Spectral and photophysical properties of the bi-chromophoric fluorene derivatives

Journal of Molecular Structure 734 (2005) 197–204 www.elsevier.com/locate/molstruc

Spectral and photophysical properties of the bi-chromophoric fluorene derivatives T. Redzimskia, J.R. Heldta,b,* a

Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57, PL 80-952 Gdansk, Poland Institute of Physics, Pomeranian Pedagogical Academy, ul. Arciszewskiego 23A, 76-200 Słupsk, Poland

b

Received 15 July 2004; revised 29 September 2004; accepted 29 September 2004 Available online 10 November 2004

Abstract A detailed investigation of the absorption and emission characteristics of bichromphoric compounds: 9-phenyl-9-fluorenol and 2-dimethylamino-9-phenyl(4 0 -dimethylamino)-9-fluorenol was carried out in non-polar and polar solvents at different temperature. The spectral data were used to obtain the outer-sphere solvent reorganization energy, louter, and the intramolecular reorganization energies: li (associated with the vibrations for which hn!kT) and li (hnOkT). They were compared with the destabilization energy Edest. The results of steady-state and kinetic investigations as well as the data of quantum chemical calculation confirm the presence of the two a and b different conformers in the ground and excited state for studied molecules. q 2004 Published by Elsevier B.V. Keywords: Fluorene derivatives; Rotamers luminescence parameters

1. Introduction It is known that the electronic spectra of bichromophoric compounds can be modified by: inductive, mesomeric and steric effects as well as by a specific interaction with the surrounding molecules of the solvents [1]. The first two effects are directly connected with the radiative electron transfer processes (ET). These phenomena are exemplified by the charge transfer (CT) absorption and emission spectra introduced by Mulliken [1], Murrel [2,3] and others [4,5] who studied many electron-donor–acceptor EDA complexes. It follows from those studies that the steric effect significantly produces a large change in the spectrum of a parent molecule mostly if the substituent causes steric distortion. The compounds studied in the present paper are a good example of the conjugate D–A p-systems. (Scheme 1).

Abbreviations: I, 9-phenyl-9-fluorenol; II, 2-dimethylamino-9(4 0 dimethylamino)phenyl-9-fluorenol; FL, fluorene; McH, methylcyclohexane; EtAc, ethyl acetate; THF, tetrahydrofuran; EtOH, ethanol. * Corresponding author. Tel.: C48 58 5529542; fax: C48 58 3413175. E-mail address: [email protected] (J.R. Heldt). 0022-2860/$ - see front matter q 2004 Published by Elsevier B.V. doi:10.1016/j.molstruc.2004.09.024

They consist of two aromatic chromophores: fluorene and a phenyl substituent (being an acceptor) linked by a single bond. Accordingly the 9-phenyl-9-fluorenol (further referred to as I) and 2-dimethylamino-9(4 0 -dimethylamino)phenyl-9-fluorenol (further referred to as II) posses a –OH group substituted in 9 position of fluorene and additionally II possesses two amino groups (being an electron donor): one in position 2 of fluorene and another in 4 0 position of the phenyl group. This specific structure of the investigated molecules gives the possibility of studying the CT phenomena and conformational processes which can occur in these systems. It was also shown that the photophysical parameters of amino derivatives of fluorene are strongly modified upon complexation with alcohol [6,7]. Our previous study proved that specific solute–solvents interaction suggested that the amino group creates the hydrogen-bond complexes in ethanol (EtOH) solution. The main objective of the present work is to evaluate the excitation wavelength and temperature dependence of the vibrational structure of emission spectra. Also, we intend to confirm the existence of isomers of 9-phenyl-9-fluorenol as it was predicted for a phenyl-amino derivative of fluorene [8].

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2. Experimental details The steady and time-resolved spectroscopic measurements of the compounds under study were performed using the same aparatures as in the previous paper [6,8,9]. Thus, we refer to them in order to learn about experimental procedures and details.

3. Results and discussion Room temperature absorption spectra of I and II derivatives of fluorene are shown in Figs. 1A and 2A, respectively. The absorption spectrum of I exhibits obvious features of the spectrum of the parent molecule fluorene [10]. Only a slight shift of the 1LB, 1B2 and 1La transitions [11] are noted in methylcyclohexane (McH) solution in comparison to the spectrum of fluorene [10]. For II, the particular UV-region bands are structureless and are Gaussian in shape. Moreover, from our previous analysis we estimated that the lowest band centered on 29,580 cmK1 can be assigned to a S1(CT))S0 transition. This band arises from the interaction between the donating group –N(CH3)2 and the aromatic chromophores: fluorene and phenyl. As can be seen on Fig. 2A, the appearance of an amino substituent in both chromophores increases the inductive effect and non-specific interaction with the solvents and it causes great changes in absorption spectra of II. The referred figures present also fluorescence excitation spectra in ethyl acetate (EtAc) solution detecting them at two wavelengths lobs. The presented spectra differ from each other and confirm the inhomogeneity of the investigated D–A system in the ground and excited state. Similar effects were observed for II derivative in an identical solution [8].

Solvent and substituent effects on the steady-state fluorescence spectra of the investigated molecules are presented in Figs. 1B and 2B. Both studied derivatives show a single fluorescence band in non-polar solvents. It shows the mirror image symmetry with the long-wavelength absorption band. In EtAc, AcN solutions this symmetry is broken and the shape of the emission band becomes Gaussian like. The fluorescence band maximum is bathochromic shifted by an increase of the solvent polarity. The maximum of the fluorescence spectra are shifted about 1800 and 4100 cmK1 for I and II, respectively, relative to the fluorene spectrum in McH solution. Additionally the second band comes into view (see Fig. 2B) in EtAc solution for II amino derivative. We proved that it results from the conformational isomer which appears in the S0 and S1 state. These isomers (rotamers) arise from the parent molecule in which the 9-phenyl(4 0 -dimethylamino) group connected in 9 position rotates with respect to the fluorene plane. The unusual long tail of the fluorescence band of both molecules in polar solvents confirms this supposition. Additionally this phenomenon for I has been confirmed by determining the fluorescence spectra of I measured as function of the solvent temperature and excitation wavelength. Fig. 3 shows the fluorescence spectra of I in EtAc obtained for two temperatures, e.g. 288 and 333 K. The spectra show that increasing temperature of the sample causes a decrease of the fluorescence band at lZ314 nm, I314, and at the same time an increase in the long wavelength band at lZ325 nm, I325. We postulate that the fluorescence band maximum at 314 and 325 nm corresponds to the a and b rotamers, respectively. It is also obvious that the change in the intensity of the particular band corresponds to the change in the number of emitting species of each conformer. From Fig. 3 follows that the intensity of the fluorescence emission is excitation wavelength dependent, i.e. increasing

Fig. 1. Absorption spectrum (in McH) and fluorescence excitation spectra (in EtAc) for lobsZ312 and 335 nm of I (A). Fluorescence spectrum of I in McH, EtAc, AcN, EtOH solution (B).

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199

Fig. 2. Absorption spectra (in McH and EtOH) and fluorescence excitation spectra (in EtAc) for lobsZ400 and 540 nm of II (A). Fluorescence spectrum of II in McH, EtAc, AcN, EtOH solution (B).

the excitation wavelength up to l values on the red-edge of absorption spectrum causes that mainly the second (b) conformer is excited. For these species the centers of fluorescence posses smaller electronic transition energies. This fact was also found in II molecule. It must be noticed that at 333 K the half-width of the fluorescence band, Dn~1=2 , increases from 2690 to 3520 cmK1 exciting the fluorophore at lexcZ280 and 310 nm, respectively. The pronounced fluorescence band excitation wavelength dependency clearly indicates that the emitting species formed spectroscopic inhomogeneous solution (McH, EtAc, EtOH and AcN) in agreement to Ref. [8]. According to the Lippert–Mataga relationship [12,13], the interaction between the solvents and solute molecules resulting in the energy differences of the absorption and fluorescence maxima, Dn~AF , can be well described by

the expression: Dn~AF Z

ðmex K mg Þ2 f ðn; 3Þ 2p30 hca3

(1)

3 K1 n2 K 1 K 2 ; 23 C 2 n C 2

(2)

f ðn; 3Þ Z

where the mex and mg are the permanent dipole moments of an excited and a ground state of the molecule, a is the effective radius of the Onsanger cavity. The constants: 30, h, c are the permittivity of vacuum (8.85!10K12 CVK1 mK1), Planck’s constant (6.626!10K34 J sK1) and the velocity of the light in vacuum (3.0!108 m sK1). In order to calculate using Eq. (1) the mex value the dipole moment of the ground

Fig. 3. Fluorescence spectra of I at 333 and 288 K in EtAc obtained for different excitation wavelength.

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state, mg, must be known. In the absence of experimental results, mg of the S0 state was calculated using Cache WS 5.04 program and the PM3 method. These calculations give the results 1.6 D for I. For the amino derivative of fluorene equals 3.15 D. This value was calculated and presented in our previous paper [9]. Making use of the experimental data we estimated that the mexZ9.2 D for II and is 3-fold larger than for I (3.1 D) confirms that the amino group increases interaction with the p clouds of two aromatic chromophores in the ground and excited state.

4. Quantum chemical calculation The equilibrium between the a and b rotamer of the compounds under study in their electronically ground and excited state has been determined by semi-empirical calculations using the PM3 method [14–17] as implemented in the CAChe WS 5.04 program. To find out how the position of the phenyl group in position 9 and dimethylamino group in position 2 of fluorenol plane influences the spectral parameters, e.g. the electronic energy value of the S0 and S1 state, we have calculated the total internal energy of both molecules as function of the QhC5–C9–C14–C20 and JhC6–C4–N17–C30 dihedral angles. The obtained results and a and b conformers for each molecule are shown in Fig. 4. As one can see the potential curves possess two minima for dihedral angles Qz25 and 808. Changing the J dihedral angle only one minimum was obtained [8]. The space structures of both molecules for the dihedral angles at which the two curve minima appear are attributed to two stable conformers a and b existing in the ground and first excited state. The energy difference DEZEK E 0 z35,600 cmK1 for I and 32,360 cmK1 for II corresponds to the transition in 225–310 nm region of its absorption spectra. The difference of the depths of the potential curves in the ground and excited state of both molecules DEa,b equals about 275 and 330 cm K1. These values are comparable with thermal energy at room temperature (kTZ200 cmK1) which is sufficient to change the position or cause a twisting torsion of the phenyl or amino-phenyl group. As a result of free rotation of the mentioned group the potential energy curve is the superposition of two curves coming from two conformers. These quantum chemical calculations confirm the existence of the two conformers being in equilibrium determined by a small activation energy DEa,b. It is obvious that its value is a function of the solution, e.g. polarity, temperature and viscosity. 4.1. Reorganization energy The CT reactions connected with the conformational changes of solute molecules in the ground and excited state can be well characterized by Marcus theory in details presented by Braunschwig [18] and others [19–21]. According to that theory the half-width of fluorescence

band, Dn~F1=2 , is related to the sum of the intramolecular reorganization energy li and li, and outer-sphere reorganization energy louter as following: ðDn~F1=2 hcÞ2 Z 2louter C 2li kT C li hni : 8 ln 2

(3)

The inner reorganization energies li and li correspond to the high-frequency motions of the solute and are associated with the changes in the molecule bond lengths and angles. li includes vibrations energy for which hni! kT, and li the intramolecular reorganization energy associated with vibrations for which hniOkT. The outersphere reorganization energy is associated with low~ 200 cmK1 ) such as reorientation frequency motions (n! of the solvent shell. The last term in Eq. (3) represents the dipolar nature reorganization energy of the solvent and solute molecules which can be easily determined using following equation: louter Z

m2e f ð3; nÞ; 4p30 a3

(4)

where f ð3; nÞ Z

3 K1 n2 K 1 K 2 : 23 C 1 2n C 1

(5)

In Eq. (4), me the dipole moment of the solute molecule is in first excited state with the assumption of the negligible small dipole moment in a ground state and a is the Onsanger radius of the solvent shell. Combining Eqs. (3) and (4), the outer-sphere reorganization energy, li and li, can be calculated for different solvents using the formula: li C

li hni ðDn~F1=2 hcÞ m2E K Z f ð3; nÞ: 16kT ln 2 4p30 a3 2kT

(6)

The respective values of the outer-sphere reorganization energy and the intramolecular vibrational and torsional reorganization energies for two fluorene derivatives are collected in Table 1. Analyzing the data we see that for the McH solution louter is only slightly higher than zero for I and II. Increasing the polarity of the solvent causes an increase of the outer-sphere and intramolecular reorganization energy at the same time. In all used solvents, the intramolecular reorganization energy dominates over the louter. The values obtained for II are about 2-fold higher than for I. This finding is a result of the increasing interaction between amino donor group –N(CH3)2 and the polar solvents which stabilizes the CT state which is a consequence of the conformational changes in the molecule. The results obtained for the EtOH solution are not adequate. The reason is the specific interactions (like hydrogen bonds) between the solvent and solute molecules. The values of calculated interaction energies can be verified for fluorene derivatives taking into account the absorption and emission bands maxima related

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Fig. 4. Potential energy curves as a function of dihedral angles Q (A) for I and (B) for II in the ground (S0) and first excited singlet state (S1) and supposed absorption and emission transitions.

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Table 1 Photophysical parameters of the studied molecules at room temperature l 0 iClihni/2kT (cmK1)

l0 (cmK1)

Edest (cmK1)

Solv.

f(3,n)

Dn~1=2 ðcmK1 Þ I

II

I

II

I

II

I

II

McH EtAc AcN EtOH

0.001 0.201 0.305 0.206

2650 2520 2811 4270

3302 3798 3666 3620

w0.1 73 113 81

w1 323 565 403

3146 2742 3388 7985

4839 5968 5323 5404

2581 2500 3065 2823

5404 10,242 8146 7017

to the CT or different conformers’ bands. According to Mataga and Kubota [22], the total destabilization energy is given by: 0K0 Edest Z hcðn~CT K n~CT Þ Z 2ðlouter C ðli C li ÞÞ

(7)

The data of Edest calculated for I are comparable with the value of the sum the outer-sphere and intramolecular reorganization energy for all used solution. Whereas for the molecules II a satisfying agreement is noted for polar solvents only. Also, from Table 1 follows that the highest value of li , li and Edest were obtained for EtOH solution. It confirms that in this solution the specific interaction exists, i.e. hydrogen bonding between the protic solvents and polar solute molecule. From the performed quantum chemical calculation as well as from the literature it is known that the existence of rotamers of bichromophoric molecules in S0 and S1 state and its intertransitions are characterized by some deactivation energy. According to the theory of Alwater et al. [23] based on the diffusion phenomenon the radiative and nonradiative deactivation process are closely correlated with the force concept in the solution and is described by:

kF h Z A C B exp½ðE1 K E2 Þ=RT; T

(8)

where kF z1=tF is the fluorescence rate constant, E1 is the activation energy of the solution viscosity, E2 zli C li : activation energy of movement into free space which is equal to the intramolecular barrier of the torsion vibration of the substituents, e.g. 9-phenyl, h: viscosity of the solvent, R: gas constant, T: temperature, and A and B constants. The E1 value equals 630 cmK1 for EtAc, it has been experimentally determined measuring the viscosity of the solution for different temperatures and fitting the data by the equation hZh 0 exp(KE1/RT) [8]. Supposing that the total fluorescence rate constant consist of the sum of decay constants of both rotamers the ratio of its fluorescence intensity determined for different temperatures is given by:   DEa;b I2b ¥exp K (9) I1a RT Measuring the intensities of the fluorescence spectra of I at different temperatures (see Fig. 5A) at constant excitation conditions, e.g. constant wavelength and intensity of the excitation beam and making use of Eq. (9) the DEa,b value has been determined. Fig. 5B shows the graph of logarithm

Fig. 5. Fluorescence spectra of I in EtAc at different temperature by lexcZ300 nm (A) and the corresponding Arrhenius plot of the rate intensity lnðI2b =I1a Þ versus TK1 (B).

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Table 2 Fluorescence decay times t1, t2 and the pre-exponential coefficient a1 a2 of I and II in different solvents I lobs [nm] / c

II 2

t1 [ns] /a1%

t2 [ns] / a2%

lobs[nm] /c2

t1 [ns] / a1%

t2 [ns] / a2%

375 1.10 435 1.08 400 1.09 512 1.26 415 1.11 490 1.09

1.51 12 1.89 52 1.76 24 1.70 31 1.67 23 1.64 29

3.41 88 4.30 48 4.39 76 6.66 69 5.92 77 6.61 71

McH

323 1.18

0.77 90

4.8 10

THF

325 1.01

1.79 100

–

EtOH

322 1.02

0.10 25

4.7 75

of the fluorescence intensity bands ratio I2b =I1a obtained for various reciprocal temperatures TK1. As it can be seen the experimental data fulfill the Arrhenius low:  b I2 DEa;b ln I a ZK RT . Thus, this graphical procedure can be 1 used to determination the sum of the E1 and E2 of the spectroscopically heterogenic mixture caused by the existence of two emitting rotamers. The logarithm of the ratio of intensity I2b =I1a versus TK1 shows good linearity with rO0.95. The activation barrier determined from the slope equals to 580 cmK1 and can be identified as the sum of intramolecular activation energy and the viscosity activation energy of the solvent, i.e. DEZE1CE2. From Fig. 4B, the depth of potential energy curves equals DEa,bz250 cmK1 in the ground state and this value corresponds to the intramolecular activation energy E2 of the conformational relaxation. Comparing this value with the sum E2 zli C li one can see that the intramolecular energy value is about one order of magnitude smaller than the sum of inner reorganization energies li and li determined using formula introduced by Marcus theory.

5. Time-resolved spectroscopy The fluorescence decay measurements were performed for McH, ThF and EtOH solution detecting the fluorescence light at the band maximum. The excitation wavelength lexcZ280 nm corresponds to the region of the middle band in the absorption spectrum. The experimental data of the fluorescence decay curves of molecule I and II are well characterized by a double-exponential function fitted with a good c2y1 and autocorrelation function values. The fluorescence decay times, tF, and the pre-exponential factors, ai, describing the contribution of the ith fluorescence decay component, are collected in Table 2. As we can see for the three representative solvents the fluorescence decay data are fitted by a two-exponential function for both molecules. This result points that the luminescent solution in the S0 and S1 established a spectroscopic inhomogeneous

medium attributed to the emissions from S1(a) and S1(b) states of the conformers. The absence of the rise time component suggests that the equilibrium between the rotamers a and b is similar in both states or is established in the sub-nanosecond time-scale as predicted for bianthryl [24,25]. It is understandable since the activation barrier between the two conformational states is small and comparable with the thermal energy at room temperature. Analyzing the fluorescence decay data collected in Table 2 follows that both molecules under study posses two decay components, a fast and slow, in all representative solvents used (except I in tetrahydrofuran (THF)). Its participation depends on the solution, which determines the partnership of both rotamers in the total fluorescence emission. Generally, it is noted that the fast decaying component in the fluorescence of I participate in the total fluorescence emission in minor degree whereas for II a reverse dependence is noticed. This finding has its reflection in the noted difference of the calculated electronic energy curves (see Fig. 4). In the case of molecule II the fast decaying component, t1, participate by about 30% in the total emission and does not depend on the emission band. The slower emission mode possesses different decay times for the short and long wavelength fluorescence bands. For all solutions of II the inequality tshort!tlong is fulfilled. This dependence is directly connected with the stabilization of the rotamers by the corresponding medium.

6. Conclusion The qualitative analysis of the data obtained from steadystate absorption and emission spectra, time-resolved fluorescence emission measurements and quantum chemical calculation point that: , The phenyl and 4 0 -dimethylaminophenyl group of the studied molecules is twisted out of plane of the parent

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molecule. It causes that conformational isomers (rotamers) exist in the ground and excited state. , We observed the red-edge effect e.g. excitation wavelength dependence of the fluorescence spectrum in EtAc solution for both molecules. , The presence of two conformer structures of the molecules under study is confirmed by the quantum chemical calculation. , The two-exponential decay of fluorescence emission can be described by a two-exponential function in neutral, aprotic and protic solvents. It points to a spectroscopic heterogeneity of the fluorescence solution under studies.

Acknowledgements The authors express their gratitude to Dr J. Karolczak from the Quantum Electronic Laboratory, Faculty of Physics, A. Mickiewicz University at Poznan for his assistance and making the fluorescence lifetime measuring set-up available.

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