ISSN: 2277-3754 Influence of Solvent Environment on the Shpol'skii Spectr of Anthrcene nd Benzonthrcene Molecules Firs J. Kdhim, As J. Al-Wtttr, Bh T. Chid, Asm N. Ahmed Deprtment of Physics, College of Science, University of Bghdd, Bghdd, Irq Astrct--- In the present work, the low temperture spectroscopy of nthrcene nd enzonthrcene molecules in different solvents hs een studied t fixed molr concentrtion. The temperture rnge hs een tken from room temperture down to liquid nitrogen temperture (77K). Polr nd nonpolr solvents; Ethnol, Isopropnol, Hexne, Heptne nd Nonne, hve een used in order to study the environment effect on the sorption nd fluorescence spectr of these molecules. Some of the spectroscopic prmeters hve een determined s functions of solvent polrity nd temperture. Thus, the fluorescence lifetime hs een mesured using the PTI instrument. The results indicte tht the nd width t FWHM increses with incresing the solvent polrity nd temperture, while the pek emission cross section decreses with incresing of solvent polrity nd tempertures. The Shpol'skii spectr of these molecules show clerly enhncement, in which shrp nd highly- resolved fine structure spectr hve een oserved t 77K nd thus, give the est results t 77K. Index Terms Anthrcene nd Benzonthrcene molecules, low-temperture spectroscopy, Fluorescence lifetime I. INTRODUCTION The temperture effect is one of the importnt prmeters tht one my vry during spectroscopic mesurements. The low temperture spectroscopy technique y Shpol skii method is discovered in 1950, nd used to study the spectroscopic properties nd structure of polytomic molecules [1]. In 2005 M.A. Hidekker studied effect of solvent polrity nd solvent viscosity on the fluorescent properties of moleculr rotors. The photo- physicl chrcteristics depend on their environment [2]. In 2008 G.I.Romnovsky wrote n rticle tht covers the history of the discovery of the Shpol skii effect on which sensitive luminescence method for determining some orgnic compounds nd nlyzing their mixtures is sed [3].In 2013 Petlehner nd A.Slenczk studied electronic spectroscopy of 9, 10 dichloronthrcene in side helium droplets. Electronic spectr of 9, 10-dichloronthrcene in helium droplets were presented nd compred with corresponding gs phse spectr to unrvel the influence of the helium environment [4].Polycyclic romtic hydrocrons (PAHs) re usully defined s group of chemicls consisting of two or more enzene rings (C 6 H 6 ) [5]. Anthrcene (C 14 H 10 ) is solid, plnr, consisting of three fused enzene rings, derived from residues of therml pyrolysis, enzonthrcene (C 12 H 18 ) consisting of four 164 fused enzene rings nd form yellow sustnce [6, 7]. They my e lso studied under other suitle conditions in solid solutions or in mtrix of nole gs [8]. For mny romtic molecules, in liquid phse, the position nd the structure of the fluorescence spectrum re strongly dependent on the solvent. The wvelength shift cn often e correlted with chnges in solvent dielectric constnt nd the solvent index of refrction [9]. The solvent shift rises ecuse the excited stte of the guest (solute) intercts with the solvent to different extend thn does the ground stte of the guest [10]. In ddition to the spectrl shifts, the virtionl structure of the fluorescence nd sorption spectr re influenced y interctions with the solvent environment. If the moleculr spectr exhiit virtionl structure in the vpor phse, this structure is normlly roder nd more diffuse in solution, nd the chnge of environment my modify the virtion spcing nd the shpe of the Frnck- Condon envelope [11]. The Shpol skii effect is of importnt scientific nd pplied vlue. First of ll, it opens new wys to study the properties nd structures of orgnic molecules, inccessile for other spectroscopic methods. In prticulr, the Shpol skii spectr llow the determintion of the structure of electronic nd virtionl levels in complex orgnic molecules with high ccurcy nd the investigtion of vrious mnifesttions of fine intr-nd intermoleculr interctions [12]. The fine liner structure of the Shpol skii spectr is responsile not only for their extreme specificity nd differences even for molecules with similr structures, ut lso for their high line intensity. The conventionl rod nds turn to nrrow lines; this determines their exclusive prcticl vlue s method for the detection nd quntittive determintion of individul orgnic compounds in complex mixtures [13,14]. This method will e used in the current reserch. In the present work, the spectroscopy of some condensed romtic compounds with different structure series re studied t low temperture down to 77K. Also the spectroscopic prmeters of these romtic in different types of solvent re investigted to descrie the solvent effect on these prmeters t low tempertures. II. EXPERIMENTAL The solution of nthrcene nd enzonthrcene molecules ws prepred y dissolved into specific solvent t constnt concentrtion (1 10-4 M). Heptne, Nonne, Hexne, Ethnol nd Isopropnol were used s
Asornce ISSN: 2277-3754 solvents nd were selected ccording to moleculr size nd polrity. The solution ws introduced to vessel V which is connected to the cell C, y mens of funnel. The smple ws frozen y immersion the vessel V in liquid nitrogen. The whole pprtus ws evcuted nd closed through the tp T. The system ws pumped down to pressure of 10-2 mr nd the t T then closed gin. The whole system could then e disconnecting from the vcuum line. The vessel ws then re- immersed in liquid nitrogen nd the system is then re-evcuted y connecting it gin to the vcuum line. The solution then ws trnsformed internlly to the cell c. The cell ws then seled off while still eing connected to the pump nd immersed in liquid nitrogen. The residul pressure ws less thn 10-2 mr prior to seling off the cell. There re two dvntges in using the vessel V; to prevent crcking of the cell fter freezing the solution nd to degs the smple. The low temperture spectroscopy down to 77K ws chieved using suitle cryostt, were mde of luminum nd consist of thermocouple of Ni Al, Ni Cr nd connected to temperture controller (Thermometer Type -K).This thermocouple ws used for precise temperture mesurements. The thermocouple ws directly ttched to the smple cell. For low tempertures (from 300-77K) liquid nitrogen ws used s refrigernt. Lowering the temperture ws chieved y controlling the liquid nitrogen flow rte. UV-Visile sorption spectr of the prepred solutions were recorded t room temperture nd t 77K, using UV-Visile doule-em (Hlogen nd deuterium lmps) spectrophotometer (Metertech SP-8001 UV/Visile) which opertes in wvelength rnge of 190 nm to 1100 nm. On the other side, the fluorescence spectr of prepred solutions were otined using spectrofluormeter setup contins GN UV-LED light source (λ ex =382nm), connected to DC power supply (PS-1502DD). The resulting fluorescence spectrum ws nlyzed using Jrrell Ash monochrometer (model 82-000, which opertes in wvelength rnge from 190 nm to 600 nm with resolution 0.2 Å in the first order). The instrument hs 0.5 meter focl length, with eight speed electric drive, nd plne reflection grting of 1180 groove/mm. The detection unit ws photomultiplier PMT (type R 666 Hmmts) which ws connected to n X-Y recorder (Siemens) to detect the output signl. The PTI (Photon Technology Interntionl) lifetime instrument ws used to mesure the fluorescence lifetime for some of the prepred smples. This instrument includes: nnosecond pulsed LED excittion sources in different rnges from 300 nm to700 nm, nd fst PMT in the rnge (185-900 nm) s detector. 165 III. RESULTS AND DISCUSSION The study of sorption spectr is the first step for descriing the excited electronic sttes of molecules s well s much informtion out to So Svn excittion trnsitions of the molecules under study. Anthrcene molecule hs sorption spectrum rng etween ~ 200-390 nm, of So Svn. Figure (1,) shows the sorption spectr of nthrcene solute in heptne solvent t room temperture nd 77K.Figure (1,) lso, shows the sorption spectr of enzonthrcene solute in nonne solvent t 300K nd 77K. 1.4 1.2 1 0.8 0.6 0.4 0.2 77K 300K 0 290 310 330 350 370 390 Wvelength (nm) Fig. (1): The Asorption Spectr of (10-4 M) of () Anthrcene in heptne () Benzonthrcene in nonne t 300K nd77k According to figure (1,), four sorption nds re oserved t round 321nm, 337nm, 355nm nd 374nm nd due to the electronic trnsition (S 0 -S vn ) of the condensed romtic hydrocrons. Clr's,[15] distinguished three types of sorption nd systems (α, p, β) for these compounds, nd clssified them y their intensity, virtionl structure nd frequency shifts. In Anthrcene molecule the α-nd ecomes oscured y
ISSN: 2277-3754 the more intense p-nd. The spectrl line width is shrp t 77K compred with 300K. This give n indiction tht the sorption trnsition strt from S 00 stte to S vn nd thus reducing the other effects such s hot nd trnsitions. c d Figure (1,) the recorded sorptions spectr of Benzonthrcene solutions s function of solvent type show four nds t round 327nm, 342nm, 360nm nd 385nm. The sorption nd t out 361nm (0-1trnsition) represents p-nd ccording to Clr's clssifiction [15], while α-nd ppers in the minimum of the sorption spectrum 387nm. It cn e seen the clerly enhncement in the spectrl line width t 77K. The sme ehvior is oserved s in Anthrcene molecules. The fluorescence mesurements of Anthrcene nd Benzonthrcene solutions, which were recorded using spectrometer fluorescence setup s descried in section experimentl, were chieved t different tempertures strting from room temperture, pssing through freezing point of ech solvent nd down to liquid nitrogen temperture. The fluorescence spectr of Anthrcene solution in different solvents re illustrted in figure (2,,, c nd d). In figure (2), it cn e seen three fluorescence nds. The nd width t full width hlf mximum is t out 378nm, 400nm nd 427nm.The fluorescence nd t 400nm is due to 0-1 trnsition [11] nd show higher intensity compred with the other two nds. Also, the fluorescence spectrum ecomes shrp nd more nrrowing s the temperture down to freezing point of ech solvent nd 77K. Thus, the enhncement in spectrl line width cn e clerly oserved in the low temperture rnge. In the sme wy, the fluorescence spectrum exhiit virtionl structure t frozen solution. Some qusi-line fluorescence spectr disply doulet or multiple structure, thus the spectr consist of two or more series of lines. Shpol'skii proposed tht this multiple structure rises from locl differences in this field in which the fluorescent solute molecules re situted [16].This structure is normlly roder nd more diffuse t 223K nd 300K. The optimum conditions for the ppernce of qusi-liner spectr of liner molecule re otined when the moleculr length of the solvent nd the solute molecule re mtched. The est result of the virtionl structure of fluorescence spectr oserved in Heptne solvent whose liner moleculr sizes 10A re close to or coincide with the sizes of the Anthrcene molecule 10A. The fluorescence spectr of enzonthrcene solute in different solvents re show in figure (3,,, c nd d). Fig.(2): The Fluorescence Spectr of (10-4 M ) of Anthrcene t Different Tempertures in () Heptne () Hexne (c) Isopropnol nd (d) Ethnol 166
Fluorescence Intensity 7 6 5 4 3 2 1 ISSN: 2277-3754 0 370 390 410 430 450 470 490 510 530 550 Wvelength (nm) 223K 180K 77K 300K In figure (3), it cn e seen three fluorescence nds is t out 395nm, 403nm nd 409nm. The fluorescence nd t 403nm is due to 0-1 trnsition nd show higher intensity compred with the other two nds [11]. The moleculr spectr exhiit virtionl structure t freezing point of ech solvent nd 77K, so tht the spectr consist of two or more series of lines. Shpol'skii proposed tht this multiple structure rises from locl differences in this field in which the fluorescent solute molecules re situted [16]. One cn oserve the highly resolved fine structure with Nonne solvent t freezing point (220K) nd 77K my e ttriuted to this solvent whose liner moleculr size (13.7A ) pproches the sizes of the enzonthrcene molecule (12.8A ). The spectrl line widths re nrrowing t frozen solutions, due to the miniml solvent-solute interction. The est result of Benzonthrcene solutions oserved t 77K, ecuse of reducing the solvent-solute interction. Figure (4, ) descrie the vrition of oth full width t hlf mximum (FWHM) nd pek emission crosssection (σ p ) with solvent polrity for Anthrcene solutions nd figure (4,c nd d) show the temperture s function of FWHM nd σ p t different solvents. c d Fig.(3): The Fluorescence Spectr of (10-4 M ) of Benzonthrcene t Different Tempertures in () Nonne () Hexne (c) Isopropnol nd (d) Ethnol 167 Fig.(4) Dependence of () FWHM nd () σ for 0-1 p trnsition of nthrcene on the polrity of solvents t
ISSN: 2277-3754 These figures, it cn e oserved clerly incresing FWHM with increse of solvent polrity nd temperture, while the pek emission cross-section decresing with increse of solvent polrity nd temperture. Figure (5, ) for Benzonthrcene solutions show the solvent polrity s function of FWHM nd σ p t different tempertures nd figure (5,c nd d) show the vrition of temperture with FWHM nd σ p t different solvents. d Fig.(5) Dependence of () FWHM nd () σ p for 0-1 trnsition of enzonthrcene on the polrity of solvents t different tempertures nd (c) FWHM nd (d) σp on the Temperture for different solvents The results indicte tht the FWHM incresing with increse of solvent polrity nd temperture, while the pek emission cross-section decresing with increse of solvent polrity nd temperture.my e ttriuted to the effect of low tempertures, when the solute molecules re semi-isolted out molecules solvent. Therefore, the ehvior mkes of these molecules s in gs phse [3]. The fluorescence decy of Anthrcene nd Benzonthrcene solutions in different solvents is show in figure (6,, ). c 168
ISSN: 2277-3754 Fig.(6): Fluorescence decy of the () Anthrcene () Benzonthrcene solution in different solvents t 300K The results indicte tht the fluorescence decy of these molecules indictes tht no considerle chnge occurs in the fluorescence decy shpe y sustitution. This indictes tht there is no specific chemicl effect of the solvents t 300K. IV. CONCLUSIONS Spectroscopic study of Anthrcene nd Benzonthrcene molecules hs een investigted s function of oth solvent polrity nd temperture. The room temperture sorption spectr of these molecules re similr in different solvent, nd indicte no specific solvent chemicl effect on the solute electronic stte. On the other side, the low temperture sorption spectr show smll lue shift with nonpolr solvent due to the specific solvent- solute interctions is miniml. The Shpol'skii spectr of these molecules show clerly enhncement, in which shrp nd highly- resolved fine structure spectr hve een oserved t 77K. Also the mtching in the moleculr size etween solvent nd solute molecules hve ply key rule for otining high resolution spectr with virtionl structure t low tempertures. Thus, heptne nd nonne solvents for Anthrcene nd Benzonthrcene solutions, respectively, give the est results compred with other solvents. REFERENCES [1] E. V. Shpolskii," Line Fluorescence Spectr of Orgnic Compounds nd Their Applictions". Soviet Physics Uspekhi, 3,522-531 (1960). [2] M.A. Hidekker "Effects of solvent polrity nd solvent viscosity on the fluorescent properties of moleculr rotors nd relted proes" Bioorgnic Chemistry 33, 415 425(2005). 169 [3] G. I. Romnovsky, "E.V. Shpol skii nd His Effect" Journl of Anlyticl Chemistry, 64,755 760, (2009). [4] D. Pentlehner nd A. Slenczk," Electronic spectroscopy of 9,10-dichloronthrcene inside helium droplets" Journl of Chemicl Physics, 138, (2013). [5] Agency for Toxic Sustnces nd Disese Registry (ATSDR). Pulic Helth Sttement, "Polycyclic Aromtic Hydrocrons" Atlnt, GA: U.S, Deprtment of Helth nd Humn Services, (1995). [6] Hns Beyer, "Orgnic Chemistry", Oswrd Schmidt KG, Germny, 1963. [7] C. A. Prker, "Photoluminescence of Solutions", Elsevier, Amsterdm, 262-267(1968). [8] M.A. Wh "Structure nd Properties of Mterils". Alph Science,1-3(2005). [9] A.F.Sfrzdeh-Amiri, "Effect of lcoholic solvents on the dectivtion of the excited stte of 4-(dicynomethylene)- 1, 2, 3,4tetrhydro-N methylquinoline"chem.phys, 12, 4999-5001(1989). [10] R. B. Cundll nd L. C. Pereir "Temperture nd solvent effects on the fluorescence of some simple romtic molecules" J. Chem. Soc., 2, 68, (1972). [11] J.B.Birks,"Photophysics of Aromtic Molecules", Wiley Interscience, London, pp. 52,54,56,58,109,118 (1970). [12] B.Meyer, "Low Temperture Spectroscopy" Americn Elsevier, New York (1971). [13] Wlter B. Wilson, Andres D. Cmpigli,"Anlysis of coeluted isomers of high-moleculr weight polycyclic romtic hydrocrons in high performnce liquid chromtogrphy nd time-resolved Shpol skii spectroscopy", Journl of Chromtogrphy A, 1218,6922 6929, (2011). [14] G. I. Romnovsky, "E.V. Shpol skii nd His Effect" Journl of Anlyticl Chemistry, 64,755 760, (2009). [15] E. Clr, "Polycyclic Hydrocrons, Acdemic Press, London, New York; Springer- Verlg, Berlin, (1964). [16] Shpol skii, E.V, Zhurn. Prikl. Spektrosk. 7, 492 (1967).