ARTICLE pubs.acs.org/JPCA
Single- and Two-Photon Excited Fluorescence in Organic Nonlinear Optical Single Crystal 3-(1,1-Dicyanoethenyl)-1-phenyl-4, 5-dihydro-1H-pyrazole Andrzej Miniewicz,* Krystyna Palewska, Lech Sznitko, and Jozef Lipinski Institute of Physical and Theoretical Chemistry, Department of Chemistry, Wybrzeze Wyspianskiego 27, Wroclaw University of Technology, 50-370 Wroclaw, Poland ABSTRACT: Fluorescence of nonlinear optical organic single crystal of 3-(1,1-dicyanoethenyl)1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) excited by a nonabsorbed light pulses from Q-switched Nd:YAG laser λ = 1064 nm as well as absorbed λ = 532 nm light is reported. Two mechanisms of two-photon excited fluorescence are considered: (i) direct two-photon excited fluorescence and (ii) single-photon excitation due to reabsorption of light generated in process of second harmonic generation (SHG) by the crystal due to its nonlinear optical properties. Strong anisotropy of fluorescence that has been observed is linked with uniaxial molecular alignment. Fluorescence decay profile shows two- exponential decay with lifetimes of emitting species of 3.7 and 5.6 ns at 293 K. The excitation and fluorescence spectra of the DCNP single crystal have been measured at 294 K and in function of temperature down to 77.4 K. The strong bathochromic shift of fluorescence spectrum in crystal with respect to fluorescence of DCNP molecule in solution is observed and interpreted with possible formation of molecular aggregates.
1. INTRODUCTION Organic nonlinear optical (NLO) crystals have recently invoked a large interest due to their potential application in the field of nanophotonics and biological imaging as the nonlinear probes in the near-field multiphoton fluorescence excitation imaging, for example, two-photon excited fluorescence (TPEF)1,2 and second harmonic generation (SHG).3,4 Specifically, SHG signals depend on the orientation, polarization, and local symmetry properties of nanocrystals in surrounding medium. Large NLO crystals are of interest as sources of a THz pulse generation through optical rectification of near-infrared laser pulses.57 Organic single crystal of 3-(1,1-dicyanoethenyl)-1-phenyl-4,5dihydro-1H-pyrazole (DCNP) has an optimum arrangement of the molecules for efficient electro-optic (Pockels) effect with large electro-optic coefficient r333 = 8.7 1011 m/V at 632.8 nm (χ333( ω;ω,0) = 2.3 109 m/V).8 The highest electro-optic coefficient of the well-known organic DAST crystal is r111 = 7.7 1011 m/V at 800 nm.9 In this paper we report on the fluorescence properties of a single crystal of DCNP, which to our knowledge has not been yet addressed in literature. The interest in fluorescence measurements was stimulated by the observation of an intense reddish luminescence emerging from the crystalline sample of the DCNP when illuminated with nanosecond pulses of Nd:YAG laser working at 1064 nm wavelength. At this wavelength, fluorescence can be excited either by two-photon absorption process or reabsorption of SHG light. Observed by us, the effect can be easily explained by the efficient generation of doubled in frequency light of Nd:YAG laser, that is, λ = 532 nm due to second harmonic generation process in a noncentrosymmetric DCNP crystal. A subsequent absorption of this light leads to an r 2011 American Chemical Society
efficient excitation of the compound which emits red light in all directions. To reveal the detailed mechanism of this process, we performed the characterization of fluorescence properties of the DCNP crystal by measuring its fluorescence and excitation spectra under various conditions of excitation as well as in the function of temperature from room temperature down to T = 77.4 K.
2. MOLECULAR AND CRYSTALLINE PROPERTIES OF DCNP 2.1. Molecular Properties. The DCNP molecule of molecular formula C13H10N4 (Mw = 222.25 g) is schematically shown in Figure 1 where atom numbering scheme and coordinate system showing molecular position with respect to the c-crystallographic axis and the main refractive index axis zopt. Molar refractivity of DCNP calculated from the bond and atom increments amounts to 68 ( 0.5 cm3, molar volume 194 ( 7.0 cm3 and mean molecular polarizability Æαijæ amounts to 27 1024 ( 0.5 1024 cm3, all these values were calculated within ACD/ChemSketch freeware version 5.12 program. DCNP represents an optimized for large molecular hyperpolarizability Æβijkæ system with internal charge transfer (CT) between donor (phenyl ring) and acceptor (dicyano group) ameliorated by the presence of a pyrazole “reduced” aromatic ring. Molecules of this type, called “pushpull” molecules, exhibit unidirectional internal charge transfer (ICT) electron displacement which is the source of their optical nonlinearity.10 Received: May 12, 2011 Revised: August 23, 2011 Published: August 26, 2011 10689
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Figure 1. Chemical structure of the molecule 3-(1,1-dicyanoethenyl)1-phenyl-4,5-dihydro-1H-pyrazole (C13H10N4). Atom numbering and direction of long molecular axis C13C3 is given with respect to the crystallographic c-axis (∼27).
Solvatochromic measurements of the DCNP molecule in organic solvents have shown its high potential as nonlinear optical chromophore with estimated mean molecular hyperpolarisabilty along the charge transfer direction βCT = 1.87 1032 m/V, (β0 = 44.7 1030 esu at zero energy11). Ground and first excited state dipole moments of DCNP μ00 and μ11 were calculated in ref 11 as μ00 = 4.4 D and μ11 = 10.0 D. In more recent papers,12,13 μ00 = 7 D in DCNP has been reported. Thirdorder nonlinear molecular optical property, namely imaginary part of molecular second hyperpolarizability representing two-photon absorption process, has been estimated by the Kerr ellipsometry technique14 and amounted to γxxxx = 2.0 1048 m5V2. Long molecular axis direction is given by a line linking C2 and C13 atoms. This line makes angle 27 with the c-crystallographic axis. Long molecular axis is near parallel with the directions of ground state μ00 and first excited μ11 dipole moments. Interesting molecular properties of DCNP molecule, that is, its high ground state dipole moment μ00 together with a large molecular hyperpolarisability βCT prompted to use this molecule as a NLO chromophore dopant to polymer matrix for fabrication of NLO active polymers by electrical poling technique.12,13,15 2.2. Single Crystal Growth and Structure Characterization. DCNP was synthesized and purified according to procedure described by Allen et al.8 where also the crystallographic structure of DCNP was determined via an X-ray analysis. It has been shown that DCNP crystallizes in the monoclinic space group Cc (point group m), though the geometry is pseudo-orthogonal. Single crystals of DCNP were grown from toluene using slow cooling of precipitated solution. The resulting single crystal plates have thicknesses between 0.5 and 1 mm and areas between 5 and 10 mm2. The optical quality of the crystals obtained by us has been checked with a polarization microscope on the (010) natural face of the crystal. Under polarizing microscope, we proved that the samples chosen for fluorescence studies are monocrystalline, that is, extinction was uniform for the whole area of the crystal. The principal zopt-axis of refractive index ellipsoid lies at an angle of 27 with respect to the crystallographic c-axis. Molecules are aligned along this zopt-axis, and the x-optic axis is orthogonal to the molecule plane (cf. Figure 1). Refractive indices of this positive optically uniaxial crystal at 633 nm have been estimated to be nx = 1.9 ( 0.1 and nz = 2.7 ( 0.1.8 In the paper of Allen et al.8 is also shown an optical absorption spectrum of the crystal taken with light polarization along the zopt-axis and perpendicular to it, for example, along the xopt-axis.
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An absorption edge for Ex is around 590 nm, while for Ez much weaker absorption (α = 17 cm1) appears already around 620 nm. The more precise structural studies of this compound by X-ray and neutron diffraction have been done by Cole et al.16 at 90, 100, 200, and 290 K. The cell parameters a = 11.8751 Å, b = 12.3735 Å, c = 7.8876 Å, β = 90.412, Z = 4 at 290 K have been measured.16 The molecules are well unidirectionally aligned defining polar axis || zopt and the structure is stabilized by the presence of two intermolecular CH 3 3 3 N hydrogen bonds. Parallel and polar alignment of molecules is directly responsible for the extremely large (r33 = 87 pm/V at 632.8 nm) electro-optic coefficient of DCNP crystals, represented by the dominating second order susceptibility term χ(2) 333(ω;ω,0) = 2.3 109 m/V.17 From the detailed analysis of the X-ray structure, the phenyl ring librations have been revealed. They are accompanied by librations of nitrile groups. In the crystal, molecules organized in a head-to-tail arrangement such that the nitrile groups in one molecule are adjacent to the phenyl groups in the next molecule. No phase transition occurs down to 90 K, and the principal thermal expansion coefficients: α11 = 12.08 105 K1, α22 = 3.43 105 K1, α33 = 0.31 105 K1 have been estimated.16
3. EXPERIMENTAL SECTION Light emitted from the crystal was collected by a multimode optical fiber, which was precisely positioned at a distance of 2 mm from the sample surface. Light collected by the fiber end was spectrally analyzed with an Ocean Optics USB 2000 spectrometer equipped with a diode array detector. The spectral resolution of the system was near 1 nm. Spectrometer output was sent to a computer for data acquisition, averaging, and presentation. For the single-photon pulsed excitation of fluorescence we used the same Nd:YAG laser with built-in NLO crystal for frequency conversion. SHG light of λ = 532 nm with pulse duration 7 ns incident normally to the sample. Optical fiber end was positioned at the input face of crystal at different distances from excitation area. For fluorescence anisotropy measurements we used the half-wave retardation plate mounted on rotatory stage and positioned on the 532 nm beam. The UVvis absorption spectra were recorded on Shimadzu UV-2500 spectrophotometer. The excitation and fluorescence spectra were carried out using a Hitachi FL-4500 fluorescence spectrophotometer within the range of 350700 nm where excitation was provided by a lamp light passing through a monochromator. The excitation light was incident at the sample under angle of 45 and the collected light made an angle of 90 with direction of excitation light. For low temperature measurements sample was placed in an Oxford Instrument cryostat cooled with liquid nitrogen down to 77 K. The luminescence lifetimes in single crystal at 293 K have been measured using time-correlated single photon counting technique on an IBH time-resolved spectrofluorimeter (IBH Consultants, Scotland, U.K.). The excitation source served 1.2 ns pulse laser diode working at the wavelength of 460 nm. Fluorescence lifetimes were estimated by fitting the decay profile using an interactive deconvolution procedure based on Marquardt algorithm with software supplied by IBH. The quantum chemical all-valence semiempirical GRINDOL18 method has been used for theoretical prediction of DCNP energy levels. Excitation energies and oscillator strengths of the DCNP molecule (in the dipole length approximation) were calculated 10690
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Figure 2. Fluorescence of DCNP single crystal at 290 K observed upon its direct excitation by 7 ns pulses of light at 1064 and 532 nm. For comparison, the fluorescence spectrum of the crystal, as obtained in a spectrometer with excitation at 468 nm, is also shown.
using the singly excited configuration interaction (CIS) approximation in our locally modified NDO-like (Neglect of Differential Overlap) method (GRINDOL program) in which oneelectron Hamiltonian matrix elements are calculated from semitheoretical formulas derived from Heisenberg equation of motion (for details see refs 19 and 20). The geometry of the DCNP molecule has been taken from the X-ray structure without further optimization. The ground and higher excited singlet state energies were calculated for the monomer of DCNP and an ensemble of four molecules in positions taken exactly from the crystallographic data at temperatures of 100 and 290 K.
4. FLUORESCENCE AND EXCITATION SPECTRA OF SINGLE CRYSTAL OF DCNP 4.1. Two-Photon Excited Fluorescence in DCNP Single Crystal. When the DCNP single crystal was illuminated with 7 ns
light pulses of wavelength λ = 1064 nm and energy density of 10 mJ/cm2, red luminescence light emerged from the crystal. In Figure 2 we show the spectrum of fluorescence of the DCNP crystal excited by 1064 nm infrared light pulses. The spectrum consists of a broad luminescence band situated at 632 nm and a less intense narrow line situated at 532 nm that evidently is a second harmonic of used fundamental wavelength 1064 nm light. Obviously, without special illumination conditions, realizing phase matching between ω and 2ω waves, the SHG light intensity could not be optimized. Small observed SHG intensity as compared to fluorescence intensity is a result of measurement geometry in which an optical fiber was not colinear with the path of fundamental frequency wave, so only the SHG scattered light was observed. In fact the highest intensity of SHG light is in the direction of infrared beam. The magnitude of SHG light is diminished also due to reabsorption of 532 nm light in the crystal bulk which exhibits a cutoff absorption wavelength at about 590 nm.8 Observation of both SHG light and fluorescence under these excitation conditions suggests that two different mechanisms could be responsible for the luminescence: (i) direct two-photon excited fluorescence process (TPEF) and (ii) reabsorption of SHG light by the crystal and subsequent onephoton excited fluorescence. Both processes may coexist only for noncentrosymmetric crystals or nanocrystals showing luminescent properties for doubled in frequency light excitation. Several research groups2124 already reported studies of materials showing both processes.
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In the case of the DCNP crystal there is not enough data to quantitatively compare the efficiency of the two mentioned above processes. However, both of them depend on the nonlinear optical molecular properties, which can be used for rough estimation of their efficiencies through comparison of corresponding cross sections σTPFE and σSHG. Typically for a onedimensional molecule with π-conjugation and resonant excitation σTPFE = 1050 cm4s/photon and σSHG = 1053 cm4s/ photon. However, axial alignment of molecular dipoles can enhance SHG response, because the growth with the square of number of molecules while TPEF growth linearly.25 Emission rate, that is, number of photons per second emitted from a certain volume of material illuminated by irradiance Iω for TPEF and SHG is dependent on respective cross sections σTPFE and σSHG:26 PTPEF ¼
1 NΦσ TPEF 3 Iω2 2
and
PSHG ¼
1 2 N σSHG 3 Iω2 2 ð1Þ
where N is the effective number of radiating molecules and Φ is the two-photon fluorescence quantum yield. The eq 1 is valid if one neglects the dependence on a particular experiment photon collection efficiency and excitation light polarization with respect to transition dipole moment. One can find in the literature expressions for two-photon absorption cross section in the following form:27 2 ω2 ð2πÞ2 μ0n μn1 σ TPFE ¼ 2 2 2 ð2Þ 2πFf ð2ωÞ 4n ε0 c n h2 3 ðωn0 ωÞ
∑
where transition dipole moments for involved states μmn and Ff density of final states have to be known. However, experimentally this coefficient could be estimated from the simple formula, if for a given molecule, the two-photon absorption is estimated by another technique: σ TPA ¼
αTPA E N
ð3Þ
where αTPA is experimentally measured two-photon absorption coefficient in m/W, N is number density of molecules in 1/cm3 and E is photon energy in J. Taking αTPA = 7 103 cm/GW, measured by nonlinear Kerr ellipsometry technique,14 number density as for a DCNP crystal8 N = 3.48 1021 cm3 and E = 1.868 1019 J, one gets σTPA = 3.7 1052 cm4s/photon ≈ 0.04 GM. In a crystal, this coefficient could be larger by several tens due to molecular interactions (in DAST NLO crystal amplification factor of ∼40 has been reported21) so one can safely assume that σTPA for DCNP crystal could be as large as 1.5 1050 GM . The cross section for second harmonic generation process for aligned molecules is approximated by the formula:26 σ SHG ¼
4n2ω ðh=2πÞω5 2 4 jβj ½m ðphoton=sÞ1 3πn2ω ε0 c5
ð4Þ
where β is molecular first hyperpolarizability and nω and n2ω are the refractive indices at the pump and SHG frequencies. Taking β value from the work of Bosshard et al.,17 β = 777.9 1030, esu = 288.75 1050 C3 m3 J2, and taking refractive index8 nω = 2.7 and assuming n2ω = 2.8, one obtains σTPA for DCNP crystal equal to 1.5 1055 cm4 s/photon. From this analysis and taking into account the scaling of SHG intensity with N2 and TPEF with 10691
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The Journal of Physical Chemistry A N one can conclude that the two effects may be equally probable as a source of observed fluorescence light due to large illuminated volume of DCNP crystal. In fact, both SHG light and fluorescence light are emerging from the crystal. Observation of SHG intensity dependence I2ω ∼ (Iω)1.6 instead of I2ω ∼ (Iω)2 may support the presence of reabsorption phenomenon of SHG light and subsequent fluorescence process. The presented rough estimations of the magnitude of the two effects will be further explored using microscopic approach for nanocrystals of DCNP. 4.2. Single-Photon Excited Fluorescence in DCNP Single Crystal. To compare the two-photon excited fluorescence with the single photon one, we switched the Nd:YAG laser to produce the SHG light λ = 532 nm and again illuminated the sample. The result of this experiment is also shown in Figure 2. One can observe a similar fluorescence band, but delicate new features have appeared: the main band maximum is red-shifted to 635 nm, and the band has narrower width at half-maximum; besides two shoulders have appeared situated at 596 nm and around 705 nm. For comparison, we measured the fluorescence spectrum of DCNP crystal using fluorescence spectrophotometer where excitation is provided by a lamp within 468 ( 5 nm range. The spectrum obtained in this way has its maximum red-shifted to 643 nm and is also showing the shoulders at 596 nm and around 705 nm. Attempting to understand the observed difference between 1064 and 532 nm excited fluorescence of DCNP crystal, we recall the optical absorption spectrum measured for single crystal of DCNP in ref 8. DCNP crystal shows dichroism in the region of 580630 nm. For light polarized along the main optical axis, the zopt-axis8 absorption coefficient amounts about α = 17 cm1, whereas for light polarized perpendicularly to this direction this coefficient is near 0 cm1. From the low temperature fluorescence spectra (cf. Figure 5), we know that there is a strong fluorescence band peaking at 592 nm. At room temperature, only its remnant is seen as a sideband of the dominating spectrum at the 640 nm band. We think that the reason for the difference is directly linked to the two sample excitations at 1064 and 532 nm light. In the case of single-photon excitation, the strong absorption of 532 nm light makes that the whole luminescence light comes from the near surface region (linear absorption coefficient α > 103 cm1). Penetration depth of 532 nm light is only a few micrometers. So the fluorescence light in the region of 600 nm is only weakly absorbed. The situation in the case of sample excitation by 1064 nm light is different because this light is very weakly absorbed. However, the intensity of SHG light that arises due to conversion of fundamental frequency ω into 2ω for any NLO crystal is dependent on the square of the optical path L of fundamental beam (i.e., ISHG L2 at perfect phase matching condition). Even if the perfect phase matching is not met, this implies that, close to the surface, intensity of 532 nm light is very small, but it could be much larger in the crystal bulk. Therefore, fluorescence light arising as a result of SHG light absorption is traversing through the crystal toward the surface where optical fiber collecting light is placed. This light polarized along the zopt-axis is much more reabsorbed in the region of 580630 nm than light at 640 nm. Simple estimation of amount of light which can reach the surface if excitation takes place at the depth of 2 mm gives the value of 3.3% assuming α = 17 cm1. This reasoning could explain the differences between the fluorescence spectra excited by 532 and 1064 nm light pulses and speaks in favor for the SHG induced fluorescence. In the next experiment we recorded the 532 nm pulse excitation fluorescence spectra of the DCNP crystal shifting the
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Figure 3. Luminescence spectra recorded for DCNP single crystal when excited by 532 nm laser pulses (7 ns duration) in the function of the optical fiber tip position with respect to the excitation laser spot. Inset: difference of the two spectra.
Figure 4. Fluorescence spectra of DCNP single crystal excited by 532 nm light and recorded by optical fiber spectrometer. Input light polarization has been rotated by 360. Inset: polar plot of fluorescence intensity at maximum vs angle between incident excitation light polarization plane and the zopt-axis.
position of the optical fiber tip with respect to the position of the excitation beam along the length of the crystal, that is, 3 mm apart. The two fluorescence spectra recorded in this way show distinct difference as is depicted in Figure 3. The shoulder that is clearly visible in the spectrum when the fiber tip is positioned very close to the excitation 532 nm beam spot disappears when the fiber tip is situated close to the end of the crystal. To highlight this feature, we present the difference of the two spectra in the inset to Figure 3. A broad band peaking at 596 nm is visible. The possible reason for the observation of different fluorescence spectra for the two positions of collecting light fiber tip in the case of 532 nm excitation may again be connected with dichroism of DCNP. For fiber position close to the excitation area reabsorption at 600 nm is weak and sideband is visible, while for the fiber end positioned at a distance of 3 mm from the excitation spot, the reabsorption at 600 nm removes the sideband from the spectrum. These results prompted us to perform more detailed spectroscopic studies of this crystal. Such a highly anisotropic crystal as DCNP should also show a pronounced fluorescence anisotropy. In Figure 4, we show the results of fluorescence anisotropy measurements for excitation with 532 nm coming from cw Nd:YAG laser doubled in frequency. Crystal remained at the fixed position, while linear polarization of excitation beam was rotated by a half-wave retardation plate. The highest fluorescence signal was observed when linearly polarized light of 532 nm wavelength has its polarization 10692
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Figure 5. Fluorescence spectra of a single crystal of DCNP excited at 468 nm wavelength in function of temperature in the range 29498 K.
Figure 6. Fluorescence emission and excitation spectra of single crystal DCNP taken at 77 K. Fluorescence was excited with λexc = 400 nm and excitation spectra were measured at λem = 640 and 700 nm.
plane parallel to the zopt-axis and the smallest in the perpendicular direction. With respect to the crystal c-axis, which is also parallel to the physical crystal edge, it makes an angle of 26. In Figure 4, the zopt-axis was chosen as the reference axis. Fluorescence anisotropy is shown in the inset to Figure 4 on a polar plot where the experimentally obtained points representing fluorescence maximum at λ = 638 nm are shown. The experimental points were fitted with the function F(α) = Amax cos2α, where α is the angle between the input light polarization and the zopt-axis. The quality of fitting is a proof that the observed luminescence is almost perfectly polarized along the long molecular axis and molecules are perfectly axially aligned. Recently, similar anisotropy for pure SHG response of DCNP crystal has been measured28 using 100 fs 80 MHz Ti-Sapphire laser working at 940 nm. The obtained SHG polarimetric results could be well fit using cos4α dependence: ISHG(α) = Imax cos4α. 4.3. Temperature Dependence of Excitation and Fluorescence Spectra in DCNP Single Crystal. Fluorescence spectra of DCNP crystal as a function of temperature were measured within a range of 77.4294 K in a spectrometer using 468 nm excitation wavelength. In Figure 5, we show how the fluorescence spectra evolve upon cooling down the crystal. One can observe the appearance of a strong band at short wavelength part of the spectrum that tends to dominate the spectrum at low temperatures. Its maximum is shifting linearly with temperature toward shorter wavelengths at the rate of 0.13 nm/K, and at 77.4 K, it is located at 592 nm. No phase transition is related to these changes
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Figure 7. Excitation spectra of DCNP single crystal measured via emission at 670 nm in function of temperature.
Figure 8. Fluorescence decay profile of DCNP single crystal observed at 647 nm at temperature 293 K. The line is a fit of experimental data to single exponential function of longer lifetime τ2 = 5.6 ns, shorter lifetime τ1 = 3.7 ns.
Figure 9. Normalized fluorescence spectra of DCNP dissolved in nhexane (3.5 mM), methanol (3.5 mM), and solid PMMA taken at 77 K. The spectrum of DCNP single crystal is presented for comparison.
as the structure of DCNP remains unchanged down to 100 K.16 At the same time, the position of the room temperature peak at 643 nm shifts negligibly with temperature. Moreover, the low intensity band is appearing at 702 nm. The three bands at 77.4 K are separated by 1320 ( 10 cm1 and the total area under those three bands remains approximately constant from room 10693
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Table 1. Positions of Main Absorption and Emission Bands in cm1 Observed Experimentally in a Single Crystal of DCNP and the Slowly Cooled Solution of DCNP in n-Hexane excitation spectrum in
excitation spectrum in
DCNP at 100 K (cm1) DCNP at 290 K (cm1) 18200 17560 16010
17220 16550
emission DCNP crystal
emission DCNP
at 100 K (cm1)
crystal at 290 K (cm1)
16790 15575
15590
n-hexane solution excitation n-hexane solution emission spectrum at 77 K (cm1)
spectrum at 77 K (cm1)
22000 (broad 3000)
20170 (monomer) 18710 (monomer) 15610 (aggregate)
14261
temperature down to 77.4 K. This suggests that there is a redistribution of excitation energy into different channels of luminescence without excessive nonradiative loss. In Figure 6, we demonstrate a comparison of the DCNP excitation spectra observed via fluorescence light emission at 640 and 700 nm and fluorescence spectrum excited with 400 nm light both taken at 77 K. Fluorescence spectrum at 77 K is quite different from the room temperature one (cf. Figure 2) and it is composed of three bands: situated at 586.8, 643.6, and the smallest one at 701 nm. The excitation spectrum of DCNP single crystal at 77 K is composed of several bands among which the most pronounced one has its maximum at 624 nm and next at 568 and 557 nm. As we noticed during measurements, the shape of the spectra depends on the emission wavelength (cf. Figure 6). There is a significant overlap between excitation and luminescence spectra in DCNP and no evident mirror symmetry is present that may suggest a difference in the molecular origin of the observed luminescence. Temperature evolution of excitation spectra are shown in Figure 7. At low temperatures, excitation spectrum observed via fluorescence at 670 nm is better resolved than that at higher temperatures. However, these spectra have relatively complicated structures. Excitations at wavelengths of 570 and 620 nm give the highest fluorescence intensity at 670 nm. The luminescence lifetimes in DCNP single crystal at 293 K have also been measured (460 nm, 1.2 ns pulse), and in Figure 8 the decay profile observed at 647 nm is shown. The fluorescence decay curve is not a single exponential function but is represented by a double exponential one. The fast decay component is characterized by the lifetime τ1 = 3.7 ns and the slow decay component by the lifetime τ2 = 5.6 ns. The two-exponential character of fluorescence decay in this crystal reflects the complex nature of the energy transfer after excitation including monomer excitation and its emission and aggregate formation and its emission. An extension of this study toward transient absorption spectroscopy measurements in function of temperature is envisaged. To understand the fluorescence spectra of DCNP single crystal, we performed spectroscopic measurements of DCNP dissolved in various matrices, namely, n-hexane, methanol, and solid PMMA. It is a frequently encountered phenomenon that, owing to strong intermolecular van der Waals-like attractive forces, dyes exhibit self-association. The aggregates in solution exhibit distinct changes in absorption and emission as compared to monomers. Generally two types of aggregates are formed, Jand H-aggregates.29,30 H- and J-aggregates are composed of parallel dye molecules stacked plane-to-plane and end-to-end and form two-dimensional dye crystals. The J-aggregates exhibit bathochromic shift of their absorption spectrum with respect to monomers and are usually highly luminescent with luminescence spectra shifted toward longer wavelengths.31 Spontaneous or forced formation of molecular J-aggregates is particularly interesting
due to their coupling with plasmon excitation in metal nanoparticles.3133 Looking at the results of fluorescence measurements for DCNP in various matrices at 77 K, gathered in Figure 9, where one observes a strong bathochromic shift, the supposition of J-aggregate formation cannot be ruled out. The DCNP molecule in n-hexane solution of 3.5 mM shows two groups of bands: one centered at 500 nm and a broad luminescence band peaking at 640 nm. Similar fluorescence, though less resolved, is observed in methanol solution of 3.5 mM with distinct broad bands centered at 550 and 640 nm. In PMMA there is only a single broad band at 550 nm. It seems that the spectra of DCNP in n-hexane and methanol depend on the rate of cooling of the solutions to the liquid nitrogen temperature. When the cooling is slow, one observes a build-up of group of bands shifted bathochromically more than 100 nm (cf. n-hexane matrix) with respect to luminescence of single molecule. When the cooling is fast, the bathochromically shifted spectrum has much lower intensity. The same behavior was observed for the methanol solution of DCNP. In Table 1, the energies of main transitions for single crystal of DCNP and in n-hexane solution are gathered. The n-hexane is a nonpolar solvent so the spectrum of molecular absorption is not strongly influenced due to solvatochromism, as in the case of the more polar methanol. J-Aggregates frequently exhibit fluorescence with a quantum yield often surpassing that of monomeric units of dyes. However, their presence is questionable as no red-shifted band narrowing is observed. Slow cooling of solution containing DCNP monomers promotes formation of molecular aggregates when confronted with a fast cooling, which freezes the molecules, thus, prohibiting the possibility of efficient aggregate formation. Apparently in the PMMA matrix the formation of aggregates is inefficient, possibly due to a large viscosity of this matrix. In closely packed structures, like molecular crystals, the coupling between molecules can lead to delocalization of the excited state of molecules, called Frenkel exciton.34 In the strong coupling case, a band of delocalized states is formed and the oscillator strength of the transitions is redistributed. The coupling between the aggregate exciton with phonons determines the line shape and the line width of the exciton transition leading frequently to exciton self-trapping, a so-called polaron state. The coupling of exciton with the lattice phonons leads to energy stabilization along the phonon coordinate.35 We think that effective excitonphonon coupling is responsible for the fluorescence spectrum changes in function of temperature in DCNP single crystal; however, exact understanding of this process is beyond the scope of the present work.
5. QUANTUM CHEMICAL CALCULATION OF ELECTRONIC PROPERTIES OF DCNP MOLECULE The quantum chemical all-valence semiempirical GRINDOL1820 method has been applied for evaluation of excitation energies of 10694
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Table 2. Transition Energies ΔE between S0 and Excited Singlet States of the DCNP Monomer, Associated Dipole Moments, Oscillator Strengths, and Transition Dipole Momentsa electronic states and nature of transition
ΔE in (cm1)
oscillator strength f of transition and transition
value and its components
dipole moments components (m) in [D]
|μ|
μx
μy
μz
f
mx
my
mz
4.46
3.74
2.43
0.26
S1 ππ* S2 ππ*
24 506/3.0383 eV 25 931/3.2151 eV
10.74 8.85
9.28 8.27
5.38 - 3.14
0.51 0.19
0.811 0.162
8.17 3.51
1.88 0.82
0.34 0.52 0.11
S0
a
dipole moment in [D] absolute
S3 nπ*
32 964/4.0871 eV
2.28
0.64
2.18
0.21
0.017
0.40
1.05
S4 mixed
36 527/4.5288 eV
2.64
0.12
2.63
0.23
0.013
0.61
0.62
0.05
S5 mixed
37 817/4.6887 eV
7.06
6.06
3.59
0.41
0.238
3.63
0.45
0.06
Positions of atoms were taken from crystallographic data measured at 100 K16 and vector components are given within this xyz-coordinate system.
Table 3. Comparison of Transition Energies between S0 and S1 and S0 and S2 States in DCNP Monomer and a Four-Molecule Cluster Taken from Crystallographic Data16 for T = 100 and 290 Ka 4 DCNP molecules as in the unit cell of crystal,
4 DCNP molecules as in the unit cell of crystal,
T = 100 K
T = 290 K
DCNP monomer T = 100 K
a
singlet excited
ΔE energy
oscillator strength
ΔE energy
oscillator strength
ΔE energy
oscillator strength
state
(cm1)
f
(cm1)
f
(cm1)
f
24214
0.1615
24436
0.1616
24389
0.0533
24612
0.0377
25095 25697
0.2359 0.8911
25175 25523
0.1532 0.4920
25933
0.1036
25805
0.0858
26116
0.0294
25933
0.0150
26387
0.5417
26211
0.1812
26804
1.9457
26813
2.7648
S1
24506
0.811
S2
25931
0.162
Each electronic state of monomer is split into four energy levels for a cluster.
DCNP molecule. The ground and higher excited singlet state energies were calculated for the monomer of DCNP and an ensemble of four near-neighbor molecules in positions taken exactly from the crystallographic data16 at temperatures of 100 and 290 K without a further optimization. The directions of dipole moments and transitions dipole moments represented by vectorial oscillator strengths all were calculated within the xyzcoordination frame adopted from ref 16. The results of calculations for the DCNP monomer are gathered in Table 2 and for the cluster of four DCNP molecules in Table 3. In the case of the DCNP monomer energies, ΔE0n were calculated for S0S1 up to S0S5 transitions. In the first excited state, S1, the dipole moment is almost 2.4 times larger than in the ground state and both are directed along the long molecular axis. Planar DCNP molecule optimized with AM1 Hamiltonian and calculated within MOPAC 6.0 program shows a ground state dipole moment |μg| = 5.2 D, while in the first excited singlet state it rises to the value of |μe1| = 8.62 D. So a strong dipoledipole interaction can be expected in the excited state. The transition dipole moment vectors for S0S1 and S0S2 define the same direction whereas the transition dipole moment for S0S3 is lying in quite different direction. For an ensemble of four molecules constituting the crystallographic unit cell, the energy splitting of original S1 and S 2 states is observed into four energy levels and in each case the lowest energy level has the highest oscillator strength.
Figure 10. Energy diagram for excited states of DCNP monomer and four-molecular cluster with atom positions taken from crystallographic data at 100 K. Energies are calculated by GRINDOL method with respect to S0 energy. Oscillator strengths f of the strongest transitions are given.
These results are graphically shown in Figure 10 and given in Table 3 Energy lowering is relatively small when compared to the monomer, but there is an inversion and enhancement in oscillator strength between monomer and an ensemble of molecules. The lowest energy transition originated from the S1 state have in a cluster considerably smaller oscillator strength than the lowest transition originated from the S2 state. 10695
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Table 4. Transition Energies between S0 and Excited Singlet States of DCNP Monomer and J-Aggregate Type Dimer and Trimer DCNP dimer JJa T = 100 K
DCNP monomer T = 100 K electronic states
ΔE energy (cm1)
oscillator strength
ΔE energy (cm1)
oscillator strength
ΔE energy (cm1)
oscillator strength
S1
24506
0.811
23874
1.8319
23554
2.7354
24783
0.0046
24385
0.0020
24852
0.0825 0.1786
S2
a
DCNP trimer JJa T = 100 K
25931
0.162
25812
0.1703
25760
25970
0.0040
25866
0.0005
25981
0.0494
Positions of all atoms have been taken from the DCNP X-ray structure at 100 K.
Figure 11. Energy diagram for excited states of DCNP monomer and J-aggregates: dimer, trimer, and an infinite chain of DCNP molecules calculated by GRINDOL method and basing on molecular geometry taken form the X-ray structure at 100 K. Oscillator strengths f of the transitions are given.
However, the calculated energies of transitions are still quite far from those observed for DCNP in solvent, matrices, and single crystal. The sources of discrepancy between observations and calculations could be due to large difference in values of dipole moments between excited (CT) state and ground state. This makes that calculated within GRINDOL method energies of excitation (absorption) for a molecule in a polar solvent (the crystal might be treated as a strong polar solvent) are too large. The discrepancy might be of the order of 1000 2000 cm1. Next we tried to calculate the transition energies of J-aggregate type dimer (head-to-tail arrangement) and trimer of DCNP molecules at positions that the molecules occupy in a single crystal at 100 K. As expected the energies showing the highest oscillator strength and originated from S0 to S1 and S0 to S2 transitions have been lowered which is shown in Table 4 and Figure 11. For an infinite number of molecules making a J-type aggregate, the energy difference can be estimated from known energies for monomer and dimer as ΔENf∞ = 2(ΔEM ΔED) = 2(24506 23874) = 1264 cm1, so in crystal the energy of HOMOLUMO transition should be lowered from 24506 cm1 (408 nm) to 23242 cm1 (430.2 nm). Presented here, calculations cannot fully explain the experimentally observed features, but they show a tendency that should be expected when going from molecule to dimer, trimer, and to an infinite chain of J-aggregates of DCNP molecule. The oscillator strengths for the lowest lying singlet transitions are rising with respect to higher energy ones and these states are usually the strongest emitters.
6. CONCLUSIONS We studied the fluorescence properties of single crystal of DCNP. The main interest in this studies relies on fact that the fluorescence in this crystal can be excited with nonabsorbed laser light of 1064 nm coming from pulsed Nd:YAG laser. Twophoton excited fluorescence process as well as reabsorption of SHG light can be responsible for the observed fluorescence as the crystal due to its noncentrosymmetric structure can generate SHG light. The fluorescence of this material is strongly anisotropic as the crystal is built-in of almost parallel aligned molecules pointing in one direction with their dipole moments. Lowtemperature measurements of fluorescence in this crystal revealed interesting features. The fluorescence and absorption bands overlap significantly and at 77 K the fluorescence spectrum of DCNP single crystal is quite different from that observed at room temperature. Fluorescence decay profile shows two exponential decay with lifetimes of emitting species of 3.7 and 5.6 ns. We link this with the formation of Frenkel excitons. The experimental findings are supplemented by the quantum chemical calculations of energy levels for the molecule of DCNP, unit cell, and its J-type aggregates. Obviously more studies are necessary to understand the curious luminescent behavior of this compound. The luminescent properties of DCNP are prospective for construction of optically pumped organic lasers, as was proven by our recent work.36 ’ AUTHOR INFORMATION Corresponding Author
*Phone: +48 71 320-35-00. Fax: +48 71 320-33-64. E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by Polish Ministry of Education and Science Grant No. N N507 475237 and in part by the Department of Chemistry, Wroclaw University of Technology. We thank Dr. Marcin Nyk for his help in fluorescence lifetime measurements and Dr. M. Zielinski (ENS Cachan) for polarimetric SHG measurements. ’ REFERENCES (1) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73–76. (2) Diaspro, A.; Corosu, M.; Ramoino, P.; Robello, M. Microsc. Res. Tech. 1999, 47, 196–205. (3) Campagnola, P. J.; Wei, M. D.; Lewis, A.; Loew, L. M. Biophys. J. 1999, 77, 3341–3349. 10696
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