical, electrical and/or optical signals. In particular, the photoresponsivc properties of photochromic compounds 6 arc especially amactivc for the manipulation of optical signals. Indeed, these molecules respond to opticc1I stimulations producing color changes. More precisely, the phocoinduccd isomcrization from one state to another alters the absorbancc in the visible region. TI1us, opticc1l inputs (incident light) can be exploited to modulate optical output.<, (transmitted light). Franc;isco M. Raymo Center for Supramolecular Science, Department ofChemistry, University of Miami We have devised an experimental protocol to modulate optical signals by operating a photochromic compound in conjunction with collections of fluorophores.; For our stud-ies, we have selected a member of a well-known family of photochrornic compounds, the spiropyrans.s Specifically, we have synthesized the spiropynm SP (Figure I) in three steps Figure 1. The interconversion of the three states SP, ME and MEH. starting from 2,3,3-trimcthyl-}H-indolc. 9 Its X-ray crystal structure (a in Figure 2) reveals the orthogonal orientation of the mean planes of the indoline and bcnzopyran halves Figure 2. X-ray crystal structures of SP (a) and MEH (b). Page 14 Volume 17 Issue 2 2004 The Spectrum imposed by the spirocenter. As a result, che two main chm-mophoric fragment£ of this molecule are isolated from each orl1f'r :ciml rb nor :cihsorh visible lighr (a in Figme 3). l Jpon 1.2 B 09 C "' € 0.6 :?, .c, <( 0.3 Wavaleng1h (nm1 a b C Figure 3. Absorpt,c~ spect·e and color chen£es of solutions of SP :1 x 10..,M, MeCN, 25 •:::) before (a) and after (b) ultravioel irradiation (254 nm, 5 min) and the subsequent addition of one equivalent of CF,CO,H (c;J. ultraviolet irradiation, however, the colorless Sr isomerizes to the purple rnerocynnine ME (Figure l ), ofter the cleav-· ~ge. nf rlw (' ,-0 oonrl ~r rhe. spimr.enrn ~nrl rhe. ris-tram isomcrization of the adjacent C-C bond. The process is accomp;mi,.-d by the appearance of an :ibsorption band ccmerecl at 563 nm (b in Figure 3). The band disappears after either storage in the dark or visible irradiation. Under both conditions, the purple ME reverts to the colorless SP (Figure L). h1sLc,1d, du: c1ddiLiu11 u( acid ciLl1cr Lciute ur after ultraviolet irrndiation results rn the formntion of the yellow-green merocyanine MEH. Concomirnnrly, a band centered clt 401 nm appears in the abwrption spectrum (c in Figure 3). The X-ray crystal structure of MEH (b in Figure 2) confirms rhe tram coniigurnrion of rhe ccnrrnl C=C bond and the coplanar arrangement of the indo-liuro and p-nitroi::henol frngmems. TI1.e ycliow-grcen MEH ~wiLd11.:s back w du: pu1vle ME, aftt:r the c1dlliLio11 uf base, or to chc colorless SP, ~ftcr visible irradiarion (figure I). Thi.' marked absorbance changes accompanying the interconversion of SP, ME and MEH (figure 3) can be ex-ploited to modulate chc emission intensity cf a compatible flnmophore. 7 TI,e fluorescence of rhc emissivc componcnr, Raymo however, should be centered at a wavelength where the absorb□.nces of the three stace~ of the photochromic com-ponf'nr ~re ,ignifirnnrly clifinenr. llnrler rhi>.sf' c.nnclirions, the froction of emitted light char is re-absorbed by the pho-tochromic swLtch 1s expected to vary with tts state. Pyrene happens co sac isl)· [hese design requiremenrs. ;, In face, rh.is particular fluorophorc emits light at 374 nm {a in Figure 4). At this wavelength, che absorbance of SP i, lower rhan tht: aµi.;roximaLdy equal aLsurLmees vi ME aJ1J MEH (pie ch arr in figure 4 ). i i ~~ 60 55 c8> ~==~~~!!!!!!!!!!!!!!!!~ a 406 456 Wa,elEngth (nm) 0 0 556 b ~a+----~----~----, o ~ o----;;:;:;;;-- o ~o Ug1l li~lll Figure 4, (a) Emission spectrum of pyrene (1 x 10 .. M, MeCN, 25 'C, '•~, = 336 rm) wit, the obsorbanco at 37 4 nm of a solution of SP (1 x 10~ M, MeCN, 25 'C) before (white) arid after (purple) ultraviclet irradiation (2!:i4 nm, ~ m n) ard the subsequent addition or one equilalent of CF,CO,H (yellow-green). (b) Flucx-escence intensity at 374 nm· of an equimoar solution ofpyrene and SP (1 x 10• M. MeCN. 25 •c 1...,.. = 336 nm) before and Etter t.;llrav101et irradiation (2!:14 rn. o min), after the subsequent addition of one equivaleni of CF,CO2H and visible irradiation (524 nm. 15 min). \Vhen cquimolar amoLrnts of pyrene and SP are co-dis-solved in acctonitrile, about 61 ¼ of the original fluorescence inrensiry is derecrerl ~t 174 nm (b in Fieure 4 ). Indeed, some The Spectrum Volume 17 Issue 2 2004 Page 15 Raymo ---=---------=::::::a-•===:..----========:::::.ac==-■aiiillKl.::'.'!-~=:..rz;::::a-ic::::::=:::s;:__=--=..=--=....::::.....::::____::::_=--=-=-=-=-of rlw c>xriring lighr, as wFII HS some> of rlw c>mirrc>d lighr, are co-adsorbed and re-adsorbed by SP respectively. After ultraviolet irradiation, SP switches to ME and the fluores-cence intensity detected at 374 nm drops to 52%, since the l-'ltuLud1ru111ic 1;Ulll1-'Ulle11L 11uw re-al~url.,~ a ~ignif1umLly larger fraction (pie chart in Figure 4) of the emitted llght. After acidification, ME switches to MEH with a negligible absorbance change at 3 74 nm ( pie chart in Figure 4) and the detected fluorescence remains approximarely constanc (bin Figure 4 ). However, it rn1se5 co about 64% after visible irradiation, since the photochromic component returns to rhe low absorbing sr::ite SP. Thus, rhe emis,ion of the Huo-rophore can be controlled by operating the photochromic switch with optical (ulcrnviolet and visible irradiation) and chemical (acid) stimulations. The i11u::r111ulecular p uce~s re~p,m~ible fur fluure~ce11ce modulation is based on d1e transfer of photons from pyrene co the photochromic component present in tbe 5ame solu-tion. The emitting and absorbing molecules involved in photon transfer, however, do not necessarily need to be co-dissolved in a single matrix. In principle, the light emitted from one can travel through space to the other. In fact, two distinct qu::irt: cells can be t>mployed to implement the very same modulation protocol with the configuration illustrated in Figure 5.7b Furthermore, mu!tipk Auorophores can be used Figure 5. Experimental setup for the modulation of the emission intensites of three fluorophorns with a single photochrome. simultaneously in this experimental setup co process multiple oµLical signals in parallel. Spccilically, wi..: have employed an cquimolar solution of naphthalene, arnluaccnc and n~rrnr.PnP in one> cc>II ::incl ::i scilminn of 8P in rhP. mlwr. ThP. three acenes absorb the light coming from the excitation source and fluoresce. The light emitted in the perpendicular direction. passes though the cell containing the photochro-mic ~wiLclt l.,efore read1iug Ute JeLecwr. Tl1e ab~urpliun properties of me photochromic element vary in respcnse to the addition of acid and optical stimulations coming from the u.ltraviolet an:! visible sources. Indeed, these three inputs control the interconversion between three scares SP, MEandMEH Naphthalene has an emission band centered at 335 nm (a in Figure 6). At this w,ivelengrh, the three srntes SP, ME and MEH abwrb equally. Thus, the naphthalene 1 2 ,; .e. 0.9 12 0,6 "' C: s b ..5 0,3 a 364 443 522 60) \Navelongth (n1'1} 100 75 t i:-·~ C: 50 .!! ..5 25 0 O Ultra1dolet 0 Aoid 0 ViE;ible 0 Light Light Figure 6. Emission spectra (1 x 10·• M. MeCN, 25 °C) of naphthalene (a, i.,,. = 275 nm), anthracene (b, ".,, = 357 nm) and tetracene (c) . .,0 = 441 nm) with the absorbances at 335, 401 and 544 nm of a solution of SP (1 x 1 D .. M, MeCN, 25 °C befo'e (white) and after (purple) ultraviolet Irradiation (254 nm, 5 min) and the sul::sequent addition of one equivalent of CF3 CD2H (yellow-green). Emission intensities detected at 335 id), '101 (el ::mrf 544 nm (fJ with thP. r.nnfig11rntion in Figurn 5 hP.fore ;int1 after ultraviolet irradiation (254 nm. 5 min). after the subsequent addition of one equivalent of CF ,CO,H and visible irradiation (524 nm, 15 min). Paye 16 Vulu111e 17 l:;:;ue 2 2004 Tilt: Spedr urn fluore~cence is always absorbed and blocked bv the photo-chromic switch positioned in front of the detector (Figure 'i). The inrensiry detectf'd at "31'i nm (din Figme 6) is al-ways low and independent of the switch state. Anrhracene emits at 401 nm (bin Figure 6) where only the two colored forms ME and MEH absorb. Therefore, irs fluorescence can travel to the detector unaffected when the photochromic switch is in state SP. However, it is absorced and blocked Ly Lhe µhowd1romic 1::fome11L wlm1 eid1er ME ur MEH ,ffe present. Con.sistencly, the inrens1ty detected at 401 nm (e in Figure 6) drops from a high to a low value upon ultraviolet irradiation, remains low after acidification and returns high after visible irradiation. Tetraeene emits at 544 nm (c in Fig11w 6) whew only ME ahsorh.~. Th11s, rlw f111orf'l>renre of this particular th1orophore can travel through the photo-chromic solution when the switch is either in scate SP or in ~Lctlt: MEH. lmLectd, il is dUsurLed ;iml LluL:ked Ly Ll1e µhu-tocluomic switch when ME is present. Consistently, the in-tensity detected at 544 nm (fin Figure 6) drops from a high to a low villue upon ultraviolet irradiation, returns high after acidincatian and remains high after o visible irradiation. Tn rhP c:nnfigi1r.irion of Fig11rP 5, one chemic:11 (~cir!) and rwo optical (ultraviolet and visible irradiation) inputs operate the photochromic switch and three optical outputs (naµlu.halt:ne, a11dmH.:t:11e a11J lelrnce11e emis,ium) reach a /1 /2 /3 01 02 03 Off Off Off Low High High Off Off On Low Low Hfr,J/1 Off On Off Low High H/g/J On Off Off Low Low Low Off On On Low High High On Off On Low Low High On On Off Low Low Low On On On Low Low High Raymo the detector. Each input can be either off or on. Tirns, a total of eight input combinations (a in Figure 7) cm address the rhorochmmic component. The cll1rr11r assoc.iatffl wirh the naphthalene emission is low for all eight coml:inations. The output corresponding tO the anthracene emission is high only for input comblnarions chat malmai.n the phom-chromic switcb in the low absorbing scatc SP. The output equivalent to the tetracene emission is low only for input cumbi11c1Lium Ll1<1l ensure Lht: IJhutudtrurnic cumpunenl Lu be in the high absorbing stare ME. Since the three inputs as well as the three outputs can only take two values, binary digits can be encoded i.n chem by assigni.ng a O co the off and law states :mcl a 1 to the on or high states. 10 The resulting rrnrh rahlP (h in Figt1re 7) .,hows rh,ir 01 is inpm irn-lP-pendent, 02 is related to 311 three inputs and 03 changes with the states of IL and 13 but 1s mdependenr oi l2. The equivalem lugic cirL:uil (c ir1 Figure 7) illustrdlt:~ d1e,1:: i11-pur-ourput correlations. The three inputs l l, 12 and l3 arc combined through two NOT gates, one OR operator and one AND function to deti.ne the state of 02. Instead, only Il and B control th.e st:J.te of OJ ofter one NOT gate and one OR opernror. The. cirrnir inpur T2 is nor ronnerrecl ro the circuit output 03. In fact, a glance at the fluorescence trace corresponding to 03 (fin Figure 6) shows chat vi,ible irradiation (12) lids 110 ir1ITue11ce ua Llte e111issiu11 illlensily. b 11 12 /3 01 02 03 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 ai Q) ii ., ., ., 03~,,~c 0 :i5 C C C ·;;; <( ., ., ., 5 ~ " " ~ 5 .J::. ~ : 5 :E .J::. i!! 12 02 Q. 'E 13 ., <( z Figure 7 co,relat1on between the inputs and outputs (a) of the configuratio, in Figure 5 and the eqLivalsnt tru'.h tat;le (b; and logic c1rcu1t (c). The Spectrum Volume 17 Issue 2 2004 Page 17 Raymo ---=---------=::::::a-•===:..----========:::::.ac==-■aiiillKl.::'.'!-~=:..m::::a-ic::::::=:::s;:__=--=..=--=....::::.....::::____::::_=--=-=-=-=-Our rPsnlrs rlPmonsrrnrP rhar inrPrmolt>c11l::1r f 11ort>s,t>ncP modulation is a versatile mechanism for the manipulation of optical signals. Indeed, the careful selection of compat-ible fluorophores and phocochromes offers the opportunity Lu expluiL i11 1ull dte pardlldbm uf dtt: pltuLUuic Jumain. 11 Binary digtts can be encoded on slgnals of distinct wave-lengths and processed simultaneously through the same region of space wimout information scrambling. It is im-porcam to stress, however, that our molecular ensembles remain far form practical applications 111 me context of signal processing. They certainly prove the viability of the conctpt, bur suffer serious limitations. In focr. methods ro reproduce these operating principles with rigid materials, rather than with liquid componencs, as well as molecular designs toleracing multiple switching cycles with no decom-µo,iLion ,mJ oUIJIJUrLing ultr,tfa~L ~wiLching ~µtt:lb H\UH be devised. Only after the identification of viable solutions to these challenging problems, practical strategies to elabornre and score information on the basis of intermolecular fluores-cence modulation might eventually emerge. Acknowledgments We rh:mk rht Narion,il SciePce Foundation (CAREER Awad CHE-0237578) and the University of Miami for financial support. References l. Guyton, A. C.; Hall, J.E. Human Physiology and Mechanisms of I)i5ease; W. B. Saunders Company: Philadelphia, 1997. 2. Sienko, T., Admatzky, A., Rambidi, N. G .. Conrad, M., Eds. Molecular Computing; MIT Press: Cambridge, 2003. 3. (a) de Silva, AP; McClean, G. n.; McClenaghan, N. 0.; Moody, T. S.; Weir, S. M. Nadir. Chem. 2001, 49,602. (b) Raymo, F. M. Adv. Mater. 2002, 14, 401. (c) Steimz, D.; Remade, F.; Levine, R. D. Chem. Phys. Chem. 2002, 3, 43. (d) Balza.ni, V.; Credi, A.; Vencuri, M. Ch~n. Phys. Chem. 2003, 4, 49. (e) de Silva, A. P.; McClenaghan, N. U. Chem. Eur. J. Z004, IC, 574. 4. (a) de Silva, AP.; McClenaghan, N. D.). Am. Chem. Soc. 2000, 122, 3965. (b) Brown, G. ].; de Silva, A. P.; Pagliari, S. Chem. Commun. 2002, 2461. 5. Stojanovic, M. N.; Stefanovic, 0. Nature Biotech. 2003, 21, 1069. 6. (a) Oiirr, H., Bouas-Laurenr, H., Eds. Phowchromism: Molecides and Sysrems; Elsevier: /\msccrdam, [990. (b) Crano, J. C., GuglidmcLti, R., Ed5. Organic Phowchromic and Thermochrcmic Compounds; Plenum PrPss: NPw York-, 199:). (c-) lriP, \-1., Fd. r.lu>m. Ri'u. 2000, 100, 1683. 7. (a) Raymo, F. M.; Giordani, S. Org. Lm. 200L, 3, 1833. (b) Raymo, F. M.;G1ordani, S.J. Am. Chem. Sue. 2002, 124, 2004. 8. (a) Bertelson, R. C. In Phocochromism; Brown, G. H., Ed.; Wiley: New York, 1971, p 45. (b) Guglielmetti. R. In Photochromism: Molecules and S)•stems; Durr, H., Bouas-Laurem, H., Eds.; Elsevier: Amsterdam, 1990, pp 314 and 855. (c) Bercelson, R. C. Organic Photochromic and Thermochromic Compounds; Crano, J.C.. Guglielrnetti, R., Eds.; Plenum Press: New York, 1999; Vol. I, p I 1. (d} Berkovic, G.; Krongauz, V.; Weiss, V. Ch.;,m. Rev. 2000, JOO, 1741. 9. (a) Raymo, F. M.; Giordani, S.J. Am. Chem. Soc. 2001, J 23, 4651. (b) Raymo, F. M.; Gion.hmi, S.; White, A. J.P.; Williams, D. J. J. Org. Chem. 2003, 6tj, 4151,. 10. Mitchell, R. J. Microprocessor Systems: An /ruroduccion; Macmillan: London, 1995. 11. Nolte, D. 0. Mind at Light Speed: A New Kind of Intelligence; Free Press: New York, 2001. About the Author Fran9isco M. Raymo recei•1ed a Laurea in Chemistry from che UniversiLy of Messina {Italy) in 1992 and a Ph.D. in Chemistry from the Universicy of Birmingham (UK) in 1996. He was a postdoctoral research fellow at the University of Birmingham (UK) in l9i6-l997 and at the University of California, Los Angeles (USA) in 1997-1999. He was appointed Assistant Professor of Chemistry at the University of Miami in 2000 and promoted co Associate Professor in 2C04. He was honored with a CAREER .Award from the Narion;;il Science Foundation (USA) in 1002- His research interests combine the design, synthesis and analy-sis of functional molecule-1:xised materials. Specifically, he i.s developing electroactive fi.lms, fluorescent coarings and photochromic glasses for chemical sensing, energv conver-sion and signal processing applications. He is the author of more than 100 publications in the areas of chemical synthesis, computational chemistry, m::iterials science and supramolecular chemistry. Paye 18 Vulu111e 17 l:;:;ue 2 2004 Tilt: Spedrurn 

Intermolecular Fluorescence Modulation: A Mechanism to Process Optical Signals With Molecules

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Intermolecular Fluorescence Modulation: A Mechanism to Process Optical Signals With Molecules

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