Journal of Chemical Crystallography

, Volume 41, Issue 9, pp 1342–1347

The Synthesis, X-ray Structure Analysis and Photophysical Characterization of 2-(9-Anthrylmethylideneamino)-5-methylphenol


  • Andrés Villalpando
    • Department of ChemistryWhittier College
  • Frank R. Fronczek
    • Department of ChemistryLouisiana State University
    • Department of ChemistryWhittier College
Original Paper

DOI: 10.1007/s10870-011-0100-0

Cite this article as:
Villalpando, A., Fronczek, F.R. & Isovitsch, R. J Chem Crystallogr (2011) 41: 1342. doi:10.1007/s10870-011-0100-0


The Schiff base 2-(9-anthrylmethylideneamino)-5-methylphenol has been prepared in good yield (55%) from commercially available starting materials. The photophysical properties of this compound and its precursor have been determined. The room temperature absorption spectra of both in various solvents exhibited two π → π* transitions that had similar maxima, one that was more intense at approximately 260 nm and a weaker one at approximately 400 nm. Solutions of both compounds in methylcyclohexane and propanenitrile were luminescent at room temperature with maxima in the range 450–500 nm. The emission spectra of both at 77 K in methylcyclohexane solution were similar and exhibited vibrational structure with maxima in the range 460–522 nm. At room temperature and 77 K both compounds had short excited state lifetimes that characterized their emission as fluorescence. The title compound, C22H17NO, crystallized in the triclinic space group \( P{\bar{\text{1}}} \) with a = 8.5533 (5) Å, b = 14.0926 (10) Å, c = 14.9382 (11) Å, α = 104.204 (6)°, β = 106.480 (5)°, γ = 105.091 (5)°, V = 1564.9 (2) Å3, T = 90 K, Dc = 1.322 Mg m−3, Z = 4, R = 0.033.

Graphical Abstract

The X-ray structure and photophysical properties of the title compound (1) were determined.


Schiff baseCrystal structureAbsorptionEmissionLifetime


Schiff bases are a diverse class of useful compounds that can be represented by the general formula RR′C = NR″, where R″ is an alkyl or aryl group that serves to stabilize the C–N double bond. Schiff bases have traditionally been synthesized via acid catalysis [1]. More recent preparations have been devised that are acid-free or utilize metal ions as templates for the construction of larger macrocyclic systems [2, 3]. One unique approach to the synthesis of Schiff bases involves microwave irradiation [4].

Schiff bases have been utilized in numerous research areas. In organic synthesis, they have found use as precursors to novel heterocyclic compounds [5]. The medicinal chemistry and pharmaceutical fields have utilized Schiff bases as antibacterial and antifungal agents [6]. Schiff bases have also been used extensively in the field of coordination chemistry. For example, they have been used to prepare recyclable and water soluble palladium(II) complexes for use in the Suzuki coupling reaction as well as novel luminescent rhenium(I) tricarbonylchloro complexes for use in solar energy collection [7, 8]. Certain Schiff bases have been found to possess nonlinear optical properties, which make them attractive materials for optoelectronics [9].

Our research explores the preparation and photophysics of Schiff bases that incorporate polycyclic aromatic hydrocarbons with the goal of applying them to light emitting diode construction. Toward this end, we have prepared and determined the X-ray structure of the novel phenolic Schiff base 2-(9-anthrylmethylideneamino)-5-methylphenol (1, Scheme 1). The photophysical properties of 1 have also been determined at room temperature and 77 K. The first detailed photophysical characterization of 9-anthracenecarboxaldehyde, the precursor to 1, has also been completed at these temperatures.
Scheme 1

The synthesis of compound 1


Reagents and Techniques

The synthetic procedure was carried out using standard techniques. Solvents and reagents were purchased from Simga-Aldrich or Acros Organics and used as received. 1H- and 13C NMR spectra were recorded on a JEOL ECX 300 MHz spectrometer using TMS as the internal standard. The IR spectrum was recorded as a KBr disk on a JASCO 460 FT-IR. Elemental analysis was done by M–H–W Laboratories of Tucson Arizona. Mass spectrometry was provided by the Washington University Mass Spectrometry Resource with support from the NIH National Center for Research Resources (Grant No. P41RR0954).

Emission and absorption spectra were recorded at room temperature and at 77 K in spectrophotometric grade methylcyclohexane and propanenitrile (99% purity) utilizing a HoribaJobinYvon FluoroMax-4 fluorometer and a Hewlett Packard 8453 diode array spectrometer. Measurements were taken from deoxygenated solutions whose concentrations were approximately 1 × 10−4 or 1 × 10−5 M. All emission spectra were corrected for detector response utilizing a correction curve supplied by the fluorometer manufacturer.

Excited state lifetimes were measured utilizing a HoribaJobinYvon Time Correlated Single Photon Counting (TCSPC) apparatus with excitation from a pulsed LED laser at 388 nm. The absorbances of the analyte solutions were kept at or below 0.1 to avoid inner filter effects. All solutions were deoxygenated prior to measurement acquisition. A multiexponential decay analysis program provided by the instrument manufacturer was used to analyze the data. Three criteria were used to assess the quality of fitted decay curves: chi-squared values (χ2) that were less than 1.2; plots of the residuals that displayed the least oscillation and varied no more than three standard deviations; and visual inspection of the goodness-of-fit between the experimental and fitted decay curves.

Synthesis of 2-(9-anthrylmethylideneamino)-5-methylphenol (1)

20 mL of methanol, 9-anthracenecarboxaldehyde (0.251 g, 1.22 mmol), and 6-amino-m-cresol (0.125 g, 1.01 mmol) were added to a 50 mL round bottom flask with a magnetic stir bar and dry 4 Å molecular sieves. The solution was refluxed over night, until it turned a dark orange/brown color. The solution was filtered hot and allowed to cool, yielding brown needle-like crystals. 0.171 g (55%) of 1 was obtained. MP 119–122 °C; IR (KBr pellet) 3445, 3394, 3047, 2935, 2867, 2822, 1624, 1578, 1259 cm−1; 1H NMR (300 MHz, DMSO-d6) ppm 9.73 (s, 1H), 9.27 (s, 1H), 8.79 (d, 3J = 8.26 Hz, 2H), 8.73 (s, 1H), 8.14 (d, 3J = 7.22 Hz, 2H), 7.57 (m, 4H), 7.25 (d, 3J = 7.91 Hz, 1H), 6.78 (s, 1H), 6.72 (d, 3J = 7.91 Hz, 1H), 2.27 (s, 3H); 13C NMR (75 MHz, DMSO-d6) ppm 159.3, 150.8, 137.8, 137.2, 131.4, 130.4, 130.3, 129.4, 128.5, 127.6, 126.1, 125.7, 120.9, 120.7, 117.3, 21.2; EI-HR-MS: m/z for [M + H]+ = 312.1374, Calcd. m/z for [M + H]+ = 312.1388 Anal. Calcd. for C22H17NO (311.4): C, 84.85; H, 5.51, N, 4.50. Found: C, 85.06; H, 5.64; N, 4.51.

X-ray Structure Determination

An orange plate of 1 (0.25 × 0.25 × 0.13 mm) was used for data collection at T = 90 K on a Bruker Kappa Apex-II CCD area detector diffractomer equipped with an Oxford Cryosystems Cryostream chiller and graphite-monochromated Cu Kα radiation (λ = 1.54178 Å). A total of 14,165 measured reflections with \( \theta_{ \max } = 6 8. 7^\circ \) yielded 5,532 unique data. The structure was solved by direct methods, and structure refinement was carried out using SHELXL-97 [10]. All hydrogen atoms were visible in difference maps, and their coordinates were refined, except for those of the methyl groups, which were idealized with refined torsional parameters. Table 1 lists the details of X-ray data collection, structure solution and refinement. Figure 2 was created using Mercury 2.3, a program for crystal structure visualization, that can be downloaded for free from the Cambridge Crystallographic Data Centre via CCDC 781653 contains the supplementary crystallographic data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via
Table 1

Crystal and experimental data



Formula weight


CCDC deposit no.


Space group

Triclinic, P\({\bar{\text{1}}}\)

Crystal dimension

0.25 × 0.25 × 0.13 mm3

Unit cell parameters



8.5533 (5) Å


14.0926 (10) Å


14.9382 (11) Å




106.480 (5)º




1564.9 (2) Å3


90.5 (5) K



Dc (Mg m−3)



CuKα (λ = 1.54178 Å)


0.63 mm−1



\( \theta_{ \max } \)


Index ranges

−10 ≤ h ≤ 10, −16 ≤ k ≤ 16, −16 ≤ l ≤ 18

Goodness of fit on F2


Reflections collected


Independent reflections


Reflections with I > 2σ(I)


Refined parameters





Bruker Kappa Apex-II CCD area detector diffractometer


Full-matrix least squares method on F2


Bruker Kappa Apex-II CCD area detector diffractometer

Extinction coefficient

0.0026 (3)

\( (\Updelta \rho )_{{{ \max },}} (\Updelta \rho )_{ \min } \)

0.21–0.16 eÅ−3

Results and Discussion

Synthesis and Characterization of 1

Compound 1 was prepared in good yield (55%) via a condensation reaction between 9-anthracenecarboxaldehyde and 6-amino-m-cresol (Scheme 1). It exhibited spectroscopic characteristics similar to other phenolic Schiff bases that exist as an enol-amine tautomer [11].

The infrared spectrum of 1 exhibited absorptions that indicated the hydroxyl group of 1 participated in hydrogen bonding. Two O–H stretching absorptions at 3445 and 3394 cm−1, which are caused by the rotational dynamics of an intermolecular hydrogen bond between two O–H groups, were observed [12]. Intramolecular hydrogen bonding between the hydroxyl proton and the neighboring nitrogen atom was indicated by the absorptions at 2867 and 2822 cm−1 [13]. Other important absorptions from the infrared spectrum include those at 1624 and 1259 cm−1, the former arising from the C–N double bond and the latter from the C–O single bond. Proton resonances were observed for the HC=N and OH moieties in the 1H NMR spectrum of 1, and the HC=N carbon resonance was observed at 159.3 ppm in the 13C NMR spectrum [14]. The identity of 1 was corroborated by an elemental analysis that yielded acceptable values and a HR-EI-MS that had an [M + H]+ peak at m/z = 312.1374, which was approximately 3 ppm less than the calculated value.

Crystallographic Study

Crystals of 1 suitable for X-ray analysis were obtained from the slow evaporation of the reaction mixture. The ORTEP representation of 1 is shown in Fig. 1 while the intermolecular interactions of the anthracene π-system of 1 are illustrated in Fig. 2. Crystal data and structure refinement data are presented in Table 1 and selected bond lengths and angles are listed in Table 2.
Fig. 1

ORTEP view of the two independent molecules of 1. Ellipsoids are represented at the 50% probability level
Fig. 2

Intermolecular interactions of the anthracene π-system of compound 1. a Anthracene C−H···π interaction (b) anthracene ring π-stacking (c) O−H···π interaction

Table 2

Selected bond lengths [Å] and angles [°] for compound 1


1.4764 (14)


1.4716 (15)


1.2791 (15)


1.2788 (15)


1.4160 (13)


1.4146 (14)


1.3638 (13)


1.3670 (14)


1.5085 (15)


1.5065 (15)


1.4204 (16)


1.4201 (16)


1.4390 (15)


1.4400 (15)


1.3956 (16)


1.3937 (16)


119.43 (9)


119.87 (9)


116.6 (7)


116.1 (8)


120.48 (10)


120.38 (10)


118.75 (10)


118.41 (10)


122.99 (10)


124.56 (10)


−176.68 (9)


−175.65 (10)

The asymmetric unit of 1 is comprised of two independent molecules of 1, each of which was found to possess an intramolecular O–H···N hydrogen bond with H···N distance of approximately 2.1 Å. The O···N distances were observed to be 2.6665 (12) Å for O1···N1 and 2.7128 (12) Å for O2···N2. This type of hydrogen bonding has been documented in similar Schiff bases and confirmed the intramolecular hydrogen bonding first observed in the infrared spectrum of 1 [13].

The π-systems of individual molecules of 1 were found to participate in different types of intermolecular interactions. For example, π-stacking about the inversion centers between adjacent phenol rings and anthracene rings was observed, the former with a centroid–centroid distance of 3.718 (2) Å and the later with a centroid–centroid distance of 3.730 (2) Å. An O–H···π interaction was observed between the hydrogen atom of the OH moiety and the anthracene π-system at a distance of 2.687 (2) Å. The anthracene ring systems were observed to be 2.689 (2) Å apart via an edge-on C–H···π interaction. These types of intermolecular interactions have been observed in similar molecules [15, 16].

The central linkage (C16–N1–C15–C1 and C38–N2–C37–C23) between the phenol and anthracene ring systems is constructed of bonds whose length correspond to alternating single and double bonds starting with C16 and C38 [17]. The bond angles of the carbon and nitrogen atoms of the C–N double bond approach 120°, and indicate their sp2 hybrid character. The linkages are fully extended, as given by observed torsion angles of −176.68 (9)° (C38–N2–C37–C23) and −175.65 (10)° (C16–N1–C15–C1). Conjugation between the anthracene and phenol groups is limited, as dihedral angles between these planes are 67.24 (1)° for the molecule containing N1 and 77.98 (1)° for the molecule containing N2.

Bond lengths for O2–C39 and O1–C17 were found to be 1.3638 (15) and 1.3670 (14) Å respectively. These values correlate to a full C–O single bond and support 1 existing in its enol-amine form in the solid state. Other bond angles and lengths observed in 1 compare well with those found in similar compounds [1720].

Photophysical Characterization

The absorption and emission data for 1 and its precursor, 9-anthracenecarboxaldehyde, are listed in Table 3. Their excited state lifetime data are listed in Table 4. Electronic absorption and emission spectra of compound 1 and 9-anthracenecarboxaldehyde in methylcyclohexane solution are presented in Figs. 3 and 4 respectively.
Table 3

Photophysical data for compound 1 and 9-anthracenecarboxaldehydea


λmax,abs nm (ε)b

λmax,em nm (λexc nm)

Compound 1


 Methylcyclohexane solution

413 (12,900)

492 (398)

262 (66,600)

 Methylcyclohexane glass at 77 K


522 (398)



 CH3CH2CN solution

404 (11,900)

493 (408)

257 (72,800)



 Methylcyclohexane solution

398 (7,670)

459 (378)

263 (96,100)

 Methylcyclohexane glass at 77 K


521 (398)



 CH3CH2CN solution

398 (6,800)

480 (408)

263 (100,200)

aSpectra were acquired at room temperature unless otherwise noted

bExtinction coefficient units M−1cm−1

Table 4

Excited state lifetimes with standard deviations and chi-squared (χ2) values for compound 1 and 9-anthracenecarboxaldehydea


\( \tau_{ 1} \left( {\text{ns}} \right) \)

\( \tau_{ 2} \left( {\text{ns}} \right) \)

\( \chi^{ 2} \)

Compound 1

 Methylcyclohexane solution

6.3 ± 0.1



 Methylcyclohexane solution at 77 K

4.1 ± 0.2

12.9 ± 0.5


 CH3CH2CN solution

3.5 ± 0.2

7.8 ± 0.1



 Methylcyclohexane solution

3.7 ± 0.2

13.2 ± 0.2


 Methylcyclohexane solution at 77 K

4.1 ± 0.2

18.0 ± 0.4


 CH3CH2CN solution

3.4 ± 0.5

8.6 ± 0.2


aExcited state lifetimes were acquired at room temperature unless otherwise noted
Fig. 3

Electronic absorption and emission spectra of compound 1 in methylcyclohexane solution. Room temperature absorption spectrum (··–··–); room temperature emission spectrum (λex = 398 nm) (- - - - -); emission spectrum at 77 K (λex = 398 nm) (——)
Fig. 4

Electronic absorption and emission spectra of 9-anthracenecarboxaldehyde in methylcyclohexane solution. Room temperature absorption spectrum (··–··–); room temperature emission spectrum (λex = 378 nm) (- - - - -); emission spectrum at 77 K (λex = 398 nm) (——)

Two bands, an intense one at approximately 260 nm and a weak one at approximately 400 nm characterized the absorption spectra of compound 1 and 9-anthracenecarboxaldehyde in methylcyclohexane and propanenitrile solution. The observed similarity was not unexpected, as the absorption spectra of polycyclic aromatic hydrocarbons have been observed to resemble those of their parent molecules [21]. Extinction coefficients for these bands were large and suggest that they originate π → π* transitions [22].

Solutions of 1 and 9-anthracenecarboxaldehyde in methylcyclohexane yielded emission spectra with broad, featureless bands centered at 492 and 459 nm respectively. The longer wavelength emission maximum of 1 is rationalized in terms of small resonance interaction between the anthracene and phenol moieties.

The emission spectra obtained from a solution of 1 in propanenitrile and methylcyclohexane were very similar, while a dilute propanenitrile solution of 9-anthracenecarboxaldehyde yielded an emission spectrum with a maximum red-shifted by 21 nm when compared to the maximum obtained in methylcyclohexane. This red-shift can be explained in terms of increased resonance interaction between the anthracene rings and the aldehyde moiety, which can arise from solvent intermolecular interactions that keep it coplanar with the aromatic system [22].

The excitation of methylcyclohexane solutions of 1 and 9-anthracenecarboxaldehyde at 77 K yielded emission spectra that had similar maxima that exhibited vibrational structure. This similarity of these spectra arose from the low temperature, which resulted in a majority of the substituents in the 9-position being locked in an angle that approached perpendicularity with the anthracene moiety. This would minimize resonance interactions between the substituent and the anthracene ring system, resulting in emission spectra originating from transitions based mainly on the anthracene rings. The energy differences between the best-resolved maxima in the emission spectra of 1 and 9-anthracenecarboxaldehyde are ca. 1100 cm−1 and ca. 1250 cm−1, both of which correspond to vibrational modes of the anthracene rings [23].

Compound 1 and 9-anthracenecarboxaldehyde, at room temperature and at 77 K, were both observed to possess excited state lifetimes on the nanosecond timescale, which characterized their emission as fluorescence (π* → π).

The excited state decay trace from a room temperature methylcyclohexane solution of compound 1 was fit to a monoexponential curve that gave a lifetime of 6.3 ± 0.1 ns. All other decay traces were fit to biexponential and triexponential curves that gave two different lifetimes (τ1 and τ2): a longer one that ranged from 8–18 ns and a shorter one that ranged from 3–4 ns. Triexponential fitted curves exhibited very short lifetimes (90–280 ps) that arose from scattered light. These were not considered in the assessment of the lifetime data [24].

The two lifetimes for 9-anthracenecarboxaldehyde are attributed to its possessing energetically similar and accessible excited states that arise from varying amounts of resonance interaction between the anthracene rings and the substituent in the 9-position [22]. The lifetimes for compound 1 can be rationalized in terms of the two lumiphores that comprise its structure: the shorter lifetime arising from the phenyl ring and the longer one from the anthracene ring system [9, 2528].


A novel Schiff base, compound 1, was synthesized and its X-ray structure determined. The X-ray structure of 1 illustrated that it is capable of intramolecular hydrogen bonding interactions and that the anthracene π-systems participated in three different intermolecular interactions. The two parts of compound 1 were also shown to be essentially perpendicular by its X-ray structure. Compound 1 and its precursor, 9-anthracenecarboxaldehyde, were found to have similar absorption spectra, each having one intense absorption at approximately 260 nm and a weaker one at approximately 400 nm. Each of these absorptions arises from π → π* transitions. Solutions of 1 and its precursor were observed to be luminescent at room temperature with maxima observed in the region of 450–500 nm. In methylcyclohexane solution at 77 K both 1 and 9-anthracenecarboxaldehyde had essentially identical emission spectra that both exhibited vibrational structure. Compound 1 and 9-anthracenecarboxaldehyde both possessed short excited state lifetimes characteristic of emission arising from fluorescence.


The authors would like to thank the Fletcher Jones Foundation for funds that allowed the purchase of the fluorometer and the TCSPC apparatus. Whittier College is acknowledged for the funds that supported this research. The Edison International Foundation is thanked for a summer research stipend for AV. The purchase of the diffractometer was made possible by grant No. LEQSF(1999–2000)-ENH-TR-13, administered by the Louisiana Board of Regents.

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